A 7-in. production casing liner is planned for an 8l/2-in. hole. Assume that a 14.0-lb/gal lignosulfonate mud will be used. Determine the critical velocity using the Bingham model (see Chapter 18) and compute the minimum acceptable flow rate in the annulus to achieve turbulence. In addition, determine the minimum acceptable flow rale if the hole is eroded to 9.5 in.
1.08(27) + 1.08 V(27)~ + 9.26(8.5 - 7)a(10)Tl4) ¡4(8.5 - 7) = 4.493 ft/sec (8.5-in. hole, no washout)
14(9.5 - 7) = 3.73 ft/sec (9.5-in. washed-out hole)
3. The minimum acceptable flow rates are: S'/a-in. hole:
Q = 2.448 (d,2 - dj')V = 2.448 (8.52 - 72) 4.493 = 255 gal/min
Spacers. Contamination of the interface between the mud and cement is a problem that can reduce the effectiveness of the cement job. The contamination can become widespread throughout the slurry and cause channeling. This problem can be controlled by separating the mud and cement with a spacer fluid.
A service company should be contacted to provide additional details on spacer fluids.
Primary Cementing Techniques. Primary cementing operations are usually conducted in single or multiple stages. The cement is pumped down the casing and up the amiulus. Some techniques use the drillstring to convey the cement to the bottom of the casing.
The single-stage method has been used traditionally for conductor, surface, intermediate, and production casing strings. Fig. 9-23 illustrates the basic features of a cement job for conductor and surface casing. Procedures for a typical intermediate job and bomb-stage cementing are as follows:
1, Drill hole to desired depth.
2. Pull drillstring and run intermediate casing.
The techniques for a continuous-displacement-stage cementing are slightly different. Large-diameter casing is usually cemented through the drillpipe (Fig. 9--24). The pipe is run to total depth, and the drillpipe is run inside the casing with an inner string stab-in or a thread adapter on the end with a centralizer. The float shoe is designed to accept the adapter, and a plug may or may not be used. Inner string cementing prevents the casing from collapsing, reduces displacement volume and time, and permits mixing until cement circulates with minimum waste.
Liner Cementing. Cementing a drilling or production liner and achieving an effective cement job is a difficult task. The difficulties arise from the small annulus between the liner and the wellbore. Problem areas are 1) small annular clearances make excess volume calculations uncertain when hole washout is present and 2) cementing aids such as scratchers and eentralizers usually cannot be used, which increases the difficulty in developing good cement encirclement and bonding. In addition, lost circulation additives such as gilsonite and cellophane cannot be used to control lost circulation since they plug the return ports on the liner hanger.
The liner is run on the bottom of the drillpipe with a hanger and setting tool. Hangers are usually set mechanically or with a hydraulic action. A typical liner assembly is shown in Fig. 9-25. Plugs sweep cement from the interior of the liner to the float collar.
Squeeze Cementing. If the primary cement job is successful, squeeze cementing will not be required. However, potential problems must be considered in the contingency plan to overcome poor primary jobs. Applications for squeeze cementing in drilling and producing operations include the following:
Each application has different characteristics with respect to slurry design and placement techniques.
Plugs. Many drilling operations require that a cement plug be set in open hole or casing. Common applications include the following:
The cement slurry and spacer design considerations are basically the same as for primary cementing.
Most plugs are spotted with the balanced plug technique (Fig. 9-26). Although simple in concept, the technique requires careful planning to ensure the plug is properly positioned. Placement failures commonly occur due to fluid backflow, slugging, or improper displacement volume calculations. The quantities that must be calculated are as follows:
1. length of the cement plug or the number of sacks of cement for a given length of plug
Fig. 9-25 Equipment typically used to install and cement a drilling liner (Courtesy BJ-Hughes Services)
(b) Cement, walrcr, and mud balanced.
(c) Pulling string above top of cement.
(c) Pulling string above top of cement.
Cement requirements can be calculated with Eq. 9.3:
N = sacks of cement L = plug length, ft Ch = hole capacity, cu ft/ft Y = slurry yield, cu ft/sack
The water volume to be pumped behind the slurry to balance the plug is computed with Eq. 9.4:
V, = spacer volume ahead of the slurry, bbl
Vb — spacer volume behind the slurry, bbl
CP = pipe capacity, cu ft/ft
Eq. 9.5 is used to calculate the plug length, Lw, before Ihe pipe is withdrawn from the slurry;
Mud volume for pipe displacement is as follows:
Vj = displacement volume, bbl Lp = total pipe length, ft Cp = pipe capacity, bbl/ft V6 = spacer volume, bbl
Pipe capacity, Cp, in Eqs. 9.4 and 9.6 has different units. Therefore, the decimal book must be consulted.
A 600-ft plug is to be placed at a depth of 8,000 ft. The open hole size is 6'/z in., and the tubing size is 2-%-in. OD (4.6 lb/ft). Ten bbl of water are to be pumped ahead of the slurry. Assume a slurry yield of 1.18 cu ft/sack. Calculate the number of sacks needed for the job, the volume of water to be pumped behind the slurry to balance the plug, and the amount of mud required to displace the spacer to the balanced point.
1, Use a decimal book to determine the following data:
hole capacity = 0.2304 cu ft/lin ft 2%-in. x 6,/z-in. annulus capacity = 0.1997 cu ft/lin ft 2%-in. capacity = 0.02171 cu ft/lin ft = 0.00387 bbl/ft
2. Determine the number of sacks required for the plug:
3. Calculate the volume of water to be pumped behind the slurry to balance the plug:
4. Determine the length of the plug before the pipe is withdrawn from the slurry:
5. Calculate the displacement volume, Vd:
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