Cement Slurry Designs
Class A Slurry
Because coals have a low mechanical strength, you must select a proper cement density to prevent the weight of the cement from fracturing the coal formations. For information on calculating proper cement density, refer to “Designing the Cementing Pro-gram” earlier in this chapter. After you have determined the correct cement density for your well, you can then select the proper cement.
Experience in the Black Warrior Basin has demonstrated that you can usually avoid potential cementing problems and accommodate the tight economic constraints of coalbed methane completions by using one of the following cement slurry applications:
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Class A Slurry•
Pozmix® Slurry•
Silicalite Slurry•
Foam Slurry•
Specialized SlurryPozmix ® Slurry
Pozzolans are siliceous or siliceous/aluminuous materials which you can use to lower the density of cement slurries, much the same as bentonite. If you are working in an area where the coal formation will support cement densities of 12 to 14 lb/gal, you can use a Pozmix®
slurry to provide zone isolation and adequate compressive strength.
A typical pozzolan blend is 50% Class A and 50% pozzolan. This mixture is commonly called "50/50 Poz." A 50/50 Poz cement has a density of 14.15 lb/gal. An advantage of pozzolan slurries is their resistance to corrosive fluids. A disadvantage is their lower compres-sive strength compared to Class A cement.
A Pozmix® cement design which has been used successfully at the Rock Creek project is listed below:
1. To mix the lead slurry, combine a 50/50 blend of Pozmix®/
Class A cement with 4% total bentonite for a slurry weight of 12.7 to 12.8 lb/gal.
2. To mix the tail cement slurry, combine the same mixture as for the lead cement, but mix at 13.5 lb/gal.
You can also mix a tail cement of 15.6 lb/gal using neat cement, if the coal formation will support this weight.
Silicalite Slurry
A Silicalite slurry is a blend of Class A, Pozmix®, and Silicalite.
Including Pozmix® and Silicalite in the blend helps reduce the density by inceasing the amount of water which may be added to the slurry.
In areas where coals will not support cement densities of 12 to 14 lb/
gal, a Silicalite cement may work effectively.You can mix a Silicalite slurry with a density from 11 to 13 lb/gal. A typical Silicalite slurry has a density of 11.5 lb/gal.
Because the properties of silicalite cement are so well suited to coalbed methane wells, some operators use this slurry even in
wellbores strong enough for a higher weight cement. The cement has excellent fluid loss characteristics, low slurry viscosity, set times
faster than Pozmix® blends, essentially no free water, and high early compressive strengths.
Operators often use foam cement slurries to cement shallow, low pressure coalbed methane wells where weak zones would break down if a normal density cement were used. If you are working in an area where wellbore integrity requires slurries under 11 lb/gal, you may consider using a foam cement.
Foam cement is usually a mixture of basic cement, foaming agents, stabilizing agents, and nitrogen. This combination provides a light-weight cement slurry with a high yield. Foam cement slurry may be the most economical if you have nearby access to nitrogen facilities.
If nitrogen is not readily available, you may consider using conven-tional cement with multistage cementing tools. When comparing the cost of using a multistage tool to the cost of using foam cement, be sure to include the drillout cost for the multistage tool.
Foam Slurry
You can use a variety of specialized slurries and additives to meet individual well requirements. For example, if you encounter a highly permeable zone that causes lost circulation, you could seal it off using a thixotropic cement, which sets very quickly. Thixotropic cements are also very effective for secondary or remedial cementing.
Some types of light weight cement achieve lower densities by utilizing additives which allow adding more water to the slurry. However, the added water lowers the ultimate compressive strength of the cement.
If you need a light cement for a primary cement job, you might use a special cement that incorporates hollow glass beads, or microspheres, with a base cement. You can add these hollow microspheres to any type Pumping foam cement at too high a rate may create a higher friction pressure in the casing annulus than would other types of cement. This increased friction pressure may offset the benefit of the lighter weight of foam cement. To fully realize the benefits of foam cement’s lighter weight, do not pump foam cement at an excessive rate.
Specialized Slurries
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of cement to produce slurries ranging in density form 9 to
12 lb/gal. This type of slurry can greatly reduce the density of the slurry without significantly reducing the compressive strength of the cured cement.
Some glass microspheres may begin to crush at pressures near 4000 psi. Because the crush resistance of glass microspheres varies, you should check with the manufacturer or supplier of microspheres before using them. Though the depth to which glass microsphere slurrries can be used is limited, most coalbed methane wells are shallow enough to use them.
Special additives are usually mixed with the base cement to alter or improve slurry properties. You can use additives to accelerate or retard cement curing, to reduce slurry density, to control fluid loss or lost circulation, or to modify other slurry properties. For example, you can add calcium chloride or sodium chloride to cement to accelerate the time required for the cement to set or to hydrate. As mentioned earlier, you also can add pozzolans or bentonite to reduce the density of the cured cement.
When designing your casing program, consult several different ce-menting company representatives who are trained and experienced in cementing coalbed methane wells. They can provide information about a variety of additives available for altering slurry properties to meet the requirements of your particular well.
In areas where leakoff is high, consider the following guidelines:
◆ Add a low fluid loss additive to the slurry. Use an additive that does not delay thickening time or increase slurry
viscosity.
◆ Add a lost circulation material such as gilsonite, cellophane flakes, or walnut shells to help prevent cement contamination of the fractured coal.
Cement Additives
Before beginning to pump cement, you should follow the procedures below:
1. Several hours before pumping the cement, meet with the service company people to discuss the goals of the cement job and the responsibilities of each person. Also discuss contin-gency plans for handling possible operational problems.
Invite questions or suggestions regarding any aspect of the operation.
2. Several hours before pumping the cement, conduct a safety meeting with all people who will be on location during the cementing job. Discuss safe operating procedures, use of safety equipment, and contingency plans in case of an emer-gency.
3. Obtain a sample of the actual dry cement mixture (with additives) that will be pumped.
Maintain this sample as a quality control check in case problems arise on the cement job. You can have it sent to a lab for analysis, if necessary.
4. Install the cementing manifold with plug(s) (from the ce-menting company) on top of the casing.
Figure 2-6 shows a cementing manifold similar to the type used to cement the wells at the Rock Creek project.
5. Pressure test all surface pumping lines with water. Test up to the maximum anticipated surface pump pressure.
The following techniques have proven effective in cementing coalbed methane wellbores in the Black Warrior Basin.
Before the Cementing Job