This paper discusses the application of foam cement technology for zonal isolation in coal bed methane (CBM) wells.
CBM reservoirs have unique cleat structures that are weaker and less stable than conventional reservoirs. As the gas storage mechanisms in the CBM reservoirs release methane and other light gases through diffusion, bedding structures can be even more destabilized. Conventional cement sheaths may not withstand annular deformation and may crack because of cyclic stress loads.
Results of recent laboratory tests and field applications have shown that foam cement can be effectively used to eliminate zonal isolation problems associated with the application of conventional cement jobs in CBM wells.
Laboratory tests have shown that a foam-cemented annulus can elastically absorb stresses from the pressure-induced expansion of internal casing and deform without failure. The foamed cement can maintain bond and overall integrity in such tests at up to 10,000 psi internal casing pressure. In cycling tests, foam cement was found to withstand 100 cycles, up to 90% of shear failure index, without noticeable damage. Results of some recent laboratory tests are presented in this paper.
Zonal isolation using foamed cement has been successfully conducted in many wells drilled in western Canada CBM reservoirs. An example of recent foamed cement application in a CBM well is also presented.
Foam cement is a mixture of cement slurry, foaming agent, foam stabilizer, and nitrogen gas. When the foam cement is properly generated, a stable and lightweight slurry that looks like gray shaving cream can form (Fig. 1). When foam slurries are properly mixed and sheared, they often contain microscopic, discrete bubbles that will not coalesce or migrate. The bubbles formed are not interconnected (Fig. 2), which results in a low-density cement matrix with low permeability and relatively high strength.
Historically, the primary purpose of foamed cement was to decrease the density of slurry. However, many other advantages have been identified, and subsequent applications of foam cement are well documented.1–7
This paper describes the most recent technology in foam cement application. Characteristics of the foam cement and laboratory test results are summarized. Guidelines for field applications are provided, and an example of a field application of foam cement in a CBM well is also presented.
The following section briefly discusses some of the favorable physical properties of foam cement.
Stable foam cement can yield 720 kg/m3 downhole density at bottomhole conditions. During primary cementing, foam cement can prevent formation breakdown, lost circulation, and post-job cement fallback. The extremely lightweight quality of foam is especially useful for lost-circulation plugs where conventional methods of cementing may not be applicable.
Foam cement has an excellent strength-to-density ratio (Fig. 3). Slurries that contain less water are usually stronger than those that carry a lot of water. With inert nitrogen gas as a filler material, slurries of even very low density can still have high solids content, which causes the ultimate strength to be relatively high.