Enhanced Coalbed Natural Gas Technologies

 

Enhanced Coalbed Natural Gas (ECBNG) is a recovery technique in which CO₂ or gas mixture, such as kiln gas (1), is injected into subsurface coalbeds to enhance the recovery of Ch₄ (2). The process is based on the assumption that CO₂ preferentially adsorbs to coal over Ch₄. Injected CO₂ travels through the cleat system and into the coal matrix replacing the sorbed Ch₄, freeing it to be produced (3). ECBNG is still evolving, with many aspects of it still not fully understood. Ideally, a successful ECBNG project will provide a positive result of increased Ch₄ production, while helping to alleviate the problem of greenhouse gases.

 

Coal Behavior During Production

 

Changing reservoir conditions during the production or injection of gases, from or into coalbeds can greatly affect permeability of the cleat system. At initial conditions the permeability of the cleat system is a function of the effective stress, a balance of overburden pressure, and reservoir pressure. Upon well completion and production, reservoir pressure of a coalbed the reservoir pressure is lowered. Lowering of pressure means an increase in effective stress, which leads to a compaction of the coalbed and closure of the cleat system, which reduces permeability (1). However, a lower reservoir pressure also leads to gas desorption, causing the coal matrix to shrink, thus opening the cleat system (2). In the case of gas injection the opposite process occurs. The processes of compaction and shrinkage counteract each other, and the relative effects of each are dependent on several variables including reservoir depth, pressure variation, and total gas production or injection. Reservoir simulators account for these processes, by using a model developed by Palmer and Mansoori (1998). The variables for the simulator will be listed later.