Quantifying and Modeling of In Situ Stress Evolutions of Coal Reservoirs for Helium, Methane, Nitrogen and CO₂ Depletions

Coalbed methane (CBM) reservoirs show unique stress responses due to the additional sorption-induced effect under subsurface in situ condition. An insight of dynamic stress evolution is important to CBM development and carbon sequestration in coals. In this study, the combined controls of geomechanical effect and sorption-induced effect were gained to determine in situ stress evolution and predict mechanical failure in coals under uniaxial strain condition for various gases. We conduct a series of experimental measurements on stress path responses with continuous gas depletion for helium, N₂, CH₄ and CO₂. The proposed model is validated by the experimental data, and then, we theoretically analyzed the stress evolution and potential mechanical failure behaviors for CBM operations. The applied horizontal stress was found to continuously decrease for all gas types with pressure depletion under uniaxial strain condition. Sorbing gas showed a higher excessive stress loss with the same decrement of gas pressure than that in non-sorbing gas depletion which is attributed to the sorption-induced effect. For helium depletion, horizontal stress variation is mainly controlled by geomechanical effect. Theoretically, coals with high gas affinity can induce relatively large horizontal stress loss. This excessive stress loss can trigger the localized instability of coal mass due to deviatoric stress trigger shear failure.