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Selecting Cement and Additives•
Running the Casing String•
Cementing the Casing StringYou should select casing hardware that is compatible with the cementing, stimulation, and completion plan for the well. Because of the marginal economics for coalbed wells, most coalbed meth-ane operators try to minimize investment in casing hardware.
However, savings on casing hardware can be easily overshadowed by formation damage or loss in well control caused by lack of proper equipment.
Before beginning your casing and cementing program you should obtain a casing and cementing handbook from one of the major oilfield service companies. This handbook provides specifications and other useful information on casing and cementing equipment and materials.
Operators in the Black Warrior Basin use a variety of casing hardware when running casing. The purpose and procedure for using several of these tools is described below:
Selecting Casing Hardware
Cement Wiper Plug
A cement wiper plug is a rubber plug (or rubber with a cast alumi-num insert) used to separate the cement slurry from the displace-ment fluid to prevent contamination and/or dilution of the tail end of the slurry. Because water is normally used as the displacement fluid in coalbed methane wells, slurry contamination is usually not a problem, but dilution could occur. The wiper plugs are mounted in a cementing head at the top of the casing so they can be released directly behind the slurry without shutting down.
Guide Shoe
A guide shoe is a short heavy-walled pipe or collar with a round nose on bottom. The shoe is installed on the bottom of the casing to prevent the casing from hanging on ledges or other borehole irregularities. The guide shoe is attached to the bottom of the production casing before running the casing into the hole.
Float Collar
A float collar contains an internal valve which prevents backflow of cement up the casing string during cementing operations. It also increases the buoyancy of the casing, thus reducing the load on the rig while running casing. In addition, the float collar serves as a stop for the cement wiper plug so that all of the cement is not inadvertently pumped out of the casing. The float collar is usually installed one joint above the guide shoe.
Casing Centralizers
Casing centralizers ensure the casing remains in the center of the wellbore during cementing operations to allow for cement coverage on all sides of the casing string. Centralizing the casing improves the probability of effective cement jobs and zone isolation. In addition, centralization reduces the negative effects of bends or doglegs in the casing which could hamper artificial lift equipment and workover operations.
When cementing across a coal seam, you should always run central-izers above and below each seam that may be produced at some future time.
The number of centralizers that you should run in the rest of the casing string depends on the hole size and the amount of hole deviation. When running 5-1/2 inch casing in a 7-7/8 inch hole, most service companies recommend running a centralizer at least every third or fourth joint. If the hole is highly deviated, you will need to space the centralizers closer
A float shoe is a combination guide shoe and float collar. It has a round nose, and it contains a check valve and may also contain a catcher for the wiper plug. A latch-down plug may be used to prevent backflow in case the check valve fails. A float shoe can be used instead of a float collar and guide shoe.
Float Shoe
Baffle plates are installed in the casing, usually instead of or along with other cementing equipment. The plates are installed between the guide shoe and the first joint of casing or between the first two joints of casing if you would like to have one joint filled with cement at the bottom of the string. Baffle plates are held in place by the pin end of the casing or tool (such as a float shoe) below them.
The latch-down plugs wipe the casing free of cement during displace-ment. The wiper plug latches in an internal catch in the baffle plate to prevent flow back into the casing after cementing.
Baffle Plates with Latch-Down Plugs
A cement basket is a tool attached to the outside of the casing to provide support for the cement column while it cures. Cement baskets can be placed above zones that have low fracture gradients to prevent them from breaking down. If cement baskets become filled with debris, they may inhibit reciprocation of casing.
Cement Basket together.
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Inadequate centralization of the casing can prevent an effective cement job.
External Casing Packer
An external casing packer, which is run in the casing string, forms a seal between the casing and the hole. If running casing in a deep well or in a well with weak coal zones (i.e., coals with low fracture gradients), you can run an external casing packer in the casing string to help support the cement column and reduce the pressure the cement exerts on the coal formation.
To operate the packer, a plug is pumped down to a seat below the packer such as a baffle plate or cementing collar. Once the plug is seated, pressure is applied above it to open the ports to the packer.
When the ports open, cement can be pumped. The cement fills the packer and inflates the packer element against the wall of the hole.
After the packer is inflated, the ports in the cementing collar above the packer can be opened by applying additional pressure, allowing cement to flow into the annulus above the packer.
External casing packers are normally used in coalbed methane wells to protect the lower-most coal seam in open hole comple-tions. This technique is described in Chapter 4 - Completing the Well.
Multi-Stage Cementing Tool
A multi-stage cementing tool is used when the required column of cement is too large to be pumped in a single slurry. The tool contains a plug catcher and side ports. To activate the tool, a plug is dropped, and then the casing is pressured up. This pressure seats the plug in the plug catcher to seal off the casing and open the side ports. Then the second cement stage is pumped, and it flows out the side ports to the annulus. This tool is run with the casing string.
It is installed in the casing at a depth above the calculated top of the primary cement and above the coal formation to be isolated from cement intrusion.
For more information on stage cementing, refer to Cementing the Casing String, later in this chapter.
Operators have used several different types of cement in coalbed methane wells. The simplest type used is Class A, which is a common portland cement. Class A cement has a density of 15.6 ib/
gal without additives. Adding bentonite to Class A cement can lower its density by increasing the maximum allowable volume of water that can be added to the cement. Adding 6% bentonite can reduce the density to 13.5 lb/gal.
You can use Class A cement for relatively shallow coals if the coal will support its density. The maximum depth recommended for Class A is 6000 ft. Class A cement is more economical than the other premium cements.