مواضيع المحاضرة: cementing
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C O N T E N T S

1.  OILWELL CEMENTS

  1.1     Functions of oilwell cement

 1.2  Classification of cement powders

 1.3  Mixwater Requirements

2. PROPERTIES OF CEMENT

3. CEMENT ADDITIVES

4. PRIMARY CEMENTING

 4.1  Downhole cementing equipment

 4.2  Surface cementing equipment

 4.3  Single Stage Cementing Operation

 4.4  Multi - Stage cementing Operation

 4.5  Inner string cementing

 4.6  Liner cementing

 4.7  Recommendations for a good cement job

5. SQUEEZE CEMENTING

 5.1  High Pressure Squeeze

 5.2  Low pressure squeeze

 5.3  Equipment used for squeeze cementing

 5.4  Testing the squeeze job

6. CEMENT PLUGS

7.  EVALUATION OF CEMENT JOBS

Cementing


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LEarNiNg OBJECTiVES : 

Having worked through this chapter the student will be able to:

  

General 

•  Describe the principal functions of cement.

Cement Slurries

•  List and describe the major properties of a cement slurry.

•  Describe the additives used in cement slurries and the way in which they affect

 the properties of the slurry.

Cementing Operations 

•  Calculate the volume of : slurry, cement, mixwater, displacing fluid required for

 a single stage and two-stage cementing operation.

•  Calculate the bottomhole pressures generated during the above cementing

 operations.

•  Describe the surface and downhole equipment used in a single, two-stage and  

 liner cementation operation.

•  Prepare a program for a single and two stage cementing operation and describe

 the ways in which a good cement bond can be achieved.

Cement Plugs 

•  Describe the reasons for setting cement plugs.

•   Describe the principal methods for placing a cement plug in casing or open  

  hole. 

•  Calculate the displacement volumes for an underbalanced cement plug.

Evaluation of Cementing Operations

•  Describe the principles involved and the tools and techniques used to evaluate

 the quality of a cementing operation.

•  Discuss the limitations of the above techniques.


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

1.   iNTrOduCTiON

Cement  is  used  primarily  as  an  impermeable  seal  material  in  oil  and  gas  well

drilling. It is most widely used as a seal between casing and the borehole, bonding

the casing to the formation and providing a barrier to the flow of fluids from, or into,

the formations behind the casing and from, and into, the subsequent hole section

(Figure 1). Cement is also used for remedial or repair work on producing wells.

It is used for instance to seal off perforated casing when a producing zone starts

to produce large amounts of water and/or to repair casing leaks. This chapter will

present: the reasons for using cement in oil and gas well drilling; the design of the

cement slurry; and the operations involved in the placement of the cement slurry.

The methods used to determine if the cementing operation has been successful will

also be discussed.

1.1   Functions of oilwell cement

There are many reasons for using cement in oil and gaswell operations. As stated

above, cement is most widely used as a seal between casing and the borehole,

bonding the casing to the formation and providing a barrier to the flow of fluids

from, or into, the formations behind the casing and from, and into, the subsequent

hole section (Figure 1). However, when placed between the casing and borehole the

cement may be required to perform some other tasks. The most important functions

of a cement sheath between the casing and borehole are:

•  To prevent the movement of fluids from one formation to another or from the

 formations to surface through the annulus between the casing and borehole.

•  To support the casing string (specifically surface casing)

•  To protect the casing from corrosive fluids in the formations.

However, the prevention of fluid migration is by far the most important function

of the cement sheath between the casing and borehole. Cement is only required

to support the casing in the case of the surface casing where the axial loads on

the casing, due to the weight of the wellhead and BOP connected to the top of

the casing string, are extremely high. The cement sheath in this case prevents the

casing from buckling.

The techniques used to place the cement in the annular space will be discussed

in detail later but basically the method of doing this is to pump cement down the

inside of the casing and through the casing shoe into the annulus (Figure 2). This

operation is known as a primary cement job. A successful primary cement job is

essential to allow further drilling and production operations to proceed.

          


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Conductor pipe

Surface casing

Cement

Perforations

Production casing

Intermediate casing

Production tubing

Liner

Normally pressured     

 

 

    Abnormally pressured

    

Figure 1 

Functions of Primary Cementing

Circulating 

mud

Pumping  spacer

and slurry

Displacing

Displacing

End of job

Top
cementing 
plug

Bottom
cementing 
plug

Centralizers 

Float 
collar 

Shoe 

Spacer 

Slurry 

Displacing 
Fluid 

Original 
mud 

Plug release pin in

Plug release pin out

Figure 2 

Primary Cementing Operations

                  


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

               

Spot cement                    Apply squeeze                  Reverse circulate 
                                             pressure          

Schematic of Bradenhead squeeze technique normally used on low pressure
formations. Cement is circulated into place down drill pipe (left), then the wellhead,
or BOP, is closed (centre) and squeeze pressure is applied. Reverse circulating 
through perforations (right) removes excess cement, or the plug can be drilled out. 

Figure 3

Secondary or Squeeze Cementing Operation

Another type of cement job that is performed in oil and gas well operations is called

a  secondary or squeeze cement job. This type of cement job may have to

be done at a later stage in the life of the well. A secondary cement job may be

performed for many reasons, but is usually carried out on wells which have been

producing for some time. They are generally part of remedial work on the well

(e.g. sealing off water producing zones or repairing casing leaks). These cement

jobs are often called squeeze cement jobs because they involve cement being forced

through holes or perforations in the casing into the annulus and/or the formation

(Figure 3).

The  specific  properties  of  the  cement  slurry  which  is  used  in  the  primary  and

secondary  cementing  operations  discussed  above  will  depend  on  the  particular

reason for using the cement (e.g. to plug off the entire wellbore or simply to plug

off perforations) and the conditions under which it will be used (e.g. the pressure

and temperature at the bottom of the well).

The cement slurry which is used in the above operations is made up from: cement

powder; water; and chemical additives. There are many different grades of cement

powder manufactured and each has particular attributes which make it suitable for

a particular type of operation. These grades of cement powder will be discussed

below. The water used may be fresh or salt water. The chemical additives (Figure


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4) which are mixed into the cement slurry alter the properties of both the cement

slurry and the hardened cement and will be discussed at length in Section 3 below.

   

CEMENT SLURRY

Retarders;

Calcium lignosulphonate

CMHEC

Saturated salt solution

Extenders;

Bentonite

Pozzolan

Fluid loss additives;

Organic polymers

CMHEC

Mud contaminants;

Diesel
NaOH

Accelerators;

CaCI2

NaCI

Heavy weight material;

Barite

Haemitite

Friction reducers (dispersants);

Polymers

Calcium ligno sulphonate

Figure 4 

Major cement additives

              

API Class   

C3S  

C2S    C3A    C4AF   CaSO4    

SQq. cm/Gram

 

  53 

  24 

  8 

  8 

  3.5    

  1600-1900

 

  44 

  32 

  5 

  12 

  2.9   

  1500-1900

 

  58 

  16 

  8 

  8 

  4.1   

  2000-2400

     D&E 

  50 

  26 

  5 

  13 

  3 

 

  1200-1500

 

  52 

  27 

  3 

  12 

  3.2   

  1400-1600

 

  52 

  25 

  5 

  12 

  3.3   

  1400-1600

  

*Plus free lime, alkali, (Na, K, Mg)

Compounds* 

   

   

            Fineness

                                       

Table 1 

Composition of API Cements

Each cement job must be carefully planned to ensure that the correct cement and

additives are being used, and that a suitable placement technique is being employed

for that particular application. In planning the cement job the engineer must ensure

that:

•  The cement can be placed correctly using the equipment available

•  The cement will achieve adequate compressive strength soon after it is placed

•  The cement will thereafter isolate zones and support the casing throughout the

  life of the well

To assist the engineer in designing the cement slurry, the cement slurry is tested

in the laboratory under the conditions to which it will be exposed in he wellbore.

Theses tests are known as pilot tests and are carried out before the job goes ahead.

These tests must simulate downhole conditions as closely as possible. They will


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

help to assess the effect of different amounts of additives on the properties of the

cement (e.g. thickening time, compressive strength development etc).

                          

 

 

          

API Class   

  Mixwater 

 

Slurry Weight

 

 

   Gals/Sk. 

 

     Lbs/Gal.

 

 

5.2     

   

 

15.6   

 

 

5.2     

   

 

15.6   

 

 

6.3     

   

 

14.8   

       D 

 

4.3     

   

 

16.4   

 

 

4.3     

   

 

16.4   

 

 

4.3     

   

 

16.2

 

 

5.0     

   

 

15.8

 

 

4.3     

   

 

16.4 

  

Table 2 

API Mixwater requirements for API cements

1.2  Classification of cement powders

There are several classes of cement powder which are approved for oilwell drilling

applications, by the American Petroleum Institute - API. Each of these cement

powders  have  different  properties  when  mixed  with  water.    The  difference  in

properties produced by the cement powders is caused by the differences in the

distribution of the four basic compounds which are used to make cement powder;

C

3

S, C

2

S, C

3

A, C

4

AF (Table 1).

Classes A and B 

- These cements are generally cheaper than other classes of cement

and can only be used at shallow depths ,where there are no special requirements.

Class B has a higher resistance to sulphate than Class A.

Class C

  - This cement has a high C

3

S content and therefore becomes hard relatively

quickly.

Classes D,E and F 

- These are known as retarded cements since they take a much

longer time to set hard than the other classes of cement powder. This retardation is

due to a coarser grind. These cement powders are however more expensive than the

other classes of cement and their increased cost must be justified by their ability to

work satisfactorily in deep wells at higher temperatures and pressures.

Class G and H

  - These are general purpose cement powders which are compatible

with most additives and can be used over a wide range of temperature and pressure.

Class G is the most common type of cement and is used in most areas . Class H has

a coarser grind than Class G and gives better retarding properties in deeper wells.

There  are  other,  non-API,  terms  used  to  classify  cement.    These  include  the

following:

•  Pozmix cement - This is formed by mixing Portland cement with pozzolan

(ground volcanic ash) and 2% bentonite. This is a very lightweight but durable

cement. Pozmix cement is less expensive than most other types of cement and due

to its light weight is often used for shallow well casing cementation operations.


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Portland 

API Class G  

API ClassH

Water,   

5.19 

 4.97 

4.29

gal./sk.

Slurry Wt

15.9 

 15.8  

16.5

lb./gal.

Slurry Vol. 

1.8 

 1.14  

1.05

cuft./sk.

Temp. (deg. F)  Pressure (psi)               Typical comp. strength (psi) @ 12hrs

 

60 

615  

 440 

 325

 

80 

1470 

 1185 

 1065

 

95 

800 

2085 

 2540 

 2110

 

110 

1600 

2925 

 2915 

 2525

 

140 

3000 

5050 

 4200 

 3160

 

170 

3000 

5920 

 4380 

 4485

 

200 

3000 

-   

 5110 

 4575

 

 

 

 Typical comp. strength (psi) @ 24hrs

 

60 

2870 

  - 

  -

 

80 

4130 

  - 

  -

 

95 

800 

4130 

  - 

  -

 

110 

1600 

5840 

  - 

  -

 

140 

3000 

6550 

  - 

 7125

 

170 

3000 

6210 

 5865 

 7310

 

200 

3000 

-   

 7360 

 9900 

                                       

Table 3 

Compressive strength of cements

•  Gypsum Cement - This type of cement is formed by mixing Portland cement

with gypsum. These cements develop a high early strength and can be used for

remedial work. They expand on setting and deteriorate in the presence of water and

are therefore useful for sealing off lost circulation zones.

•  Diesel oil cement - This is a mixture of one of the basic cement classes (A, B, G,

H ), diesel oil or kerosene and a surfactant. These cements have unlimited setting

times and will only set in the presence of water. Consequently they are often used

to seal off water producing zones, where they absorb and set to form a dense hard

cement.

1.3  Mixwater Requirements

The water which is used to make up the cement slurry is known as the mixwater.  

The amount of mixwater used to make up the cement slurry is shown in Table 2.

These amounts are based on :

•  The need to have a slurry that is easily pumped.

•  The need to hydrate all of the cement powder so that a high quality hardened

 cement is produced.

•  The need to ensure that all of the free water is used to hydrate the cement  

 powder and that no free water is present in the hardened cement.


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

The amount of mixwater that is used to make up the cement slurry is carefully

controlled. If too much mixwater is used the cement will not set into a strong,

impermeable cement barrier. If not enough mixwater is used :

• The slurry density and viscosity will increase.

• The pumpability will decrease

• Less volume of slurry will be obtained from each sack of cement

The quantities of mixwater quoted in Table 2 are average values for the different

classes of cement. Sometimes the amount of mixwater used will be changed to

meet the specific temperature and pressure conditions which will be experienced

during the cement job.

2.   PrOPErTiES OF CEMENT

The properties of a specific cement slurry will depend on the particular reason for

using the cement, as discussed above. However, there are fundamental properties

which must be considered when designing any cement slurry.

(a)   Compressive strength

The casing shoe should not be drilled out until the cement sheath has reached a

compressive strength of about 500 psi. This is generally considered to be enough

to support a casing string and to allow drilling to proceed without the hardened

cement sheath, disintegrating, due to vibration. If the operation is delayed whilst

waiting on the cement to set and develop this compressive strength the drilling rig

is said to be “waiting on cement” (WOC). The development of compressive

strength is a function of several variables, such as: temperature; pressure; amount

of mixwater added; and elapsed time since mixing.

The setting time of a cement slurry can be controlled with chemical additives, known

as accelerators. Table 3 shows the compressive strengths for different cements

under varying conditions.

(b)   Thickening time (pumpability)

The thickening time of a cement slurry is the time during which the cement slurry

can be pumped and displaced into the annulus (i.e. the slurry is pumpable during

this time). The slurry should have sufficient thickening time to allow it to be:

• Mixed

• Pumped into the casing

• Displaced by drilling fluid until it is in the required place

Generally 2 - 3 hours thickening time is enough to allow the above operations to

be completed. This also allows enough time for any delays and interruptions in the

cementing operation. The thickening time that is required for a particular operation

will be carefully selected so that the following operational issues are satisfied:

•  The cement slurry does not set whilst it is being pumped

•  The cement slurry is not sitting in position as a slurry for long periods,

 potentially being contaminated by the formation fluids or other contaminants


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•  The rig is not waiting on cement for long periods.

Wellbore conditions have a significant effect on thickening time. An increase in

temperature, pressure or fluid loss will each reduce the thickening time and these

conditions will be simulated when the cement slurry is being formulated and tested

in the laboratory before the operation is performed.

(c) Slurry density

The standard slurry densities shown in Table 2 may have to be altered to meet

specific operational requirements (e.g. a low strength formation may not be able

to support the hydrostatic pressure of a cement slurry whose density is around 15

ppg). The density can be altered by changing the amount of mixwater or using

additives to the cement slurry. Most slurry densities vary between 11 - 18.5 ppg.

It should be noted that these densities are relatively high when the normal formation

pore pressure gradient is generally considered to be equivalent to 8.9 ppg. It is

generally the case that cement slurries generally have a much higher density than

the drilling fluids which are being used to drill the well. The high slurry densities

are however unavoidable if a hardened cement with a high compressive strength

is to be achieved.

(d) Water loss

The slurry setting process is the result of the cement powder being hydrated by

the mixwater. If water is lost from the cement slurry before it reaches its intended

position in the annulus its pumpability will decrease and water sensitive formations

may be adversely affected. The amount of water loss that can be tolerated depends

on the type of cement job and the cement slurry formulation.

Squeeze cementing requires a low water loss since the cement must be squeezed

before the filter cake builds up and blocks the perforations. Primary cementing is

not so critically dependent on fluid loss. The amount of fluid loss from a particular

slurry should be determined from laboratory tests. Under standard laboratory

conditions (1000 psi filter pressure, with a 325 mesh filter) a slurry for a squeeze

job should give a fluid loss of 50 - 200 cc. For a primary cement job 250 - 400 cc

is adequate.

(e) Corrosion resistance

Formation water contains certain corrosive elements which may cause deterioration

of the cement sheath. Two compounds which are commonly found in formation

waters are sodium sulphate and magnesium sulphate. These will react with lime

and C

3

S to form large crystals of calcium sulphoaluminate. These crystals expand

and cause cracks to develop in the cement structure. Lowering the C

3

A content of 

the cement increases the sulphate resistance. For high sulphate resistant cement

the C

3

A content should be 0 - 3%

(f) Permeability

After the cement has hardened the permeability is very low (<0.1 millidarcy). This

is much lower than most producing formations. However if the cement is disturbed

during setting (e.g. by gas intrusion) higher permeability channels (5 - 10 darcies)

may be created during the placement operation.


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  Cement   

Gel 

 

Mixwater 

                     Slurry Density           Slurry Volume

    Class 

gal/sk. 

cu. ft/sk 

ppg 

pcf 

cu. ft/sk

 

44.0 

4.96 

0.663 

15.9 

118.70 

1.14

 

65.2 

7.35 

0.982 

14.3 

107.00 

1.49

 

88.4 

9.74 

1.302 

13.3 

99.77 

1.83

 

12 

107.2 

12.10 

1.621 

12.7 

94.83 

2.18 

 

16 

128.8 

14.50 

1.940 

12.2 

91.24 

2.52 

 

 

 

 

 

  

 

 

 

   

  

SLURRY COMPOSITION

                                       

  Cement   

Gel 

Time   

 

Class 

hrs. 

80 deg F  100 deg F 

120 deg F  140 deg F 

160 deg F

 

24 

1800 

3050 

4150 

5020 

6700

 

24 

860 

1250 

1830 

1950 

2210 

 

24 

410 

670 

890 

1090 

1340 

 

 

 

 

 

 

 

 

 

  

 

 

 

   

  

COMPRESSIVE STRENGTH, psi 

                                       

  Cement   

Gel                                 Casing Schedules, Hrs; mins.

    Class 

2000 ft 

4000ft 

6000ft 

8000ft 

10000ft

 

 

 

91 deg F  103 deg F  113 deg F 

126 deg F  144 deg F

 

4:30 

2:50 

2:24 

1:50 

1:20

 

4:10 

2:18 

1:51 

1:27 

  0:57 

 

5:00 

2:43 

2:06 

1:38 

  1:04 

   

 

  

 

 

 

   

  

THICKENING TIME

                                       

Table 4

Cements with bentonite

3.   CEMENT addiTiVES

Most cement slurries will contain some additives, to modify the properties of the

slurry and optimise the cement job. Most additives are known by the trade-names

used by the cement service companies. Cement additives can be used to:

• Vary the slurry density

• Change the compressive strength

• Accelerate or retard the setting time

• Control filtration and fluid loss

• Reduce slurry viscosity

Additives may be delivered to the rig in granular or liquid form and may be blended

with the cement powder or added to the mixwater before the slurry is mixed. The

amount of additive used is usually given in terms of a percentage by weight of the

cement powder (based on each sack of cement weighing 94 lb). Several additives

will affect more than one property and so care must be taken as to how they are used

(Figure 4).


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1

It should be remembered that the slurry is mixed up and tested in the laboratory

before the actual cement job.

(a)   Accelerators   
Accelerators are added to the cement slurry to shorten the time taken for the cement

to set. These are used when the setting time for the cement would be much longer

than that required to mix and place the slurry, and the drilling rig would incur WOC

time. Accelerators are especially important in shallow wells where temperatures are

low and therefore the slurry may take a long time to set. In deeper wells the higher

temperatures promote the setting process, and accelerators may not be necessary.

The most common types of accelerator are:

• Calcium chloride (CaCl

2

) 1.5 - 2.0%

• Sodium chloride (NaCl) 2.0 - 2.5%

• Seawater

It  should  be  noted  that  at  higher  concentrations  these  additives  will  act  as

retarders.

(b)   Retarders
In deep wells the higher temperatures will reduce the cement slurry’s thickening

time. Retarders are used to prolong the thickening time and avoid the risk of the

cement  setting  in  the  casing  prematurely.   The  bottom  hole  temperature  is  the

critical factor which influences slurry setting times and therefore for determining

the need for retarders. Above a static temperature of 260 - 275 degrees F the effect

of retarders should be measured in pilot tests.

The most common types of retarders are:

• Calcium lignosulphanate (sometimes with organic acids) 0.1 - 1.5%

• Saturated Salt Solutions

(c)   Lightweight additives (Extenders)
Extenders are used to reduce slurry density for jobs where the hydrostatic head

of the cement slurry may exceed the fracture strength of certain formations. In

reducing the slurry density the ultimate compressive strength is also reduced and

the thickening time increased. The use of these additives allows more mixwater

to be added, and hence increases the amount of slurry which is produced by each

sack of cement powder (the yield of the slurry). Such additives are therefore

sometimes called extenders.

The most common types of lightweight additives are:

•  Bentonite (2 - 16%) - This is by far the most common type of additive used to

lower slurry density. The bentonite material absorbs water, and therefore allows

more mixwater to be added.  Bentonite will also however  reduce compressive

strength and sulphate resistance. The increased yield due to the bentonite added is

shown in Table 4.


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•  Pozzolan - This may be used in a 50/50 mix with the Portland cement. The result

is a slight decrease in compressive strength, and increased sulphate resistance.

•  Diatomaceous earth (10 - 40%) - The large surface area of diatomaceous earth

allows  more  water  absorption,  and  produces  low  density  slurries  (down  to  11

ppg).

(d) Heavyweight additives
Heavyweight additives are used when cementing through overpressured zones. The

most common types of additive are:

•  Barite (barium sulphate) - this can be used to attain slurry densities of up to

18ppg. It also causes a reduction in strength and pumpability.

•  Hematite (Fe

2

O

3

) - The high specific gravity of hematite can be used to raise slurry

densities to 22 ppg. Hematite significantly reduces the pumpability of slurries and

therefore friction reducing additives may be required when using hematite.

• Sand - graded sand (40 - 60 mesh) can give a 2 ppg increase in slurry density.

(e)   Fluid loss additives
Fluid  loss  additives  are  used  to  prevent  dehydration  of  the  cement  slurry  and

premature setting. The most common additives are:

• Organic polymers (cellulose) 0.5 - 1.5%

• Carboxymethyl hydroxyethyl cellulose (CMHEC) 0.3 - 1.0%

(CMHEC will also act as a retarder)

(f) Friction reducing additives (Dispersants)
Dispersants are added to improve the flow properties of the slurry. In particular

they will lower the viscosity of the slurry so that turbulence will occur at a lower

circulating pressure, thereby reducing the risk of breaking down formations. The

most commonly used are:

• Polymers 0.3 - 0.5 lb/sx of cement

• Salt 1 - 16 lb/sx

• Calcium lignosulphanate 0.5 - 1.5 lb/sxg)

(g)  Mud contaminates
As well as the compounds deliberately added to the slurry on surface, to improve

the slurry properties, the cement slurry will also come into contact with, and be

contaminated by, drilling mud when it is pumped downhole. The chemicals in the

mud may react with the cement to give undesirable side effects. Some of these

are listed below:

    Mud additive                   

Effect on cement        

   barite  

increases density and reduces

   

compressive strength

   caustic  

acts as an accelerator


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   calcium compounds

decrease density

 diesel oil

decrease density

   thinners

act as retarders

           

   

The mixture of mud and cement causes a sharp increase in viscosity. The major

effect of a highly viscous fluid in the annulus is that it forms channels which are

not easily displaced. These channels prevent a good cement bond all round the

casing.

To prevent mud contamination of the cement a spacer fluid is pumped ahead of the

cement slurry.

4.   PriMarY CEMENTiNg

The objective of a primary cement job is to place the cement slurry in the annulus

behind the casing. In most cases this can be done in a single operation, by pumping

cement down the casing, through the casing shoe and up into the annulus. However,

in longer casing strings and in particular where the formations are weak and may

not be able to support the hydrostatic pressure generated by a very long colom of

cement slurry, the cement job may be carried out in two stages. The first stage is

completed in the manner described above, with the exception that the cement slurry

does not fill the entire annulus, but reaches only a pre-determined height above the

shoe. The second stage is carried out by including a special tool in the casing

string which can be opened, allowing cement to be pumped from the casing and into

the annulus. This tool is called a multi stage cementing tool and is placed in the

casing string at the point at which the bottom of the second stage is required. When

the second stage slurry is ready to be pumped the multi stage tool is opened and

the second stage slurry is pumped down the casing, through the stage cementing

tool and into the annulus, as in the first stage. When the required amount of slurry

has been pumped, the multi stage tool is closed. This is known as a two stage 

cementing operation

and will be discussed in more detail later.

The height of the cement sheath, above the casing shoe, in the annulus depends

on the particular objectives of the cementing operations. In the case of conductor

and surface casing the whole annulus is generally cemented so that the casing is

prevented from buckling under the very high axial loads produced by the weight

of the wellhead and BOP. In the case of the intermediate and production casing

the top of the cement sheath (Top of Cement - TOC) is generally selected to be

approximately 300-500 ft. above any formation that could cause problems in the

annulus of the casing string being cemented. For instance, formations that contain

gas which could migrate to surface in the annulus would be covered by the cement.

Liners are generally cemented over their entire length, all the way from the liner

shoe to the liner hanger.

4.1  Downhole cementing equipment

In order to carry out a conventional primary cement job some special equipment

must be included in the casing string as it is run.


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• Guide shoe - A guide (Figure 5) shoe is run on the bottom of the first joint of

casing. It has a rounded nose to guide the casing past any ledges or other irregularities

in the hole .

          

             

.

                             

Drillable
material

Float  valve

Float  shoe

Guide  shoe

                     

Figure 5 

Guide shoe and float shoe

• Float collar - A float collar (Figure 6) is positioned 1 or 2 joints above the guide

shoe. It acts as a seat for the cement plugs used in the pumping and displacement

of the cement slurry. This means that at the end of the cement job there will be

some cement left in the casing between the float collar and the guide shoe which

must be drilled out.

The float collar also contains a non-return valve so that the cement slurry cannot

flow back up the casing. This is necessary because the cement slurry in the annulus

generally has a higher density than the displacing fluid in the casing, therefore a

U-tube effect is created when the cement is in position and the pumps are stopped.

Sometimes the guide shoe also has a non-return valve as an extra precaution. It is

essential that the non-return valves are effective in holding back the cement slurry.


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Drillable
material

Float  valve

       

                        

Figure 6 

Float collar

The use of a non-return valve means that as the casing is being run into the borehole

the fluid in the hole cannot enter the casing from below. This creates a buoyancy

effect which can be reduced by filling up the casing from the surface at regular

intervals while the casing is being run (every 5 - 20 joints). This filling up process

increases  the  running  in  time  and  can  be  avoided  by  the  use  of  automatic  or

differential fill up devices fitted to the float collar or shoe. These devices allow a

controlled amount of fluid to enter the casing at the bottom of the string. The ports

through which the fluid enters are blocked off before the cement job begins. The

use of a differential fill-up device also reduces the effect of surge pressures on the

formation .

                                               

•    Centralisers  -  these  are  hinged  metal  ribs  which  are  installed  on  the  casing 

 string as it is run (Figure 7). Their function is to keep the casing away from the 

 borehole so that there is some annular clearance around the entire circumference 

   of the casing 

The proper use of centralisers will help to:

•    Improve  displacement  efficiency  (i.e.  place  cement  all  the  way  around  the 

 casing)

• Prevent differential sticking

• Keep casing out of keyseats


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Centralisers are particularly required in deviated wells where the casing tends to lie

on the low side of the hole. On the high side there will be little resistance to flow,

and so cement placement will tend to flow up the high side annular space. Mud

channels will tend to form on the low side of the hole, preventing a good cement

job. Each centraliser is hinged so that it can be easily clamped onto the outside of

the casing and secured by a retaining pin. The centraliser is prevented from moving

up and down the casing by positioning the centraliser across a casing coupling or a

collar known as a stop collar. The spacing of centralisers will vary depending on

the requirements of each cement job. In critical zones, and in highly deviated parts

of the well, they are closely spaced, while on other parts of the casing string they

may not be necessary at all. A typical programme might be:

1 centraliser immediately above the shoe

1 every joint on the bottom 3 joints

1 every joint through the production zone

1 every 3 joints elsewhere

                                                 

                                            

Figure 7 

Casing Centraliser

•  Wipers/scratchers - these are devices run on the outside of the casing to remove 

 mud cake and break up gelled mud. They are sometimes used through the  

 production zone.


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Cutting table

Hopper

Screen

Slurry tub

Mixing manifold

To triplex pump
slurry suction

Fluid end

HP hoses

To centrifugal 
pump

Discharge gooseneck

Jet mixer

Figure 8 

Cement unit showing jet mixer

4.2   Surface cementing equipment

Mixing and pumping facilities:
On  most  rigs  cement  powder  and  additives  are  handled  in  bulk,  which  makes

blending  and  mixing  much  easier.    For  large  volume  cement  jobs  several  bulk

storage bins may be required on the rig. On offshore rigs the cement is transferred

pneumatically from supply boats to the storage bins.

For any cement job there must be sufficient water available to mix the slurry at the

desired water/cement ratio when required. The mix water must also be free of all

contaminants.  

The water is added to the cement in a jet mixer (Figure 8). The mixer consists of

a funnel shaped hopper, a mixing bowl, a water supply line and an outlet for the

slurry. As the mixwater is pumped across the lower end of the hopper a venturi

effect is created and cement powder is drawn down into the flow of mixwater and

a slurry is created. The slurry flows into a slurry tub where its density is measured.

The density of the slurry should be regularly checked during the cement job since

this is the primary means by which the quality of the slurry is determined. If the

density of the slurry is correct then the correct amount of mixwater has been mixed

with the cement powder. Samples can be taken directly from the mixer and weighed

in a standard mud balance or automatic devices (densometers) can also be used.

Various types of cement pumping units are available. For land based jobs they

can be mounted on a truck, while skid mounted units are used offshore. The unit

normally  has  twin  pumps  (triplex,  positive  displacement)  which  may  be  diesel

powered or driven by electric motors. These units can operate at high pressures

(up to 20,000 psi) but are generally limited to low pumping rates. Most units are

capable of mixing and displacing 50 - 70 cubic feet of slurry per minute. In order

to minimise contamination by the mud in the annulus a preflush or spacer fluid is

pumped ahead of the cement slurry. The actual composition of the spacer depends


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on the type of mud being used. For water based muds the spacer fluid is often just

water, but specially designed fluids are available. The volume of spacer is based on

the need to provide sufficient separation of mud and cement in the annulus (20 - 50

bbls of spacer is common).

                           

Hex plug

Cap

Body

Bull plug

Bail assy.
w/lock bolt

Manifold
assembly:
2" pipe 
fittings

         

                

Figure 9 

Cement Head

Cementing heads:
The  cement  head  provides  the  connection  between  the  discharge  line  from  the

cement  unit  and  the  top  of  the  casing  (Figure  9).    This  piece  of  equipment  is

designed to hold the cement plugs used in the conventional primary cement job.

The cement head makes it possible to release the bottom plug, mix and pump down

the cement slurry, release the top plug and displace the cement without making or

breaking the connection to the top of the casing. For ease of operation the cement

head should be installed as close to rig floor level as possible. The cement jobs will

be unsuccessful if the cement plugs are installed in the correct sequence or are not

released from the cementing head.

Mud is normally used to displace the cement slurry. The cement pumps or the rig

pumps may be used for the displacement. It is recommended that the cement slurry

is displaced at as high a rate as possible. High rate displacement will aid efficient

mud displacement. It is highly unlikely that it will be possible to achieve turbulence

in the cement slurry since it is so viscous and has such a high density. However, it

may be possible to generate turbulence in the spacer and this will result in a more

efficient displacement of the mud.


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4.3   Single Stage Cementing Operation 

The single stage primary cementing operation is the most common type of cementing

operation that is conducted when drilling a well. The procedure for performing a

single stage cementing operation (Figure 10) will be discussed first and then the

procedure for conducting a multiple stage and stinger cementing operations

will be discussed.

Circulating 

mud

Pumping  spacer

and slurry

Displacing

Displacing

End of job

Top
cementing 
plug

Bottom
cementing 
plug

Centralizers 

Float 
collar 

Shoe 

Spacer 

Slurry 

Displacing 
Fluid 

Original 
mud 

Plug release pin in

Plug release pin out

 

Figure 10 

Single Stage Cementing Operation

In the case of the single stage operation, the casing with all of the required cementing

accessories such as the float collar, centralisers etc. is run in the hole until the shoe is

just a few feet off the bottom of the hole and the casing head is connected to the top

of the casing. It is essential that the cement plugs are correctly placed in the cement

head. The casing is then circulated clean before the cementing operation begins (at

least one casing volume should be circulated). The first cement plug (wiper plug)

shown in Figure 11, is pumped down ahead of the cement to wipe the inside of the

casing clean. The spacer is then pumped into the casing. The spacer is followed by

the cement slurry and this is followed by the second plug (shut-off plug) shown

in Figure 12. When the wiper plug reaches the float collar its rubber diaphragm is

ruptured, allowing the cement slurry to flow through the plug, around the shoe, and

up into the annulus. At this stage the spacer is providing a barrier to mixing of the

cement and mud. When the solid, shut-off plug reaches the float collar it lands on

the wiper plug and stops the displacement process. The pumping rate should be

slowed down as the shut-off plug approaches the float collar and the shut-off plug

should be gently bumped into the bottom, wiper plug. The casing is often pressure

tested at this point in the operation. The pressure is then bled off slowly to ensure

that the float valves, in the float collar and/or casing shoe, are holding.


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The displacement of the top plug is closely monitored. The volume of displacing

fluid necessary to bump the plug should be calculated before the job begins. When

the pre-determined volume has almost been completely pumped, the pumps should

be slowed down to avoid excessive pressure when the plug is bumped. If the top

plug does not bump at the calculated volume (allowing for compression of the mud)

this may be because the top, shut-off plug has not been released. If this is the case,

no more fluid should be pumped, since this would displace the cement around the

casing shoe and up the annulus. Throughout the cement job the mud returns from

the annulus should be monitored to ensure that the formation has not been broken

down. If formation breakdown does occur then mud returns would slow down or

stop during the displacement operation.

The single stage procedure can be summarised as follows:

1. Circulate the casing and annulus clean with mud (one casing volume pumped)

2. Release wiper plug

3. Pump spacer

4. Pump cement

5. Release shut-off plug

6. Displace with displacing fluid (generally mud) until the shut-off plug lands on

 the float collar

7. Pressure test the casing

                                        

Rupture Disk

Moulded 
Elastomer 

Aluminium 
Core

Figure 11 

Bottom Plug (wiper plug)


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Figure 12 

Top Plug (shut off plug)

4.4  Multi - Stage Cementing Operation

When a long intermediate string of casing is to be cemented it is sometimes necessary

to split the cement sheath in the annulus into two, with one sheath extending from the

casing shoe to some point above potentially troublesome formations at the bottom

of the hole, and the second sheath covering shallower troublesome formations. The

placement of these cement sheaths is known as a multi-stage cementing operation

(Figure 13). The reasons for using a multi-stage operation are to reduce:

•  Long pumping times

•  High pump pressures

•  Excessive hydrostatic pressure on weak formations due to the relatively high

 density of cement slurries.


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Figure 13 

Multi-Stage Cementing Operation

             

The procedure for conducting a multi-stage operation is as follows:

First stage

 

The procedure for the first stage of the operation is similar to that described in

Section 4.3 above, except that a wiper plug is not used and only a liquid spacer is

pumped ahead of the cement slurry. The conventional shut-off plug is replaced by

a plug with flexible blades. This type of shut-off plug is used because

            

it has to pass through the stage cementing collar which will be discussed below. It

is worth noting that a smaller volume of cement slurry is used, since only the lower

part of the annulus is to be cemented. The height of this cemented part of the

annulus will depend on the fracture gradient of the formations which are exposed in

the annulus (a height of 3000' - 4000' above the shoe is common).

Second stage
The second stage of the operation involves the use of a special tool known as a

stage collar (Figure 14), which is made up into the casing string at a pre-determined

position. The position often corresponds to the depth of the previous casing shoe.

The ports in the stage collar are initially sealed off by the inner sleeve. This sleeve

is held in place by retaining pins. After the first stage is complete a special dart is

released form surface which lands in the inner sleeve of the stage collar. When a

pressure of 1000 - 1500 psi is applied to the casing above the dart, and therefore

to the dart, the retaining pins on the inner sleeve are sheared and the sleeve moves


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down, uncovering the ports in the outer mandrel. Circulation is established through

the stage collar before the second stage slurry is pumped.

The normal procedure for the second stage of a two stage operation is as follows:

1  Drop opening dart

2  Pressure up to shear pins

3  Circulate though stage collar whilst the first stage cement is setting

4  Pump spacer

5  Pump second stage slurry

6  Release closing plug

7  Displace plug and cement with mud

8  Pressure up on plug to close ports in stage collar and pressure test the casing.

                                                    

Closing 
sleeve

Lock 
ring

Shear 
pin

Drillable
opening
seat

Opening 
sleeve

Ports

Figure 14 

Multi-Stage Cementing Collar

                                                  

To prevent cement falling down the annulus a cement basket or packer may be run

on the casing below the stage collar. If necessary, more than one stage collar can

be run on the casing so that various sections of the annulus can be cemented. One

disadvantage of stage cementing is that the casing cannot be moved after the first

stage cement has set in the lower part of the annulus. This increases the risk of

channelling and a poor cement bond.


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4.5   Inner string cementing

For  large  diameter  casing,  such  as  conductors  and  surface  casing,  conventional

cementing techniques result in:

• The potential for cement contamination during pumping and displacement

• The use of large cement plugs which can get stuck in the casing

• Large displacement volumes

• Long pumping times

• Large volume of cement left inside the casing between float collar and shoe.

An alternative technique, known as a stinger cement job, is to cement the casing

through a tubing or drillpipe string, known as a cement stinger, rather than through

the casing itself.  

In the case of a stinger cement job the casing is run as before, but with a special

float shoe (Figure 15) rather than the conventional shoe and float collar. A special

sealing adapter, which can seal in the seal bore of the seal float shoe, is attached to

the cement stinger. Once the casing has been run, the cementing string (generally

tubing or drillpipe), with the seal adapter attached, is run and stabbed into the float

shoe. Drilling mud is then circulated around the system to ensure that the stinger

and annulus are clear of any debris. The cement slurry is then pumped with liquid

spacers  ahead  and  behind  the  cement  slurry.    No  plugs  are  used  in  this  type  of

cementing  operation  since  the  diameter  of  the  stinger  is  generally  so  small  that

contamination of the cement is unlikely if a large enough liquid spacer is used.

The cement slurry is generally under-displaced so that when the seal adapter on

the stinger is pulled from the shoe the excess cement falls down on top of the shoe.

This can be subsequently drilled out when the next hole section is being drilled.

Under-displacement  however  ensures  that  the  cement  slurry  is  not  displaced  up

above the casing shoe, leaving spacer and drilling mud across the shoe. After the

cement has been displaced, and the float has been checked for backflow, the cement

stinger can be retrieved. This method is suitable for casing diameters of 13 3/8" and

larger. The main disadvantage of this method is that for long casing strings rig time

is lost in running and retrieving the inner string.


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Tool joint 
adapter

Sealing 
adapter

Sealing 
sleeve

Super seal
float shoe

Special
float shoe

Casing

Cement

Drillpipe
or tubing

Figure 15 

Stinger Cementing Operation

 

4.6   Liner cementing

Liners are run on drillpipe and therefore the conventional cementing techniques

cannot be used for cementing a liner. Special equipment must be used for cementing

these liners.

As with a full string of casing the liner has a float collar and shoe installed. In

addition there is a landing collar, positioned about two joints above the float collar

(Figure 16). A wiper plug is held on the end of the tailpipe of the running string by

shear pins.


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Float shoe

Cementing 
head 

Setting tool

Hanger slips

Centralizers

Float collar

Landing collar

Wiper plug

Slick joint

Packoff

Tie- back 
sleeve

Cementing 
manifold

Figure 16 

Liner Cementing Equipment

                              

The liner is run on drillpipe and the hanger is set at the correct point inside the

previous casing string. Mud is circulated to ensure that the liner and the annulus is

free from debris, and to condition the mud. Before the cementing operation begins

the liner setting tool is backed off to ensure that it can be recovered at the end of the

cement job. The cementing procedure is as follows:

1  Pump spacer ahead of cement slurry

2  Pump slurry

3  Release pump down plug

4  Displace cement down the running string and out of the liner into the annulus

5  Continue pumping until the pump down plug lands on the wiper plug.

6  Apply pressure to the pump down plug and shear out the pins on the wiper  

 plug. This releases the wiper plug

7  Both plugs move down the liner until they latch onto landing collar

8  Bump the plugs with 1000 psi pressure

9  Bleed off pressure and check for back flow

Since there is no bottom plug in front of the slurry the wiper plug cleans off debris

and mud from the inside of the liner. This material will contaminate the cement

immediately ahead of the wiper plug. The spacing between the landing collar and

the shoe should be adequate to accommodate this contaminated cement, and thus

prevent  it  from  reaching  the  annulus  where  it  would  create    a  poor  cement  job

around the shoe.


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To promote a good cement job, cement in excess of that required to fill the annulus

between the liner and the borehole is used. This excess cement will pass up around

the liner top and settle on top of the liner running assembly. Once the cement is in

place the liner setting tool is quickly picked up out of the liner. With the tail pipe

above the liner top the excess cement can be reverse circulated out. The setting tool

can then be retrieved.

                                 

In practice it is very difficult to obtain a good cement job on a liner. The main

reasons for this are:

(a) Minimal annular clearances
A 7" OD liner run in an 8 1/2" hole gives a clearance of only 3/4" (assuming the

liner is perfectly centred). This small clearance means that:

• It is difficult to run the liner (surge pressure)

• High pressure drops occur during circulation (lost circulation problems)

• It is difficult to centralise the liner

• Cement placement is poor (channeling)

(b) Mud contamination
When the cement comes in contact with mud or mud cake it may develop high

viscosity. The increased pump pressure required to move this contaminated cement

up the annulus may cause formation breakdown. Fluid loss additives must be used

to prevent dehydration of the cement which may cause bridging in the annulus.

(c) Lack of pipe movement
Due to risk of sticking the setting tool, most operators want to be free of the liner

before cementing begins. By disconnecting the setting tool the liner cannot be moved

during the cement job. This lack of movement reduces the efficiency of cement

placement. Due to these problems it is often necessary to carry out a remedial

squeeze job at the top of the liner (Figure17). It is becoming more common these

days to remain latched on top of the liner and rotate the liner whilst the cement is

being displaced into position. A special piece of liner running equipment, known

as a rotating liner assembly, is used for this purpose.


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Tubing

Retainer or
retrievable packer

Top of liner

Cement

Figure 17 

Remedial squeeze job on a liner 

  

4.7  Recommendations for a good cement job

The main cause for poor isolation after a cement job is the presence of mud channels

in the cement sheath in the annulus. These channels of gelled mud exist because

the mud in the annulus has not been displaced by the cement slurry. This can occur

for many reasons. The main reason for this is poor centralisation of the casing in

the borehole, during the cementing operation. When mud is being displaced from

the annulus the cement will follow the least path of resistance. If the pipe is not

properly centralised the highest resistance to flow occurs where the clearance is

least. This is where mud channels are most likely to occur (Figure 18).

In  addition,  field  tests  have  shown  that  for  a  good  cement  bond  to  develop  the

formation should be in contact with the cement slurry for a certain time period

while the cement is being displaced. The recommended contact time (pump past

time) is about 10 minutes for most cement jobs. To improve mud displacement and

obtain a good cement bond the following practices are recommended:

•  Use centralisers, especially at critical points in the casing string

•  Move  the  casing  during  the  cement  job.  In  general,  rotation  is  preferred  to

reciprocation, since the latter may cause surging against the formation. A specially

designed swivel may be installed between the cementing head and the casing to

allow rotation. (Centralisers remain static and allow the casing to rotate within

them.)

•  Before doing the cement job, condition the mud (low PV, low YP) to ensure

good flow properties, so that it can be easily displaced.


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•  Displace the spacer is in turbulent flow. This may not be practicable in large

diameter casing where the high pump rates and pressures may cause erosion or

formation breakdown.

•  Use spacers to prevent mud contamination in the annulus.

                

            

Cement

100% Standoff

Mud

75% Standoff

 50% Standoff

    

      

Figure 18 

Effect of centralisation on channeling

5.  SQuEEZE CEMENTiNg

Squeeze  cementing  is  the  process  by  which  hydraulic  pressure  is  used  to  force

cement slurry through holes in the casing and into the annulus and/or the formation.

Squeeze  cement  jobs  are  often  used  to  carry  out  remedial  operations  during  a

workover on the well (Figure 3). The main applications of squeeze cementing are:

•  To seal off gas or water producing zones, and thus maximise oil production  

 from the completion interval

•  To repair casing failures by squeezing cement through leaking joints or

 corrosion hole

•  To seal off lost circulation zones

•  To carry out remedial work on a poor primary cement job (e.g. to fill up the  

 annulus)

•  To prevent vertical reservoir fluid migration into producing zones (block

 squeeze)

•  To prevent fluids escaping from abandoned zones.


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During squeeze cementing the pores in the rock rarely allow whole cement to enter

the formation since a permeability of about 500 darcies would be required for this

to happen. There are two processes by which cement can be squeezed:

•  High pressure squeeze - This technique requires that the formation be fractured.

which then allows the cement slurry to be pumped into the fractured zone.

•  Low pressure squeeze - During this technique the fracture gradient of the formation

is not exceeded. Cement slurry is placed against the formation, and when pressure

is applied the fluid content (filtrate) of the cement is squeezed into the rock, while

the solid cement material (filter cake) builds up on the face of the formation.

5.1   High Pressure Squeeze

In a high pressure squeeze the formation is initially fractured (broken down) by

a solids free breakdown fluid. A solids free fluid is used because a solids laiden

fluid such as drilling mud will build up a filter cake and prevent injection into the

formation. Solids free fluids such as water or brine are recommended. The direction

of the fracture depends on the rock stresses present in the formation. The fracture

will occur along a plane perpendicular to the direction of the least compressive

stress (Figure 19). In general, the vertical stress, due to the overburden, will be

greater than the horizontal stresses. A vertical fracture is therefore more likely.

In practice the fracture direction is difficult to predict since it may follow natural

fractures in the formation. Since squeeze cementing is often used to isolate various

horizontal zones a vertical fracture is of little use (vertical fluid movement is not

prevented).

 


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Effect of well depth and vertical-horizontal formation stresses on type of hydraulic fracture 
induced by injected fluid. Horizontal fracture pressure is less than overburden pressure, 
this is usually the case at depths greater than 3,000 feet.

Wellbore fracture pressure PF

 PF

 PF

Vertical stress 

σv

Horizont

al stress

 σh1

Induced horizontal fracture

Induced vertical fracture

PF>σv ; σv<σH1 or σH2 

PF>σH1or σH2 ; σH1or σH2< σv

              

Figure 19 

Horizontal and vertical fracturing

After the formation is broken down a slurry of cement is spotted adjacent to the

formation, and then pumped into the zone at a slow rate. The injection pressure

should gradually build up as the cement fills up the fractured zone. After the cement

has been squeezed the pressure is released to check for back flow. The disadvantages

of this technique are:

•  No control over the orientation of the fracture

•  Large volumes of cement may be necessary to seal off the fracture

•  Mud filled perforations may not be opened up by fracturing, so the cement  

 may not seal them off effectively.

5.2   Low Pressure Squeeze

It is generally accepted that a low pressure squeeze is a more efficient method of

sealing off unwanted perforated zones. In a low pressure squeeze the formation is

not fractured. Instead a cement slurry is gently squeezed against the formation. A

cement slurry consists of finely divided solids dispersed in a liquid. The solids are

too large to be displaced into the formation. As pressure is applied, the liquid phase


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Cementing

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is forced into the pores, leaving a deposit of solid material or filter cake behind. As

the filter cake of dehydrated cement begins to build up, the impermeable barrier

prevents further filtrate invasion. The filtrate must then be diverted to other parts of

the perforated interval. This technique therefore creates an impermeable seal across

the perforated zone. Fluid loss additives are important to perform this technique

successfully. Neat cement has a high fluid loss, resulting in rapid dehydration which

causes bridging before the other perforations are sealed off. Conversely a very low

fluid loss means a slow filter cake build up and long cement placement time. Key

factors which affect the build up of cement filter cake are:

• Fluid loss (generally 50 - 200 cc)

• Water to solids ratio (0.4 by weight)

• Formation characteristics (permeability, pore pressure)

• Squeeze pressure

Only a small volume of cement is required for a low pressure squeeze. Perforations

must be free from mud or other plugging material. If the well has been producing

for some time these perforations have to be washed out, sometimes with an acid

solution. The general procedure for a low pressure squeeze job is:

1  Water is pumped into the zone to establish whether the formation can be squeezed

(injectivity test). If water cannot be injected the squeeze job cannot be done without

fracturing the formation

2  Spot the cement slurry at the required depth

3  Apply moderate squeeze pressure

4  Stop pumping and check for bleed off

5  Continue pumping until bleed off ceases for about 30 mins

6  Stop displacement of cement and hold pressure

7  Reverse circulate out excess cement from casing

A properly designed slurry will leave only a small cement node inside the casing

after  removing  the  excess  cement.    Throughout  the  procedure  squeeze  pressure

is kept below the fracture gradient. A running squeeze is where the cement is

pumped slowly and continuously until the final squeeze pressure is obtained. This

is often used for repairing a primary cement job. A hesitation squeeze is where

pumping is stopped at regular intervals to allow time for the slurry to dehydrate and

form a filter cake. Small volumes of cement (1/4 - 1/2 bbl) are pumped each time

separated by a delay of 10 - 15 mins. This technique is dangerous if the cement is

still in contact with the drillpipe or packer.

5.3   Equipment Used for Squeeze Cementing

The high pressure and low pressure squeeze operations can be conducted with or

without packers.

(a) Bradenhead squeeze

This technique involves pumping cement through drill pipe without the use of a

packer (Figure 20). The cement is spotted at the required depth. The BOPs and the

annulus are closed in and displacing fluid is pumped down, forcing the cement into

the perforations, since it cannot move up the annulus. This is the simplest method

of placing and squeezing cement, but has certain disadvantages:


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•  It is difficult to place the cement accurately against the target zone

•  It  cannot  be  used  for  squeezing  off  one  set  of  perforations  if  other 

 perforations are to remain open

•  Casing is pressured up, and so squeeze pressure is limited by burst resistance

A Bradenhead squeeze is only generally used for a low-pressure squeeze job.

    

             

Spot cement                    Apply squeeze                  Reverse circulate 
                                             pressure          

Schematic of Bradenhead squeeze technique normally used on low pressure formations. 
Cement is circulated into place down drill pipe (left), then the wellhead, or BOP, is closed 
(centre) and squeeze pressure is applied. Reverse circulating through perforations (right) 
removes excess cement, or the plug can be drilled out. 

Figure 20 

Bradenhead technique

(b) Squeeze using a packer

The use of a packer makes it possible to place the cement more accurately, and apply

higher squeeze pressures. The packer seals off the annulus, but allows communication

between drill pipe and the wellbore beneath the packer. (Figure 21)

            


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Tubing

Tailpipe

Perforated Zone

Packer

Figure 21 

Squeeze cementing using a packer with or without a tailpipe

Two types of packer may be used in this type of operation:

(i) Drillable packer (cement retainer)

This type of packer contains a back pressure valve which will prevent the cement

flowing back after the squeeze. These are mainly used for remedial work on primary

cement jobs, or to close off water producing zones. The packer is run on drill pipe

or wireline and set just above or between sets of perforations. When the cement

has been squeezed successfully the drill pipe can be removed, closing the back

pressure valve. The advantages of these packers are:

• Good depth control

• Back pressure valve prevents cement back flowing

• Drill pipe recovered without disturbing cement

The major disadvantage is that they can only be used once then drilled out.

(ii) Retrievable packer (cement retainer)

These can be set and released many times on one trip. This makes them suitable for

repairing a series of casing leaks or selectively squeezing off sets of perforations.

By-pass ports in the packer allow annular communication, but these ports are closed

during the squeeze job. When the packer is released there may be some backflow,

and the cement filter cake may be disturbed. If this happens the packer should be

re-set and the squeeze pressure applied until the cement sets.

 

The basic procedure for squeezing with a retrieveable packer is:

1. run the packer on drillpipe and set it at required depth with by-pass open

2. pump the cement slurry (keep back pressure on annulus to prevent cement falling

The packer setting depth should be considered carefully. If positioned too high

above the perforations the slurry will be contaminated by the wellbore fluids and

large volumes of fluid from below the packer will be pumped into the formation


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ahead of the cement. If the packer is set too low it may become stuck in the cement.

Generally the packer is set 30 - 50 ft above the perforations.

Sometimes a tail pipe is used below the packer to ensure that only cement is squeezed

into the perforations, and there is less chance of getting stuck (Figure 21). Bridge 

plugs

 are often set in the wellbore, to isolate zones which are not to be treated.

They seal off the entire wellbore, and hold pressure from above and below. Bridge

plugs can either be drillable or retrievable.

                                       

       

Condition mud

rotation pipe

Displace cement

and fluids

Spot balanced 

plug

Pull pipe 

slowly

Drill 
pipe 

Scratch
centralizer

Spacer

Spacer
and 
preflush

Cement

Cement
plug

Preflush

Figure 22 

Balanced Plug Cementation

5.4 Testing the Squeeze Job

After the cement has hardened it must be pressure tested. The tests should include

both positive and negative differential pressure. The following should be considered

when making a test:

•  A positive pressure test can be performed by closing the BOPs and pressuring 

 up on the casing. (Do not exceed formation fracture gradient.)

•  A negative pressure test (or inflow test) can be performed by reducing the 

 hydrostatic pressure inside the casing. This can be done using a DST tool or 

 displacing with the well to diesel. This test is more meaningful since mud filled 

 perforations may hold pressure from the casing, but may become unblocked 

 when pressure from the formation is applied.


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Wire line

Cement

Casing

Dump bailer

Dump release

Mud

Bridge plug 
or obstuction

Figure 23 

Dump Bailer Plug Cementation

6.   CEMENT PLugS

At some stage during the life of a well a cement plug may have to be placed in

the wellbore. A cement plug is designed to fill a length of casing or open hole to

prevent vertical fluid movement. Cement plugs may be used for:

•  Abandoning depleted zones

•  Seal off lost circulation zones

•  Providing a kick off point for directional drilling (eg side- tracking around  

 fish)

•  Isolating a zone for formation testing

•  Abandoning an entire well (government regulations usually insist on leaving

 a series of cement plugs in the well prior to moving off location).

The  major  problem  when  setting  cement  plugs  is  avoiding  mud  contamination

during placement of the cement. Certain precautions should be taken to reduce

contamination.

•  Select a section of clean hole which is in gauge, and calculate the volume required 

 (add on a certain amount of excess). The plug should be long enough to allow 

 for some contamination (500' plugs are common). The top of the plug should 

 be 250' above the productive zone

•  Condition the mud prior to placing the plug

•  Use a preflush fluid ahead of the cement

•  Use densified cement slurry (ie less mixwater than normal)


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After the cement has hardened the final position of the plug should be checked by

running in and tagging the cement. There are three commonly used techniques for

placing a cement plug:

(a) Balanced plug (Figure 22)

This method is aimed at achieving an equal level of cement in the drillpipe and

annulus. Preflush, cement slurry and spacer fluid are pumped down the drillpipe

and displaced with mud. The displacement continues until the level of cement

inside and outside the drillpipe is the same (hence balanced). If the levels are not

the same then a U-tube effect will take pace. The drillpipe can then be retrieved

leaving the plug in place.

(b) Dump bailer (Figure 23)

A  dump  bailer  is  an  electrically  operated  device  which  is  run  on  wireline.   A

permanent bridge plug is set below the required plug back depth. A cement bailer

containing the slurry is then lowered down the well on wireline. When the bailer

reaches the bridge plug the slurry is released and sits on top of the bridge plug. The

advantages of this method are:

•  High accuracy of depth control

•  Reduced risk of contamination of the cement

the disadvantages are:

•  Only a small volume of cement can be dumped at a time - several runs may be 

 necessary

•  It is not suitable for deep wells, unless retarders used.

7.   EVaLuaTiON OF CEMENT JOBS

A primary cement job can be considered a failure if the cement does not isolate

undesirable zones. This will occur if:

•  The cement does not fill the annulus to the required height between the casing 

 and the borehole.

•  The cement does not provide a good seal between the casing and borehole and 

 fluids leak through the cement sheath to surface.

•  The cement does not provide a good seal at the casing shoe and a poor leak off 

 test is achieved

When any such failures occur some remedial work must be carried out. A number of

methods can be used to assess the effectiveness of the cement job. These include:

    


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90ºF 

100ºF  110º 

120ºF

700'

800'

900'

1000'

1100'

1200'

1300'

1400'

PROBABLE CEMENT
 

TOP

        

Figure 24 

Estimating top of cement in annulus by running a temperature log

                                   

Radiation Intensity Increases

5800'

5900'

6000'

6100'

Cement top

Base Run
After Run

Figure 25 

Estimating top of cement by running radioactivity log


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0

detecting Top of Cement (TOC)

(a) Temperature surveys (Figure 24)

This involves running a thermometer inside the casing just after the cement job.

The thermometer responds to the heat generated by the cement hydration, and so

can be used to detect the top of the cement column in the annulus.

(b) Radioactive surveys (Figure 25)

Radioactive tracers can be added to the cement slurry before it is pumped (Carnolite

is commonly used). A logging tool is then run when the cement job is complete.

This tool detects the top of the cement in the annulus, by identifying where the

radioactivity decreases to the background natural radioactivity of the formation.

detecting Top of Cement (TOC) and the Measuring the Quality of 
the Cement Bond

(a) Cement bond logs (CBL)

The cement bond logging tools have become the standard method of evaluating

cement jobs since they not only detect the top of cement, but also indicate how

good the cement bond is. The CBL tool is basically a sonic tool which is run on

wireline. The distance between transmitter and receiver is about 3 ft (Figure 26).

The logging tool must be centralised in the hole to give accurate results. Both the

time taken for the signal to reach the receiver, and the amplitude of the returning

signal, give an indication of the cement bond. Since the speed of sound is greater

in casing than in the formation or mud the first signals which are received at the

receiver are those which travelled through the casing (Figure 27). If the amplitude

(E

1

) is large (strong signal) this indicates that the pipe is free (poor bond). When

cement is firmly bonded to the casing and the formation the signal is attenuated,

and is characteristic of the formation behind the casing.

             

3 feet

Formation

Cement

Mud

T

R

Shortest path
Longest path

    

Figure 26 

Schematic of CBL tool


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1

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(b) the Variable Density Log (VDL)

The CBL log usually gives an amplitude curve and provides an indication of the

quality of the bond between the casing and cement. A VDL (variable density log),

provides the wavetrain of the received signal (Figure 28), and can indicate the

quality of the cement bond between the casing and cement, and the cement and

the formation. The signals which pass directly through the casing show up as

parallel, straight lines to the left of the VDL plot. A good bond between the casing

and cement and cement and formation is shown by wavy lines to the right of the

VDL plot. The wavy lines correspond to those signals which have passed into and

through the formation before passing back through the cement sheath and casing

to the receiver. If the bonding is poor the signals will not reach the formation and

parallel lines will be recorded all across the VDL plot.

The interpretation of CBL logs is still controversial. There is no standard API scale

to measure the effectiveness of the cement bond. There are many factors which can

lead to false interpretation:

•  During  the  setting  process  the  velocity  and  amplitude  of  the  signals  varies 

 significantly. It is recommended that the CBL log is not run until 24 - 36 hours  

 after the cement job to give realistic results.

•  Cement composition affects signal transmission

•  The thickness of the cement sheath will cause changes in the attenuation of the signal

•  The CBL will react to the presence of a microannulus (a small gap between 

 casing and cement). The microannulus usually heals with time and is not a critical  

 factor. Some operators recommend running the CBL under pressure to eliminate 

 the microannulus effect

                   

Transmitter

Amplitude

mV

Casing

arrivals

Formation arrivals                       Mud arrivals

E1

t (µ sec)

Figure 27 

Signals picked up by receiver


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TRANSIT TIME 

GAMMA RAY 

API UNITS

 

CASING BOND 

BONDING CODE 

VARIABLE DENSITY 

MICROSECONDS

SPACING

3'

200

100

200

100

MICROSECONDS

SPACING

5'

1200

200

50

0

GOOD BOND

POOR BOND

GOOD

FAIR

POOR

Casing Collars

Corrected Depth 

DEPTH

2200

2250

2300

Figure 28 

Example of CBL/VDL

 

CEMENTiNg CaLCuLaTiONS

The following calculations must be undertaken prior to a cementation operation:

•  Slurry Requirements

•  No. of sacks of Cement

•  Volume of Mixwater

•  Volume of Additives

•  Displacement Volume Duration of Operation


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These calculations will form the basis of the cementing programme. They should

be performed in this sequence as will be seen below.

1. Cement Slurry Requirements :
Sufficient cement slurry must be mixed and pumped to fill up the following (see Fig

29):

A -  the annular space between the casing and the borehole wall,

B -  the annular space between the casings (in the case of a two stage

   cementation operation)

C -  the openhole below the casing (rathole)

D -  the shoetrack

The  volume  of  slurry  that  is  required  will  dictate  the  amount  of    dry  cement,

mixwater and additives that will be required for the operation.

                                      

Casing/Casing Annulus

Casing/Hole Annulus

Shoetrack

Rathole

Figure 29 

Single Stage Cementing Operation

In addition to the calculated volumes an excess of slurry will generally be mixed

and pumped to accommodate any errors in the calculated volumes. These errors

may arise due to inaccuracies in the size of the borehole (due to washouts etc.).

It  is  common  to  mix  an  extra  10-20%  of  the  calculated  openhole  volumes  to

accommodate these inaccuracies.

The volumetric capacities (quoted in bbls/linear ft or cuft/limear ft or m3/m) of

the annuli, casings, and open hole are available from service company cementing

tables.. These volumetric capacities can be calculated directly but the cementing

tables are simple to use and include a more accurate assessment of the displacement

of the casing for instance and the capacities based on nominal diameters.

In the case of a two stage operation (Figure 30) the volume of slurry used in the first

stage of the operation is the same as that for a single stage operation. The second

stage slurry volume is the slurry required to fill the annulus between the casing and

hole (or casing/casing if the multi-stage collar is inside the previous shoe) annular

space.


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Casing/Hole Annulus

2nd. Stage Annulus
Multi Stage Collar

Shoetrack

Rathole

                      

Figure 30 

Two-Stage Cementing Operation

2.  Number of Sacks of Cement
Although cement and other dry chemicals are delivered to the rigsite in bulk tanks

the amount of dry cement powder is generally quoted in terms of the number of

sacks (sxs) of cement required. Each sack of cement is equivalent to 1 cu. ft of

cement.

The number of sacks of cement required for the cement operation will depend on

the amount of slurry required for the operation (calculated above) and the amount of

cement slurry that can be produced from a sack of cement. The amount of cement

slurry  that  can  be  produced  from  a  sack  of  cement,    known  as  the  yield  of  the

cement,  will  depend  on  the  type  of  cement  powder  (API  classification)  and  the

amount of mixwater mixed with the cement powder. The latter will also depend

on the type of cement and will vary with pressure and temperature. The number of

sacks of cement required for the operation can be calculated from the following

No. of Sacks =   

Total Volume of Slurry

    

   

       

      Yield of Cement 

3.  Mixwater Requirements
The mixwater required to hydrate the cement powder will be prepared and stored

in specially cleaned mud tanks. The amount of mixwater required for the operation

will depend on the type of cement powder used. The volume of mixwater required

for the cement slurry can be calculated from:

Mixwater Vol. = Mixwater per sack x No. sxs

4. Additive Requirements
Their are a variety of additives which may be added to cement.. These additives

may be delivered to the rigsite as liquid or dry additives. The amount of additive

is generally quoted as a percentage of the cement powder used. Since each sack of

cement weighs 94 lbs, the amount of additive can be quoted in weight (lbs) rather

than volume. This can then be related to the number of sacks of additive. The

number of sacks of additive can be calculated from:


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Number of sacks of additive = No. sxs Cement x % Additive

Weight of additive = No. sxs of  Additive x 94(lb/sk)

The amount of additive is always based on the volume of cement to be used.

5.  Displacement Volume
The volume of mud used to displace the cement from the cement stinger or the

casing during the cementing operation is commonly known as the displacement

volume. The displacement volume is dependant on the way in which the operation

is conducted.

a. Stinger Operation :

The displacement volume can be calculated from the volumetric capacity of the

cement stinger and the depth of the casing shoe. The cement is generally under 

displaced

by 1-2 bbls of liquid.

Displacement  Vol.  =  Volumetric  capacity  of  stinger  x  Depth  of  Casing  - 
1bbl

b. Conventional Operation :

In a conventional cementing operation the displacement volume is calculated from the

volumetric capacity of the casing and the depth of the float collar in the casing.

Displacement  Vol.    =  Volumetric  Capacity  of  Casing  x  Depth  of  Float 
Collar

c. Two-stage Cementing Operation:

In a two stage operation the first stage is firstly displaced by a volume of mud,

calculated in the same way as a single stage cement operation described above.

The second stage displacement is then calculated on the basis of the volumetric

capacity of the casing and the depth of the second stage collar.

Ist Stage :
Displacement  Vol.    =  Volumetric  Capacity  of  Casing  x  Depth  of  Float 
Collar

2nd stage :
Displacement  Vol.    =  Volumetric  Capacity  of  Casing  x  Depth  of  Multi-
stage collar

The amount of mud to be pumped during the displacement operation may be quoted

in terms of a volume (bbls, cuft etc.) or in terms of the number of strokes of the mud

pump required to pump the mud volume. It will therefore be necessary to determine

the volume of fluid pumped with each stoke of the pumps (vol./stroke). The number

of strokes required to displace the cement will therefore be calculated from:

Number of strokes = Volume of displacement fluid/Vol. of fluid per stroke


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6.  Duration of Operation

The duration of the operation will be used to determine the required setting time

for the cement formulation. The duration of the operation will be calculated on the

basis of the mixing rate for the cement, the pumping rate for the cement slurry and

the pumping rate for the displacing mud. An additional period of time, known as a

contingency time, is added to the calculated duration to account for any operational

problems during the operation. This contingency is generally 1 hour in duration.

The duration of the operation can be calculated from:

Duration =  Vol. of Slurry + Vol. of Slurry + Displacement Vol. + Contingency Time (1hr.)
    

        Mixing Rate      Pumping Rate    Displacement Rate

EXAMPLE OF  CEMENT VOLuME CaLCuLaTiONS

The 9 5/8” Casing of a well is to be cemented in place with a single stage cementing

operation. The appropriate calculations are to be conducted prior to the operation.

The details of the operation are as follows:

9 5/8" casing set at:  

13800',

12 1/4" hole:  

13810'

13 3/8" 68 lb/ft casing set at :  6200'

TOC outside 9 5/8" casing:

3000' above shoe

Assume gauge hole, add 20% excess in open hole

                                                                                                                                         

The casing is to be cemented with class G cement with the following additives:

                                                                         

0.2% D13R (retarder)

1 % D65 (friction reducer)

Slurry density

= 15.9 ppg

                          

Casing/Casing Annulus

Casing/Hole Annulus
(0.3132 ft

3

/ft)

Shoetrack
(0.411 ft

3

/ft)

Rathole
(0.8185 ft

3

/ft)

13 3/8 Shoe @ 6200'

TOC @ 10800'

Float Collar @ 13740'

9 5/8" Shoe @ 13800'

12 1/4" Hole @ 13810'

                  

               

Figure 31  

Example of Cementing Calculation


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1.  Slurry Volume Between The Casing and Hole:

9 5/8" csg/ 12 1/4" hole capacity = 0.3132 ft

3

/ft

annular volume

= 3000 x 0.3132

     

= 939.6 ft

3

plus20% excess

=187.9ft

3

     

= 1127.5ft

=> 1128 ft

3

2. Slurry Volume Below The Float Collar:
 

Cap. of 9 5/8, 47 lb/ft csg

= 0.4110 ft

3

/ft

shoetrack vol.  

= 60 x 0.411

Total    

= 25 ft

3

3. Slurry volume in the  rathole

    

Cap. of 12 1/4" hole  

= 0.8185 ft

3

/ft

rathole vol.

= 10 x 0.8185

     

= 8.2 ft

3

               

plus 20%

= 1.6 ft

3

 

Total    

= 9 .8 ft

3

 => 10 ft

3

Total cement slurry vol.

= 1128 + 25 + 10

       

 

 

= 1163 ft

                                                          

4.  Amount of cement and mixwater

Yield of class G cement for density of 15.9 ppg  = 1.14 ft

3

/sk  

mixwater requirements  

= 4.96 gal/sk

No. of sks of cement  

= 1163       

 

 

= 1020 sx               

           

 

 

     1.14

Mixwater required  

= 1020 x 4.96 gal

     

= 5059 gal

= 120 bbls

5. Amount of Additives:

 

Retarder D13R (0.2% by weight)

     

= 0.2 x 1020 x 94 (lb/sk) = 192 lb     

100

   

Friction reducer (1.0% D65 by weight)

     

= 1  x 1020 x 94(lb/sk) = 959 lb

         

100


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6.  Displacement Volume:

Displacement vol.

= vol between cement head and float collar

     

= 0.4110 x 13740 = 5647 ft

= 1006 bbl

(add 2 bbl for surface line)

= 1008 bbl

For Nat. pump 12-P-160, 7" liner 97% eff, 0.138 bbl/stk

   

                  

No. of strokes  

= 1008   

 

=  7300 strokes 

   

0.138

 

EXERCISE 1  Cementing Calculations - Stinger Cementation

The 0" casing of a well is to be cemented to surface with class ‘C’ high early strength 
cement + % Bentonite using a stinger type cementation technique. Calculate the 
following for the 0" casing cementation :

a.  The number of sacks of cement required (allow 100% excess in open hole).

b.   The volume of mixwater required.

c.  An estimate of the time taken to carry out the job.(Note: use an average mixing/ 
 

pumping time of  bbls/min.)

 

0" Casing 

 

: 0 - 00 ft.

 

0" Casing  lb/ft 

 

: 0 - 00 ft

 

0" Casing 1 lb/ft   

: 00 - 100 ft.

 

" Open hole Depth   

: 10 ft.

 

Stinger  

 

: " 1." drillpipe 

 

 

 

Class ‘C’ Cement + % Bentonite

 

Density 

 

:  1.1 ppg 

 

Yield 

 

:  1. ft

/sk            

 

Mixwater Requirements 

:  1. ft/sk

EXERCISE   Cementing Calculations - Two Stage Cementation

The 1 /" casing string of a well is to be cemented using class ‘G’ cement. Calculate 
the following:

a.  The required number of sacks of cement for a 1st stage of 00 ft. and a nd    
 

stage of 00 ft.(Allow 0% excess in open hole)

b.  The  volume of mixwater required for each stage.

c.  The total hydrostatic pressure exerted at the bottom of each stage of cement    
 

(assume a 10 ppg mud is in the well when cementing).


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

d.  The displacement volume for each stage.

 

0" Casing shoe 

 

:  100 ft

 

1 /" Casing 

 lb/ft 

:  0 - 1000 ft

 

1 /" Casing 

 lb/ft 

:  1000 - 000 ft.

 

1 1/" open hole Depth 

:  00 ft.

 

Stage Collar Depth 

 

:  100 ft.

 

Shoetrack 

 

:  0 ft.

 

Cement stage 1 

(000-00 ft.)  

 

 

Class ‘G’ 

 

Density 

 

:  1. ppg 

 

Yield 

 

:  1.1 ft

/sk            

 

Mixwater Requirements 

:  0. ft/sk

 
 

Cement stage  

(100-1000 ft.)

 

Class ‘G’ + % bentonite

 

Density 

 

:  1. ppg 

 

Yield 

 

:  1. ft

/sk            

 

Mixwater Requirements 

:  1. ft/sk

VOLuMETriC CaPaCiTiES

 

 

 

 

bbls/ft 

  

ft

3

/ft

 

drillpipe

 

" drillpipe : 

 

 

0.01  

 

0.0   

 

Casing

 

1 /"  lb/ft : 

 

 

0.10 

 

0.1

 

1 /"  lb/ft : 

 

 

0.1 

 

0.1

 

Open Hole

 

" Hole 

 

 

0. 

 

.

 

1 1/" Hole 

 

 

0. 

 

1.0

 

annular Spaces

 

" hole x 0" Casing:  

 

0.1 

 

1.0

 

1 1/" hole x 1 /" Casing: 

 

0.1 

 

0.

 

0" Casing x 0" Casing: 

 

0.0 

 

.0

 

0" Casing x 1 /" Casing: 

 

0.11 

 

1.01


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0

SOLuTiON TO EXErCiSES

Exercise 1   Cementing Calculations - Stinger Cementation

The  surface  (20”)  casing  of  a  well  is  normally  cemented  to  surface  (continue

pumping cement until it is seen at surface). In order to determine the volume of

slurry required one calculates the annular space between the conductor (30”) and the

surface string (20”) and between the surface string and the openhole. The volume

of rathole is added to the above and the slurry volume is translated via the yield of

the cement recipe to the number of sacks of cement required for the entire job.

The volume of mixwater required is specified in the slurry recipe in terms of cu

ft. per sack of cement and will be determined on the basis of a required cement

strength, setting time and allowable free water content.

The time required for the cement job will include the mixing and pumping time

(assuming that the slurry is not batch mixed), the time to displace the cement from

the cement stinger (since this type of job would normally be carried out using a

stinger cementation technique) and 1 hr. contingency time to allow for operational

problems during the job. The operation duration will be used to design the slurry so

that the cement is set as soon as possible after the job is complete.

                             

400'

1500'
1530'

26" Hole

5" d.p

30"

a.  No. sxs cement

Slurry volume between the 20" casing and 30" casing:

 20" casing/30" casing capacity

= 2.0944 ft

3

/ft

 annular volume

= 400 x 2.0944

     

= 838 ft

3

   

 

 

     

Slurry volume between the casing and hole:

 20" csg/ 26" hole capacity  

= 1.5053 ft

3

/ft

 annular volume

= 1100 x 1.5053

     

= 1656 ft

3

 plus100% excess  

= 1656 ft

3

                      

 Total

= 3312 ft

3

                      


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Cementing

1

Institute of Petroleum Engineering,  Heriot-Watt University  

Slurry volume in the rathole

 Cap. of 26" hole

= 3.687 ft

3

/ft

 rathole vol.  

= 30 x 3.687  

     

= 111 ft

3

               

 plus 100%  

= 111 ft

3

 

 Total

= 222 ft

3

  TOTAL SLURRY VOL.  :   

 

 

= 4372 ft

3

 Yield of class C cement for density of 13.1 ppg

= 1.88 ft

3

/sk

  TOTAL No. SXS CEMENT : 

   4372/1.88   = 2326 sxs

b. Mixwater Requirements

 

Mixwater requirements for class C cement with 6% Bentonite  

     

= 1.36 ft

3

/sk

 

Mixwater required  

= 2326 x 1.36                 

       

 

 

 

 

 

= 3163  ft

3

 

c. Displacement Time

Total  Displacement  time    =  Time  to  mix  and  pump  cement  +  time  to  displace

cement

 Total Volume of Cement

= 4372 ft

3

     

= 779 bbl

Displacement vol. = vol to displace down drillipipe leaving 1 bbl under displaced

 d.p. capacity  

= 0.01776 bbl/ft

 Displacement to 1500 ft

= 0.01776 x 1500

     

= 26.6 bbl

 (underdisplace by 1 bbl )

= 25.6 bbl

 

                                          

Total Volume to mix and displace = 779 + 25.6  = 804.6 bbls

  Total time @ 5 bbl/min 

 

 

 

= 804.6/5

   

   

 

 

 

 

 

= 160.9 = 2.7 hrs


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Exercise 2  Cementation Calculations - Two Stage Cementation

    

                                 

1000'

1500'

6300'

6940'

7030'

7000'

77 lb/ft

72 lb/ft

20" Shoe

17 1/2" Hole

TOC

                

a.  No. sxs cement

Stage 1:

Slurry volume between the casing and hole:

 13 3/8" csg/ 17 1/2" hole capacity

= 0.6946 ft

3

/ft

 annular volume

= 700 x 0.6946

     

= 486 ft

3

 plus20% excess

= 97 ft

3

                                         

Total

= 583 ft

3

 Slurry volume below the float collar:

 Cap. of 13 3/8, 72 lb/ft csg  

= 0.0.8314 ft

3

/ft

 shoetrack vol.

= 60 x 0.8314

 Total

= 50 ft

3

 

 Slurry volume in the rathole

 Cap. of 17 1/2" hole

= 1.6703 ft

3

/ft

 rathole vol.  

= 30 x .6703  

     

= 50.11 ft

3

               

 plus 20%  

= 10.02 ft

3

 

  Total

= 60 ft

3

  TOTAL SLURRY VOL. STAGE 1 : 

= 693   ft

3

 Yield of class G cement for density of 15.9 ppg = 1.18 ft

3

/sk

  TOTAL No. SXS CEMENT STAGE 1: 

693/1.18 = 587 sxs


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Cementing

Institute of Petroleum Engineering,  Heriot-Watt University  

Stage 2:

 20" csg/ 13 3/8" csg  

= 1.0194 ft

3

/ft

 annular volume

= 500 x 1.0194

     

= 508 ft

3

  TOTAL SLURRY VOL. STAGE 2 : 

 

508 ft

3

 Yield of class G cement for density of 13.2 ppg

= 1.89 ft

3

/sk

  TOTAL No. SXS CEMENT STAGE 2:  

 

508/1.89 = 269 sxs   

b. Mixwater Requirements

 

Stage 1:

mixwater requirements for class G cement for density of 15.9 ppg

     

= 0.67 ft

3

/sk

 

Mixwater required  

= 587 x 0.67

                 

       

 

 

 

 

 

= 393  ft

3

   

Stage 2:

 mixwater requirements for class G cement for density of 13.2 ppg

     

= 1.37 ft

3

/sk

 

Mixwater required  

= 270 x 1.37

       

 

 

 

 

 

= 370 ft

3

 

 

c. Hydrostatic Head

Stage 1:

Mud Hydrostatic (0 - 6300 ft) + Cement Hydrostatic (6300 - 7030 ft)

     

= 6300 x 10 x 0.052 + 730 x 15.9 x 0.052

       

 

= 3880 psi

 

Stage 2:

Mud Hydrostatic (0 - 1000 ft) + Cement Hydrostatic (1000 - 1500 ft)

     

= 1000 x 10 x 0.052 + 500 x 13.2 x 0.052

       

 

 

= 863 psi

A knowledge of the hydrostatic pressure exerted by the cement slurry when it is

place will ensure that the formation fracture pressure will not be exceeded during

the cement job.


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d. Displacement Volumes

Stage 1:

Displacement vol. = vol between cement head and float collar

     = 0.1463 x 1000 (77 lb/ft casing) + 0.148 x 5940 (72 lb/ft casing)

   

= 1025 bbl

     (add 2 bbl for surface line)

= 1027 bbl

Stage 2:

Displacement vol. = vol between cement head and stage collar

     = 0.1463 x 1000 (77 lb/ft casing) + 0.148 x 500 (72 lb/ft casing)

   

= 220 bbl

     (add 2 bbl for surface line)

= 222 bbl




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المشاهدات: لقد قام 10 أعضاء و 1061 زائراً بقراءة هذه المحاضرة








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