Selecting Casing Weight and Grade

Burst Pressure

Burst pressure refers to a condition of unbalanced internal pressure.

Burst pressure is probably the most important factor in designing the coalbed casing string because the pipe will likely experience the greatest pressures during fracturing stimulations, when treating pressures can exceed 5000 psi. You can estimate the treating pressures required by using the fracture gradients you predicted when determining casing setting depth (step 3 above). Once you have estimated fracture gradients for the coal seams of interest, you can select the proper casing weight and grade. For more informa-tion on casing specificainforma-tions, refer to a service company casing handbook.

Tensile Load

Tensile load is the force exerted on a joint by the weight of the joints below it. Because each joint supports all the weight below it, the greatest tension occurs at the top of the string. Most coalbed wells in the Black Warrior Basin are shallow; therefore, tensile load is not a primary consideration for this area.

Production casing is usually available in sizes ranging from 4.5 inches to 7.0 inches and in a variety of weights and grades. Casing is also Collapse pressure is the unbalanced external pressure imposed on the pipe. The worst operational case is for the pipe to be empty with a normal hydrostatic pressure gradient exerted on it from the outside. The greatest differential pressure exerted on the casing is most likely to occur during flowback of a fracture treatment or during the later stage of production when pressure inside the wellbore decreases significantly. You should design the casing string for this worst case scenario.

Typically, water levels in coalbed wells are pumped down to minimize hydrostatic pressure and optimize gas production. The collapse pressure becomes a more significant factor in deeper coalbed wells. Because of the relatively shallow wells (500-3500 feet) in the Black Warrior Basin, casing collapse has posed few problems in this area. However, the collapse strength of the casing may be reduced by mechanical operations such as slotting or high density perforating.

Collapse Pressure

classified as API (American Petroleum Institute) standard casing or limited service casing. API standard meets all specifications for wall thickness, outside diameter, inside diameter, drift, collapse, internal yield, and joint yield strength ratings for its respective grade.

Limited service casing is also called “mill reject” because one or more specifications does not meet API standards. However, limited service casing may also be tested to 80 percent of the minimum yield as set forth by API specifications. Therefore, to reduce cost you may choose to use limited service casing for some applications.

Typical casing grades are F-25, H-40, J-55, K-55, C-75, N-80, C-95, and P-110. These grades represent the strength of the casing. A variety of casing weights and wall thicknesses is also available for use according to well conditions. Select the size, weight, and grade of production casing based on the individual well design and completion technique. For more information on completing coalbed methane wells, refer to Chapter 4.

Before ordering casing, find out the limitations of casing weight and length for the rig you will use to run the tubulars. By ordering Range Two casing and tubing, which have lengths of 28-32 feet, you may be able to use a smaller, less costly rig.

Most Black Warrior Basin operators complete coalbed methane wells simply using a production string set through a shallow sur-face casing. They generally run 5-1/2 inch casing in a 7-7/8 inch hole. The surface casing usually consists of 300 feet of 8-5/8 inch casing set in a 12-1/4 inch hole.

Casing Used in the Black Warrior Basin

Using casing smaller than 4-1/2 inch (O.D) limits the size of production tubing you can run inside it. If the casing/tubing annulus is too small, the flow path for gas will be restricted and the annulus can easily plug.

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To select the most effective drilling technique for your area of interest, you must consider the geologic and reservoir conditions of the coal basin. Generally, wells drilled in the eastern United States target shallow coal beds (less than 4000 feet) in geologically older (Pennsylvanian) and more competent formations. Operators in this area usually employ relatively simple drilling techniques. In contrast, complex drilling techniques are used to drill wells in the western United States, which usually target younger (Cretaceous) formations that are deeper, over-pressured, and less competent.

Operators in the Black Warrior Basin frequently drill coalbed wells using the rotary-percussion technique, with air or air-foam mist as the circulating fluid. Figure 2-5 shows a comparison of the con-ventional rotary and the rotary-percussion drilling techniques.

Rotary-percussion drilling has become a standard technique in the Black Warrior Basin because it typically yields higher penetration rates and lower drilling costs than conventional rotary drilling. In addition, the rotary-percussion technique minimizes formation damage because it uses no drilling mud.

In the northern end of the Black Warrior Basin, where the surface formations are hard, coalbed wells are often drilled from surface to total depth using the rotary-percussion technique. In this area, drilling with a tri-cone rotary bit yields lower penetration rates because at shallow depths it is not possible to apply sufficient weight on the bit.

In the southern end of the Black Warrior Basin, however, where the softer Cretaceous formations are encountered from surface to as deep as 500 feet, the surface hole must be drilled using a tri-cone rotary bit with drilling fluid (usually water) to prevent hole col-lapse. After drilling through the Cretaceous formations and setting surface casing, drillers usually switch to rotary-percussion drilling to achieve greater penetration rates in the harder formations.

Most of the coalbeds in the Black Warrior Basin are water satu-rated, low pressure, low permeability formations. In some parts of the basin, little formation water flows into the wellbore during drilling, and air circulation can easily remove not only cuttings, but any produced water as well. When the wells at the Rock Creek site