Stress response during in-situ gas depletion and its impact on permeability and stability of CBM reservoir

Abstract

Coalbed methane (CBM) reservoir is generally believed to be under the uniaxial strain condition. Because of this stress/strain controlled boundary, the horizontal stress is passively changed with gas depletion. This dynamic stress evolution has great impact on permeability and stability of the CBM reservoir. Even though it is generally known that the stress depletion is both poromechanics and desorption controlled process, however, how the gas pressure and the gas type affect the horizontal stress profile and its induced coal failure is still unclear. In this work, we experimentally simulated the uniaxial strain condition in the lab. Both the horizontal stress and permeability were continuously monitored during of different gases (He, N2, CH4 and CO2) depletion. The results show that regardless of gas type, the horizontal stress decreases with the reduction of gas pressure. And gas with a higher adsorption capacity corresponds to a greater horizontal stress loss. During the depletion, the effective vertical stress rises, while effective horizontal stress show various trends for different gases. The effective horizontal stress increases during helium depletion, which leads to a reduction of the permeability. And it decreases during methane and carbon dioxide depletion, which corresponds to a significant increase of the permeability. While for nitrogen, the permeability decreases first and followed by a slight uptick. Both the lab test and modeling results show that a significant non-linearity exists in effective horizontal stress change during adsorptive gas depletion, which is expected to affect the reservoir stability. To evaluate the reservoir stability during depletion, a stability factor (SF) was put forward, and the influencing factors on this parameter were analyzed. With this parameter, the critical reservoir pressure indicating reservoir failure can be determined with appropriate geomechanical parameters input. This research will advance the understanding of the geomechanics change during CBM production.