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Cementing Techniques
نویسنده : رضا سپهوند - ساعت ۱۱:٥٧ ‎ق.ظ روز ۱۳٩٤/٧/٥
 

Curtin University Master of Petroleum Engineering
Department of Petroleum Engineering Drilling Engineering
Chapter 8 – Cement and Cementing Techniques
Table of Contents
8.1 Cement and Cementing Techniques..............................................8-1
8.2 Composition and Properties Of Cement........................................8-1
8.2.1 Basic Composition......................................................................8-1
8.3 Cement Properties...........................................................................8-3
8.3.1 Slurry Density..............................................................................8-3
8.3.2 Thickening Time..........................................................................8-3
8.3.3 Pumpability.................................................................................8-4
8.3.4 Compressive Strength.................................................................8-4
8.3.5 Permeability................................................................................8-4
8.3.6 Water or Fluid Loss.....................................................................8-4
8.3.7 Expansion...................................................................................8-5
8.3.8 Corrosion Resistance..................................................................8-5
8.4 Cement Additives.............................................................................8-5
8.4.1 Lightweight Additives..................................................................8-6
8.4.2 Heavy Weight Additives..............................................................8-7
8.4.3 Accelerators For Cement Slurries...............................................8-7
8.4.4 Retarders for Cement Slurries....................................................8-9
8.4.5 Low Water Loss Control Additives............................................8-10
8.4.6 Lost Circulation Additives..........................................................8-11
8.5 Special Cements............................................................................8-12
8.5.1 'Gunk'........................................................................................8-12
8.5.2 Silica Flour Cement...................................................................8-12
8.5.3 Resin Cement...........................................................................8-12
8.5.4 Latex Cement............................................................................8-12
8.5.5 Sodium Chromate-Para Formaldehyde.....................................8-12
8.6 Cementing Techniques..................................................................8-13
8.6.1 Single Stage/Two Plug Casing Cementation............................8-13
8.6.2 Two Stage Cementation............................................................8-19
8.6.3 Inner String Cementing for Large Diameter Casing..................8-22
8.6.4 Liner Cementations...................................................................8-24
8.6.5 Subsea 2 Plug Cementing System............................................8-26 CHAPTER 8
Cement and Cementing Techniques
Curtin University Master of Petroleum Engineering
Department of Petroleum Engineering Drilling Engineering
CHAPTER 8
Cement and Cementing Techniques
Curtin University Master of Petroleum Engineering
Department of Petroleum Engineering Drilling Engineering
Chapter 8 – Cement and Cementing Techniques
8.1 Cement and Cementing Techniques
The reasons for running casing were discussed in section 7, however it is usually only able to perform those functions if it fills the annular void and preferably is bonded to the formation. The reasons for cementing the anulus between the casing and the formation are:
(1) Provides vertical and radial support for the casing.
(2) The well is totally controlled and hence drilling ahead can resume.
(3) Protects the casing against corrosion, e.g. from sour formation water.
(4) Isolates porous formations.
(5) Permits zonal treatment, stimulation and production.
Cement fulfils these above requirements as it is:
(a) Relatively strong and resistant.
(b) Cheap.
(c) Reasonably chemically inert.
(d) Flexible in formulation for specific applications through the use of additives.
8.2 Composition and Properties Of Cement
8.2.1 Basic Composition
Oilwell cement is manufactured by grinding and then firing in a rotary kiln, a mixture of limestone and clay or shale, i.e. the basic constituents are silica, lime, alumina and iron. In the final stages some gypsum is added to control the setting properties.
The four major chemical compounds in the produced cement are:
(a) TRICALCIUM ALUMINATE - which controls the initial setting of the cement. However, its presence lowers the sulphate resistance of the cement.
(b) TRICALCIUM SILICATE - which produces the initial strength.
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Cement and Cementing Techniques
Curtin University Master of Petroleum Engineering
Department of Petroleum Engineering Drilling Engineering
(c) DICALCIUM SILICATE - which produces the longer term strength due to its slow rate of hydration,
(d) TETRACALCIUM ALUMINO FERRITE - which has little effect on cement properties.
Basic cement composition can be altered to adjust the relative quantities of these compounds in the cement and this gives rise to various grades. Table (8.1). A.P.I. Recommended Practice 10B lists the properties of basic cements and these are summarised in Table (8.2). These basic types of Portland cement cover a wide range of well applications and the setting depths and recommended well temperature ranges as given in Table (8.2) are a useful guide to their application. Table (8.1) Basic cement compositions
Table (8.2) A.P.I. cement classification
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Cement and Cementing Techniques
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8.3 Cement Properties
nly the main properties, which are important in the control of a cement job
.3.1 Slurry Density
he basic cements when mixed as per Table (8.2) yield a certain slurry
O
will be discussed here.
8
T
weight. However, it may be necessary to adjust the slurry weight for a specific well application. For example it may be necessary to lighten the cement weight when cementing a formation which is causing lost circulation or alternatively is so weak that formation breakdown may occur. Conversely it may be necessary to increase the slurry weight for cementing in an over pressured formation.
Figure (8.1) Heavy weight additives for cement
8.3.2 Thickening Time
The thickening time is the time taken for the cement to thicken sufficiently so that it no longer behaves as a slurry e.g. does not pour or is rigid enough to support a vertical rod. Its importance therefore is that it gives a guideline as to the time available for placement of the slurry within the casing/hole annulus. (Allowance must be made for delays in the cement job and thus a safety factor must be used).
The thickening time is reduced by both increasing pressure and temperature (see Halliburton or Dowell cement tables). The thickening time can be chemically controlled by the addition of additives such as accelerators and retarders.
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Curtin University Master of Petroleum Engineering
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8.3.3 Pumpability
The flow properties of the cement slurry are important for a number of reasons:
(a) Flow regime influences the efficiency of borehole wall and annulus cleaning – mud removal.
(b) The flow regime influences the pressure drop in the hole and the use of higher pressures can give rise to losses, rapid setting due to increased water loss, etc.
(c) As a minor aspect the flow regime may influence the extent of mixing ahead of and behind the cement slurry.
(d) The flow regime may influence the mixing within the cement slurry.
The addition of friction reducers will be discussed later.
8.3.4 Compressive Strength
The level of compressive strength of the cement is important both on the completion of the cement job and subsequently throughout the life of the well, e.g. after cementing sufficient compressive strength must have developed in the cement before drilling operations can resume. The level of compressive strength depends on the time elapsed since mixing, the amount of water in the slurry and the class of cement used.
Accelerators can be used to improve the early strength development of cement slurries.
8.3.5 Permeability
After it has set, cement is almost impermeable (e.g. < 0.5md). However, cement can become 'cut' when cementing through gas zones and in such instances it will then possess some degree of permeability.
8.3.6 Water or Fluid Loss
The setting mechanism for cement involves dehydration or removal of free water from the slurry. The normal reaction is one of chemical absorption of water, however, physical separation of water can decrease the thickening time. The measurement of fluid loss is done on a filter pressurising a 325 mesh screen at 1,000 psi. for a period of 30 minutes. For primary cementation a fluid loss of 150 to 400 c.c. is desirable. For squeeze cementing, it is necessary to have a low fluid loss (50-200 c.c.) so that the squeeze operation can be completed before the setting reaction is too far advanced.
Fluid loss can be controlled by chemical additives.
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Cement and Cementing Techniques
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8.3.7 Expansion
Expansion characteristics of cements can be important in improving the bonding of cement.
8.3.8 Corrosion Resistance
The tricalcium aluminate content of a cement lowers the resistance of the cement to sulphates. The reaction of the tricalcium aluminate with the sulphates in the formation water forms calcium alumino sulphate which causes expansion and cracks.
The more dilute a slurry is (i.e. the higher is the water content), the lower is the cements' resistance to sulphates. For very high resistance to sulphates, the cement should contain less than 3% tricalcium aluminate.
8.4 Cement Additives
Table (8.3) illustrates the range of additives available for oil well cements. In most areas cements are available in bulk either with or without additives. Additionally, additives are also available in sacks or drums.
Table (8.3) Cement additives
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8.4.1 Lightweight Additives
For applications where it is necessary to reduce the hydrostatic head of the cement column to avoid losses or formation breakdown, additives are available to reduce the slurry weight. In general as the slurry weight is reduced the compressive strength is lowered and the thickening time is increased. In primary cementing operations, where the cement has to be lightened a neat cement is usually 'tailed’ in (300-500 ft.) to provide a good cement bond at the casing shoe.
(a) Bentonite
Bentonite is a colloidal clay which can absorb large quantities of water. It can therefore be employed as a lightening additive since it allows larger quantities of water to be used. As a guideline 1% of bentonite will absorb 3-5% additional water. Although up to 25% bentonite can be used, the average value is usually about 4% with the best technique being a pre- hydration of the bentonite in the mix water for 12-24 hours before being required. The use of bentonite in cement can cause a lowering of the compressive strength and sulphate resistance.
(b) Pozzolans
Pozzolanic material is ground volcanic lava and is generally mixed 50:50 with Portland cement. This type of cement can be accelerated or retarded and although the early strengths developed by the cement are lower than neat cement there appears to be little loss of ultimate strength. This type of cement shows increased resistance to attack by sulphates.
(c) Diatomaceous Earth
This type of naturally occurring material has a very high specific area and accordingly it can absorb large quantities of water and hence by water dilution produces a low weight cement slurry. Slurry weight down to 11 P.P.G. can be achieved. However, because of the water dilution the strengths of these cements are low.
(d) Gilsonite
Gilsonite is a lightweight occurring asphalt like material which in use displays tendencies to 'bridge'. It is therefore useful not only as a lightweight additive but also for lost circulation zones. However, it can pose problems by bridging across cement plugs and other restrictions. The lightening effect is due to the low S.G. of the material it can be used in additions up to 50 lbs Gilsonite/SK of cement.
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(e) Expanded Perlites
This type of material occurs naturally and after heating under pressure it forms a cellular product of low density. It can be used for bridging in lost circulation or to reduce slurry weights.
8.4.2 Heavy Weight Additives
It may be necessary to increase the slurry weight when cementing in an abnormally pressured formation. The following additives should be correctly pre- blended in dry cement.
Barytes
Barium sulphate can be used to weight up cement slurries to 18 P.P.G. However its use leads to some compressive strength reduction and a reduction in thickening time.
Sand
Graded sand (40-60 mesh) can be used as a weighting material and yields up to 2ppg. increase in cement slurry weight. The weight increase comes about due to the fact that additional water is not required.
llmenite (Iron Arsenate)
This material has a higher S.G. than barytes and requires exceptionally low quantities of water. It has negligible effect on the thickening time of slurries and additionally provides greater compressive strengths due to the increased solids content. It is possible to achieve slurry weights up to 20 P.P.G.
Hematite
This has a higher S.G. than Ilmenite and accordingly can be more effective. It is also more readily available.
8.4.3 Accelerators For Cement Slurries
Accelerators are principally employed to reduce the time 'waiting on cement' (W.O.C.) prior to recommencing drilling operations after a cement job.
Calcium Chloride
Calcium Chloride is the most widely used accelerator generally at an addition rate of 2% (based on Regular grade) where it approximately doubles the normal 24 hour compressive strength. CaCI 2 is available as a Regular grade, i.e. 77% or Anhydrous grade, i.e. 96%.
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Above approximately 3% addition there is little increase in the acceleration obtained and it has been reported that above 4% the action is reversed and retarding takes place. Table (8.4) illustrates the effect of accelerator on Class H cement.
Table (8.4) effect of accelerators on class H cement
Sodium Chloride
In small concentrations of 2-5% by cement weight, CaCI2 provides an acceleration effect on cement (the optimum effect is produced at 2-2.5%). However, since it is not as effective as CaCI it is only recommended if CaCl2 is not available. Table (8.5) illustrates the effect of NaCI on Class H cement.
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Table (8.5) Effect of NaCl on class H cement
Sea Water
Sea water contains small quantities of NaCI, MgCl2, CaCl2 generally of the order of 20-60,000 P.P.M., i.e. 2-6%, and accordingly sea water has an accelerating effect on cement slurries.
8.4.4 Retarders for Cement Slurries
This type of additive may be useful to control the setting time of cement for deep or hot wells. Most of the retarders can be added to the cement in a dry form or to the mix water. Retarders are used both with basic Portland cements A and B as well as retarded cements such as D, E and F. The guideline for retarder usage depends on the bottom hole temperature.
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Lignins (Calcium Ligno sulphonates)
This type of retarder is most effective and Figure (8.2) illustrates the use of a calcium lignosulphonate retarder marketed by Halliburton under the name HR-4. The retarding action of the lignin is achieved by a dispersion mechanism.
Figure (8.2) Effect of retarder on setting time of cement
CMHEC
This material not only acts as an effective retarder but also as a water loss control agent.
Chloride Salts
As indicated above, the accelerating effect of CaCl2 and NaCI can be reversed if it is used in too high quantities, i.e. saturated. For example 3.1 lbs NaCI per gallon of water will retard the setting time sufficiently to allow Class A cement to be used to 10,000'. However, additionally the use of such quantities of salt reduces the amount of cement required since the slurry weight is increased approximately 1 P.P.G.
8.4.5 Low Water Loss Control Additives
The reason for using such additives depends on the type of application being considered. Consider the following operations:
(a) Squeeze Cementing
The use of water loss control additives is important for the following reasons:
1. Reduction in premature dehydration in tubing and casing whilst squeezing off perforations.
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Curtin University Master of Petroleum Engineering
Department of Petroleum Engineering Drilling Engineering
2. Build up of filter cake in perforations whilst using a hesitation squeeze technique.
3. Low water loss limits the amount of bentonite clays in the filtrate which can be lost interstitially into the formation.
(b) Casing Cementation
1. Reduces premature dehydration which could cause bridging during the cement displacement.
2. Protects water sensitive shales.
3. Maintains pumpability in the slurry.
4. Helps to reduce the possibility of formation plugging due to bentonite clays.
Additives for water loss control are as follows:
Organic Polymers
These comprise high molecular weight polymers which provide effective bridging across cement particles hence reducing the fluid filtration under pressure. An addition rate of the order of 1% is required to achieve useful results. The polymers are usually heat stable and hence of use in deep wells.
Carboxy methyl hvdroxyethyl cellulose C.M.H.E.C.
Again this a long chain material which provides effective bridging between cement particles. However, as mentioned previously C.M.H.E.C. is also a retarder and hence it may be necessary to add an accelerator to achieve the desired slurry properties.
8.4.6 Lost Circulation Additives
Lost circulation may be encountered whilst carrying out a primary cementation or alternatively the cement job may be done to cure lost circulation in the drilling operation. In such instances it may be necessary to consider using lost
circulation additives.
There are 3 main classes of lost circulation materials:
(a) Fibrous materials
This class of materials comprises shredded wood, bark and sawdust. It should be noted that some L.C.M. for mud systems cannot be used for cement as they contain tannins which have a retardation effect.
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(b) Granular materials
In this class of material are nut shells, gilsonite and perlites which effectively help bridge off lost circulation.
(c) Laminated materials
This type of bridging material includes mica, cellophane, etc.
8.5 Special Cements
8.5.1 'Gunk'
Gunk is prepared from a mix of 50:50 by weight of cement and bentonite mixed in diesel oil. The bentonite in this material sets immediately it contacts water and is therefore useful to shut off water zones or repair loss circulation zones. After the initial set, created by the bentonite, the cement sets to give a longer term strength.
8.5.2 Silica Flour Cement
The addition of up to 40% of silica flour (SiO2.) prevents the strength retrogression of the cement at high temperatures, i.e. above 230 F. High ultimate strength can be achieved.
8.5.3 Resin Cement
This type of cement is a mixture of water, liquid resin and Portland Cement. It is useful where other cementing materials have proved unsatisfactory. The cement possesses increased resistance to corrosion.
8.5.4 Latex Cement
This type of cement utilises a synthetic latex emulsion and the slurry demonstrates low fluid loss, greater resilience, increased corrosion resistance and good bonding performance.
8.5.5 Sodium Chromate-Para Formaldehyde
This is an additive intended to counteract the retarding properties of some organic chemicals used in muds, e.g. C.M.C., lignite. The recommended addition rate is 1 lb/sack of cement.
CHAPTER 8 Page 8-12
Cement and Cementing Techniques
Curtin University Master of Petroleum Engineering
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8.6 Cementing Techniques
8.6.1 Single Stage/Two Plug Casing Cementation
In most cases it is necessary to displace cement into the annulus behind a casing string. However, the cement slurry is generally heavier than the mud used to drill through the section. For a cementation of this type it is therefore necessary to have a back pressure valve in the casing string which prevents the slurry u-tubing back into the casing from the annulus. This valve is incorporated into the float collar, Figure (8.3), which is usually located 1-3 joints above the 'shoe' or bottom of the casing string. To assist in the running of the casing, a guide shoe or float shoe is run at the bottom of the casing string. The shoe has a bull nose which assists in the running of the casing especially through deviated sections of the hole. The shoe is available with or without a non-return valve. Figures (8.4) and (8.5).
Figure (8.3) Float collar
Figure (8.4) Stab in guide shoe
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Figure (8.5) Cement guide shoe
Whilst running the casing to the setting depth, it is necessary to ensure that the casing is filled with fluid to avoid evacuation of the casing as it could lead to casing collapse or blowout. The filling of the casing can either be done from surface every few joints or continuously using an automatic fill up shoe or collar (Figure (8.6)), which has a small side mounted orifice which allows fluid flow into the string but is subsequently blocked during the cementing by one of the plugs.
Figure (8.6)(a) Automatic fill up shoe
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Figure (8.6)(b) Circulating differential fill float collar
Figure (8.6)(c) Differential equipment operation
The displacement of the cement slurry is done between two plugs which are loaded in a plug container (Figure 8.7) at surface and are released before and after the cement slurry enters the casing. The bottom plug is released just
CHAPTER 8 Page 8-15
Cement and Cementing Techniques
Curtin University Master of Petroleum Engineering
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before the cement slurry enters the casing and it travels ahead of the slurry until it lands in the float collar. The bottom plug (Figure 8.8) thus separates the slurry from the fluid displaced out of the casing and annulus. The top plug is released behind the slurry and separates the cement from the displacing fluid, usually mud, Figure (8.9), and additionally, it removes cement from inside the casing. After landing in the float collar the bottom plug ruptures at a set pressure allowing the cement to be displaced through the float collar and eventually the top plug latches into the float collar. It is useful just prior to 'bumping' the top plug to note the pump pressure as this gives a useful indication of the Top of Cement T.O.C. in the annulus.
The displacement is shown in Figure (8.10)
Figure (8.7) Plug container
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Figure (8.8) Bottom plug
Figure (8.9) Top plug
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Figure (8.10) Standard casing cement job
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8.6.2 Two Stage Cementation
With 2 stage cementations it is possible to displace cement around the lower part of the casing through the shoe and also subsequently to place cement higher up in the casing annulus. The technique makes use of a multiple stage cementer as depicted in Figure (8.11), which is run with the casing string at the depth required for the bottom of the upper column of cement.
Figure (8.11)(a) Stage cementing collar
Figure (8.11)(b) Multi stage cement collar
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The operation is depicted in Figure (8.12) and utilises a bottom plug displaced ahead of the first stage of cement and a shut off plug displaced after the first slurry.
Figure (8.12) Two stage cement job
To commence the second stage an opening bomb is dropped (or alternatively an opening plug is pumped down) which shears a pin in the stage cementer and hence opens the ports. The second stage cement can then be pumped into the annulus followed by a closing plug which latches into the stage cementer. It may be necessary to run a metal petal basket (Figure 8.14), below the stage cementer to avoid cement dropping down the annulus. Alternatively there is a version available of the stage cementer which incorporates a packer below the cementing ports. By using two stage cementers, the cementation can be performed in 3 stages.
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Figure (8.13) Bomb type plugs for 2 stage cementing
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Figure (8.14) Cement basket
Reasons for Multi-Stage cementation
(1) Allows effective cementation of a long casing string without excessive pumping times and thus avoids displacement / setting problems.
(2) Reduces pump pressures since a long column of cement is not required to be supported in the annulus.
(3) Allows cementing of casing opposite specific formations for efficient isolation.
(4) Minimises losses or risk of formation breakdown by reducing hydrostatic head in the annulus.
8.6.3 Inner String Cementing for Large Diameter Casing
The cementing of large diameter casings poses certain problems:
(1) Large displacement volumes and times.
(2) Inefficient displacement, e.g. large amount of mixing of cement and other fluids, if plugs are not used.
The technique of Inner String Cementing is used to overcome these problems. The method consists of running casing as before but prior to cementing, running in a string of tubing or drill pipe which latches into the float collar or shoe using a sealing adaptor. The seal adaptor can either be incorporated
CHAPTER 8 Page 8-22
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Curtin University Master of Petroleum Engineering
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into the float shoe or collar design (Figure (8.15)) or alternatively onto the inner string (Figure (8.16)). The process is shown in Figure (8.17).
In this technique after landing off the casing, the inner string is run and latched into the sealing adaptor. The cement can then be displaced down the inner string. After checking that the float shoe, collar, or latch down plug is holding cement column pressure, the inner string is disconnected and the string contents circulated out prior to retrieving the inner string.
This type of technique is useful for 13 3/8" and larger casing sizes.
Figure (8.15) Float collar adapted for cementing stinger
Figure (8.16) Float shoe with cement stinger attached
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Figure (8.17) Cementing through a stinger
8.6.4 Liner Cementations
The procedure for running a liner is covered in section 9. The cementation of a liner after it has been landed off is quite difficult to achieve successfully.
Some of the factors affecting the cement job are listed below:
(1) Hole size and washout.
(2) Liner size.
(3) Type of liner hanger and Pack off achieved.
(4) Type of pipe movement used (rotation or reciprocation).
(5) Pipe centralisation.
(6) Slurry properties.
(7) Circulation rate.
(8) Preflush or prior hole cleaning.
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Normal Cementing procedure (Figure 8.18)
After running the liner to the setting depth and setting the hanger, the drill pipe running tool is raised 12-18 inches. Circulation is then established before cementing operations begin with the packer set. At the end of the cement mixing operation, the drill pipe wiper plug in the cement head is released and is then displaced down the string until it lands in the liner wiper plug at the top of the liner. Increasing pump pressure shears the retaining pin and the wiper plug moves down the liner until it latches into the floating equipment where it acts as a backpressure valve. Upon bumping the plug, the packer mechanism collapses and the string is then free to be pulled up above the liner top and reverse circulation removes any excess cement.
Figure (8.18) Liner cement job
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Problem Areas in Liner Cementing
The main problems in liner cementations are:
(1) Minimal annular clearance, e.g. 4 ½" OD liner in 6 81" diameter hole - increases differential pressure and required small cement volumes.
(2) Inefficient hole cleaning, i.e. mud filter cake removal.
(3) Operation in deviated or horizontal holes.
(4) Pump horsepower limits displacement rates due to deep holes.
(5) Usually cementing in oil or gas zones.
(6) Require rotation and/or reciprocation to improve placement in high angle wells
(7) Pump horsepower limits displacement rate due to increased hp for heavy slurry weights.
The main problems appear to be numbers 1, 2, 4, 7.
Delayed setting technique (Figure (8.19))
This technique developed by Halliburton involves placing a cement slurry into the open hole using a drill pipe stinger. The cement slurry is highly retarded. After retrieving the stinger, the liner is run into the cement and a more homogeneous placement is achieved due to less channelling. A drill pipe stinger is then run down and excess cement is reversed out of the liner.
8.6.5 Subsea 2 Plug Cementing System (Figure (8.20))
This system involves running the casing on a drill pipe running string and landing the casing off in the previous casing housing. The physical make up of the casing to the drill pipe is via a left hand threaded running tool screwed onto the casing housing. Below the running tool inside the casing housing is the plug assembly attached to the drill pipe running string. The two plugs can be sequentially released from surface using darts/ball/plugs. However, as installed downhole the plugs provide a full bore flow for circulation prior to cementing.
The cementing procedure is as follows:-
(1) Circulate casing string when it is landed off.
(2) Release the bottom launching ball which drops into the ball catcher on the bottom plug.
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(3) Displace cement. (Bottom plug shears out under pressure)
(4) After pumping the cement slurry into the drill pipe running string, the top dart plug is released. During the slurry displacement, this plug wipes the inside of the drill pipe and then lands inside the top plug. Again increased pump pressure will shear out the retaining pins on this plug, which will then travel down the inside of the casing and latches into the float collar.
Advantages of this system
(1) The drill pipe running string incorporates a bumper sub, which compensates for the heave of the rig. This makes the cementation technique of great value for operations from floating rigs.
(2) There is no requirement to use casing extension strings, which must be retrieved after the cement job. This advantage can significantly reduce the time required for a casing job in deeper waters.
Figure (8.19) Delayed setting cement for liner cementing
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Figure (8.20) Three plug system for cementing subsea wells