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322 chapter 5

  1. 1. CHAPTER-5 CEMENTING Oil well cementing falls into three categories. -primary cementing job on a casing string -squeeze cementing -plugs Primary Cementing: Casing strings are usually cemented : -to isolate troublesome behind the casing from deeper formations to be drilled, -to isolate high-pressure formations below the casing from the weaker shallow zones behind the casing, -to isolate producing zones from water bearing sands. The cement is normally placed behind the casing in a single or multi-stage technique. The single stage technique pumps cement down the casing and up to annulus. The heavier cement in the annulus is prevented from U-tubing by backpressure valves in the bottom of the casing string. The initial stage of multistage job is usually planned as if it were a single stage effort. Cement is pumped down and up to annulus. The next stage is pumped through a special port collar at the desired location up to annulus. The port is opened after the initial stage is cemented. 71
  2. 2. Squeeze Cementing: A common method for repairing faulty primary casing jobs or performing remedial operations on the hole is squeeze cementing. Major applications: -supplement a faulty primary casing cement job, -reduce water-oil, water-gas and gas-oil ratio -repair casing leaks, -stop lost circulation in an open hole while drilling -bring a well under control. Placement techniques and slurry design are important considerations squeeze operations. Supplementing a faulty or ineffective primary casing cement job is the most prominent application for squeeze cementing. Cement Characteristics The cement slurry pumped into oil and gas wells includes cement, special additives and water. Portland cement is most commonly used. The additives are used to control characteristics such as thickening time, density and compressive strength. Water is an important agent in the cementing. Portland Cement: Portland cement is manufactured by calcining limestone, clay, shale, and slag together at 2000-2600 oF in a rotary kiln. The resulting material, clinker, is cooled and inters ground with small percentages of gypsum to form Portland cement. In addition to the raw materials, other components such as sand, bauxite, 72
  3. 3. and iron oxide may be added to adjust the chemical composition of the clinker for the different types of Portland cement. The principal components of the finished Portland cement are lime, silica, alumina, and iron. Each component affects the slurry in a different manner. When water is added to cement, setting and hardening reactions begin immediately. The chemical compounds in the cement undergo hydration and re-crystallization, resulting in a set product. The API has established a classification system for cements used in oil and gas operations. Slurry Features Variables involved in the design of the slurry include: yield, density, mix water, thickening time, compressive strength, fluid loss and downhole temperature. -The yield of the cement in cubic ft per sack, is the volume of space that will be occupied by the dry cement, water and additives when the slurry is mixed according to design specifications. A major factor affecting the slurry yield is the density, since water must be added in significant volumes to achieve low weight cements that will not fracture shallow, weak zones. -The density of cement is an important design criterion. It must be sufficient to prevent kick and blow-outs yet it should not cause lost circulation. -The mixing water requirements will vary, depending primarily on cement class and slurry density. Most cement jobs use well site water. Quality of mixing water is an important parameter in cement planning. The hydration and curing of the 73
  4. 4. slurry will react differently with varying amounts of salt, calcium, or magnesium the mix water. -Thickening time is the amount of time that cement remains pumpable with reasonable pressures. This is the perhaps the most critical property in the displacement process. Factors affecting the thickening time include cement composition and temperature. The compressive strength is measured in pounds per square inch. A 500psi minimum compressive strength is generally recommended before drilling operations resume, but higher strengths are preferred. -Temperature affects the compressive strength of the cement. Higher temperatures reduce the time for the cement slurry to reach some compressive levels. However, at temperatures above 230 o C, cement strength begins to decrease. -Fluid loss is the water lost from the slurry to the formation during slurry placement operations. If a large volume of water is lost, the slurry becomes too viscous or dense to pump. Neat cement, or cement with no special additives has a fluid loss rate in excess of 1000 cc/30 min. 0-200 cc/30 min 200-500 cc/30 min 500-1000 cc/30 min Over 1000 cc/30 min Good control Moderate control Fair control No control 74
  5. 5. Cement Additives Neat slurry is a mixture of water and cement only. Special chemicals are often added to the slurry to achieve some desired purposes. These additives are: Accelarators, retarders, density adjuster, dispersants, fluid loss additives Accelarators: Most operators wait for cement to reach a minimum of 500 psi compressive strength before resuming operations. At temperatures below 100 oF common cement may require a day or two to develop 500 psi strengths. Accelerators are useful at reducing the amount of waiting-on-cement (WOC) time. Low concentration of cement accelerators, usually 2-4 % by weight of cement, shorten the setting time of cement and promote rapid strength development. Calcium chloride is perhaps the most commonly used chemical for this purpose. Retarders: High formation temperatures associated with increased well depths necessitate the use of chemicals that retard the setting time of the cement; i.e. increase the pumping time. The most common retarder may be calcium lignosulfonate. Its effectiveness is limited in temperatures above 200 oF. Other retarders such as carboxymethyl-hydoxyethylcellulose, can be used to about 240 o F. Density Adjusters: High formation pressures for neat slurry densities require additions in cement density. Formations with low fracture gradients require reductions in cement weight. Dispersants as an additive can increase slurry 75
  6. 6. densities to 17.5 ppg due to their effect on viscosity. Adding more water to the slurry and adding materials to prevent solid separation achieve density reductions. Dispersants: Dispersants provide several beneficial features for the slurry. -reduce slurry viscosity -allow slurry turbulence at lower pump rates -assist in providing fluid loss control for densified slurries Fluid Loss Additives: Fluid loss agents are used in cement slurries for the following reasons: -minimize cement dehydration in the annulus -reduce gas migration -improve bonding -minimize formation damage. Slurry Design A well plan is not complete until the cement slurry has been designed. Major aspects of the design are as follows: -Volumetric requirements for the casing and annulus -cement -mixing water -density selection 76
  7. 7. Calculation of slurry density or “weight” usually expressed in pounds per gallon, is based on the following equation. Slurry weight = (lb cement + lb water + lb addit.) / (gal cement + gal water + gal addit.) Cement has a bulk density of 94 lb/cu ft, an absolute density of 94/0.48 = 195.8 lb/cu ft and an specific gravity of 195.8/62.4 = 3.14. The absolute volume of all solid constituents must be calculated in gallons, where: Absolute Volume, gal = (lb of material) / (8.34 lb/gal x spec. grav. of material) The volume of slurry to be realized from 1 sack of cement when mixed with a specified amount of water and possibly other additives is called the yield. The yield in cubic feet per sack of cement is : Yield = (gal cement + gal water + gal additive) / 7.48 gal/cu ft Example 4-1 Calculate the weight, percent mix and yield or set volume of a slurry given? Water-cement ratio = 5.5 gal/sx Spec. Grav. of cement = 3.14 1 sx = 1 cu ft = 94 lb Density of water = 8.34 ppg Solution: 77
  8. 8. Slurry weight = (lb cement + lb water + lb additive) / (gal cement + gal water + gal additive) Slurry weight = [(94 lb/sx + (5.5 gal/sx x 8.33 lb/gal)] / [(94 lb/sx / 8.33 lb/gal x 3.14) + 5.5 gal/sx] Slurry weight = 15.4 lb/gal Yield = (gal cement + gal water + gal additive) / 7.48 gal/cu ft Yield = [(94 lb/sx / 8.33 lb/gal x 3.14) + 5.5 gal/sx] / 7.48 gal /cu ft Yield = 1.215 cu ft Absolute Volume, gal = (lb of material) / (8.34 lb/gal x spec. grav. of material) Absolute Volume = 94 lb/sx / (8.34 lb/gal x 3.14) Absolute Volume = 3.6 gal/sx Percent Mix = (5.5 gal/sx x 8.34 lb/gal x 100) / 94 lb/sx Percent Mix = 48.8 % by weight of cement Example 4-2 Calculate the number of sacks of cement and bentonite required to obtain cement returns on surface casing. Volume of 9 5/8 inch 40 lb/ft casing = 0.4256 cuft / lin ft Class-A cement with 4 % gel Water-cement ratio = 7.73 gal/sx Slurry weight = 14.10 lb/sx 78
  9. 9. Casing to be landed at 1400 ft Excess cement required = 35 % Solution: Cement left in casing = 30 ft x 0.4256 cu ft / ft Cement left in casing = 12.77 cu ft Cement required to fill annulus = 1400 ft x 0.3469 cu ft / ft x 1.35 Cement required to fill annulus = 655.64 cu ft Total Cement required = 12.77 cu ft + 655.64 cu ft = 668.41 cu ft Sacks of cement required = 668.41 / 1.536 = 435 sx Pounds of cement = 435 sx x 94 lb/sx = 40890 lb Bentonite required = 40890 x 4/100 = 1636 lb or 163.6 sx Cement planning involves evaluating and selecting equipment to be used with the cementing process. The down-hole equipment includes shoes and collars that are run as integral sections of the casing string. In addition, many cementing aids attached to the exterior of the pipe may be used, i.e., centralisers, scratchers and cement baskets. Casing Shoe: A casing shoe is a short, heavy walled pipe run on the bottom of the casing string. It has a rounded “nose” to guide the casing into the hole. The shoe is screwed on the casing and generally is “glued” with a thread-locking compound. Casing shoes are generally available in three types. 79
  10. 10. i) guide shoe; ii) float shoe and iii) differential fill shoe. Figure 4-1 Guide shoe (Courtesy World Oil’s Cementing Book) Figure 4-2 Float Collar (Courtesy World Oil’s Cementing Handbook) A guide shoe contains an orifice through the centre that allows mud to pass freely. A float shoe contains a back pressure valve that prevents mud from flowing into 80
  11. 11. the casing from the bottom yet allows fluid to be pumped through the shoe. Float valve prevents surface casing pressure resulting from cement U-tubing. The driller must fill, or partially fill, the casing with mud periodically to prevent casing collapse as the annulus hydrostatic pressure increases with depth. Differential fill shoe are similar in concept to float shoes. Figure 4-3 Centralizers (Courtesy World Oil’s Cementing Handbook) Collars: A cementing collar is typically run as an integral part of the string and is placed at the top of the first or second casing joint. The collar serves as a stop for the cement wiper plug so that all the cement is not inadvertently pumped completely out of the casing and into the annulus. Multi-stage cementing requires special collars with sliding sleeves and ports. The sleeves are usually closed during the primary stage of cementing. The sleeves are activated with either the free-fall or displacement methods. 81
  12. 12. Centralizers: Centralizers are placed on the exterior of the casing string to provide stand-off distance between the well bore and the pipe in an effort to assist in attaining cement encirclement of the pipe. Numerous types of centralizers are available. The bow spring type is most common. Scratchers: To achieve an effective cement job, the slurry must bond to the formation. Scratchers assist by scraping and scratching the mud cake on the formation to promote bonding to the virgin formation. Cement Baskets: Cement baskets provide support for the column of cement while it cures, or hardens. The baskets are often placed above lost circulation zones that cannot support a full column of cement. Plugs: The cement slurry is normally separated from the mud column by plugs that minimise interface contamination. The bottom plug has a diaphragm that is ruptured with pump pressure after it seats on the collar or shoe. The top plug has a solid aluminium insert. The plugs are mounted in a cementing head at the top of the casing. 82
  13. 13. Displacement Process Pumping the cement into the annulus is an important to the successful cementing program as the slurry design. The displacement rate affects the flow regime in the annulus. High flow rates convert the flow regime from laminar to turbulent. Although annular turbulent flow is not desirable in most drilling operations, it is desirable in cementing operations because it erodes the mud cake on the formation. Contamination of the interface between the mud and cement is a problem that can reduce the effectiveness of the cement job. This problem can be controlled by separating the mud and cement with a spacer fluid (Figure 4-4, 4-5). Primary Cementing Technique Primary cementing operations are usually conducted in single or multiple stages. The single stage method has been used traditionally for conductor, surface, intermediate and production casing strings. Procedure: 1.Drill hole to desired depth. 2.Pull drill string and run intermediate casing. 3.Circulate hole with rig pump. 4.Attach cementing head with plugs to casing. 83
  14. 14. Figure 4-4 Diagram of a casing cementing job (Courtesy World Oil’s Cementing Handbook) 84
  15. 15. Figure 4-5 Equipment typically used to install and cement a drilling liner (Courtesy BJ-Hughes Services) 85
  16. 16. Figure 4-6 Setting and cementing casing (Courtesy Oil & Gas Journal) 5.Connect lines to pump truck and cementing head. 6.Start circulation with pump track. 7.Release bottom plug. 8.Pump spacer to remove mud. 9.Mix cement and displace until all cement is mixed and in casing. 10.Release plug. a) Release top plug for a single-step job. b) Release bottom shut-off plug for second-stage job (Figure 4-7). 86
  17. 17. 11.Pump until sharp pressure increase is noted on pump truck gauge, indicating top plug has bumped. Step 12-16 is for stage cementing. 12.Drop bomb, open ports. 13.Circulate any excess cement around the stage tool. 14.Wait at least 6 hr. for cement to gain initial strength. 15.Mix second stage cement and displace until all cement is mixed and in casing. 16.Release top closing plug and displace until a sharp increase is noted on the pump truck gauge, indicating the plug has bumped. 17.Release pressure to determine if single stage or stage tool is holding. Liner and Squeeze Cementing The liner is run on the bottom of the drill pipe with a hanger and setting tool. Hangers are usually set mechanically or with a hydraulic action. A typical liner assembly is given. Plugs sweep cement from the interior of the liner to the float collar. If the primary cement job is nor successful, squeeze cementing will not be required. However, potential problems must be considered to overcome poor primary jobs. Application for squeeze cementing in drilling and producing operations include: -casing shoe; -liner top -perforation -plug a producing zone or sections of the zone -seal lost circulation problems. 87
  18. 18. Example 4-3 Assume that a 10000-ft well is in an area where the geothermal gradient is 1.8 o F/100 ft. Determine the bottom hole temperature (BHT) if the ambient temperature is 70 oF. Solution: BHT = (D / 100 x G) + TA BHT = (10000 / 100 x 1.8) + 70 BHT = 250 oF Figure 4-7 Cementing plugs: (a) top and (b) bottom plugs (Courtesy World Oil’s Cementing Handbook) 88
  19. 19. Example 4-4 A cementing engineer was preparing to runcompressive strength tests on a cement slurry prior to upcoming casing job. The following data wre available from the logging engineer. The well had been circulated for 6 hr prior to logging. Run No 1 2 3 Time Diff. Btw Runs, hr 7 4.5 8 Temp. oF 220 225 228 Estimate the bottom hole static temperatuıre (BHST) ? Solution: TD = time after circulation / (time of circulation + time after circulation) Run 1, TD = 7 / (6 + 7) = 0.538 Run 2, TD = (7 + 4.5) / (6 + 7 + 4.5) = 0.657 Run 3, TD = (7 + 4.5 + 8) / (6 + 7 + 4.5 + 8) = 0.765 Plot the data and extrapolate to a BHST of 235 oF. Figure 4-8 Estimation of BHST 89
  20. 20. Example 4-5 A 7 5/8 inch 39 lb/ft production casing string will be run inside 51 lb/ft, 10 ¾ surface casing set at 2000 ft. The bottom of the 9 inch hole is at 9100 ft (casing seat). Compute the volume of casing and annulus. A 6 ½ inch x 18 inch duplex pump will be used to pump the cement plıg against the float shoe. If the pump operates at 90 % efficiency, how many strokes will be required? After the job was completed, the drilling engineer at the well site observed that 1990 strokes were required to bump the plug. What is the actual pump efficiency? Solution: 7 5/8 inch pipe capacity = 0.2394 cu ft / lin ft; 0.0426 bbl / lin ft 7 5/8 in. x 9 inch hole annulus = 0.1247 cu ft / lin ft 7 5/8 in. x 9.85 inch annulus = 0.2148 cu ft / lin ft ; 0.0382 bbl / lin ft Compute the pipe and annulus capacities: 7 5/8 inch pipe capacity = 910 ft x 0.0426 bbl / lin ft = 387.6 bbl 7 5/8 in. x 9 inch hole annulus = (9100 – 2000) ft 7 5/8 in. x 9.85 inch annulus = 2000 ft x 0.0382 bbl / lin ft = 76.4 bbl x 0.0222 bbl/lin ft = 157.6 bbl The output of the 6 ½ inch x 18 inch duplex pump is obtained as: 0.2280 bbl/stroke = 100 % efficiency 0.2052 bbl/stroke = 90 % efficiency 90
  21. 21. Determine the pump stroke requirements to bump the plug. 387.6 bbl / (0.2052 bbl / stroke) = 1888 stroke If the pump required 1990 strokes, determine the output. 387.6 bbl / 1990 strokes = 0.1948 bbl / stroke Determine the actual efficiency. (0.1948 bbl / stroke) / (0.2280 bbl / stroke) x 100 = 85.4 % Example 4-6 A 3000 ft 13 3/8 inch surface casing is to be cemented in a 17.5 inch hole. The 1000 ft tail slurry is 14.2 lb/gal Class-A cement with 4 % gel.The remaining lead slurry is 12.2 lb/gal Class-A cement with 16 % gel. Use 100 % volumetric wash out. Compute the cement, water and gel requirements. Solution: The annulus volume is computed as: 0.6946 cu ft / lin ft x 2000 ft = 1389.2 cu ft 0.6946 cu ft / lin ft x 1000 ft = 694.6 cu ft Accounting for 100 % wash outs: (2000 ft lead slurry) : 1389.2 (1000 ft lead slurry) : 694.6 x x 2 = 2778.4 cu ft 2 = 1389.2 cu ft Lead slurry calculations are as follows: Cement: 2778.4 cu ft / (2.55 cu ft / sx) = 1089.5 sx of cement 91
  22. 22. Gel: (1089.5 sx) (16 % gel) (94 lb/sx) = 16386 lb gel = 163.86 sx of gel Water : 14.7 gal/sx x 1089.5 sx = 16015 gal = 381 bbl Tail slurry calculations are as follows: Cement: 1389.2 cu ft / (1.52 cu ft / sx) = 913.9 sx of cement Gel: (913.9 sx) (4 % gel) (94 lb/sx) = 3436 lb gel = 34.4 sx of gel Water : (913.9 sx) x 7.57 gal/sx = 6918 gal = 164.7 bbl Applications of API Cements: Class-A -Used at a depth range of 0 – 6000 ft. -Used at a temperature of up to 170 oF. -Intended for use when special properties are not required; well conditions permit. -Economical compared with premium cements. Class-B -Used at a depth range of 0 – 6000 ft. -Used at a temperature of up to 170 oF. -Intended for use when moderate to high sulfate resistance is required; well conditions permit. -Economical compared with premium cements. 92
  23. 23. Class-C -Used at a depth range of 0 – 6000 ft. -Used at a temperature of up to 170 oF. -Intended for use when early strength is required; its special properties are required. -High in tricalcium silicate. Class-D & E -Class-D is used at a depth range of 6000 – 10000 ft. -Class-E is used at a depth range of 10000 – 14000 ft. -Class-D is used at a temperature of 170 oF to 260 oF. -Class-E is used at a temperature of 170 oF to 290 oF. -Intended for use when moderately high temperature and high pressure are encountered; its special properties are not required. -Available in types that exhibit regular and high resistance to sulfate. -Retarded with an organic compound, chemical composition and grind. -More expensive than Portland cement. Class-F -Used at a depth range of 10000 – 16000 ft. -Used at a temperature of 230 oF to 320 oF. -Intended for use when extremely high temperature and high pressure are encountered; its special properties are not required. -Available in types that exhibit moderate and high resistance to sulfate. 93
  24. 24. -Retarded with an organic compound, chemical composition and grind. Class-G & H -Used at a depth range of 0 – 8000 ft. -Used at a temperature up to 200 oF without modifiers. A basic cement compatible with accelerators or retarders. -Useable over the complete range of classes A to E with additives. Class-J -Used at a depth range of 12000 – 16000 ft. -Used at a temperature of 170 oF to 320 oF without modifiers. -Useable with accelerators and retarders. -Will not set at temperature less than 150 oF if used as a neat slurry. API Cement Composition API Class A B C C3 S C2 S C3A C4AF Fineness Sq cm/g 53 47 70 24 32 10 8 3 3 8 12 13 1500-1900 D G H J 26 52 52 53.8 54 32 32 - 2 8 8 8.8 12 12 12 1500-1900 20002400 1100-1500 1400-1600 1200-1400 - 94 12402480 Water / Cement 0.46 0.46 0.56 0.38 0.44 0.38 0.44
  25. 25. API Cement Properties Cement Class Mix Water Gal /s x Slurdy density Ppg Slurry yield Cuft / sx Thicken. Time 113 oF, hr Comp. Streng. 110 oF, psi A C G H 5.2 6.3 5.0 4.3 15.6 14.8 15.8 16.5 1.18 1.32 1.15 1.05 2½ 1¾ 1¾ 2 4000 2700 3000 3700 Effect of Temperature on Compressive Strength Curing Time 8 12 24 80 oF 203 750 1570 100 oF 1100 1710 2720 120 oF 2320 2600 3740 140 oF 2235 3420 4580 160 oF 2900 4150 5190 Effect of Gel Additions on Class H Slurries % Gel 0 4 8 12 16 Mixing Water gal/sx 5.18 7.57 9.96 12.4 14.7 Slurry Density lb/gal 15.7 14.2 13.3 12.6 12.2 95 Slurry Volume cu ft /sx 1.17 1.52 1.86 2.21 2.55