Modeling Studies Of Coupled Thermal–Hydrological–Mechanical ?THM? Process Of CO₂–Enhanced Coalbed Methane Recovery And Their Applications

CO₂-enhanced coalbed methane recovery,also known as CO₂-ECBM,is a po-tential win-win approach for enhanced methane production while simultaneously se-questering injected anthropogenic CO₂to decrease CO₂emissions into the atmosphere.Based on constant confining pressure tests of coal samples,a relationship between per-meability and pore pressure of was investigated by in-house gas permeability exper-iment system.A physical similar simulation of CO₂enhanced coalbed methane was conducted.The code bugs of subroutine SOLIT in TOUGH2-7C?ECBM?is fixed.CO₂-ECBM is simulated using a coupled thermal–hydrological–-mechanical?THM?nu-merical model that considers multiphase?gas and water?flow and solubility,multi-component?CO₂and CH₄?diffusion and adsorption,heat transfer and coal deforma-tion.An anisotropy permeability model for fractures is also developed to predict the evolution of permeability on the different directions.Than,a sensitivity analysis is designed to understand the effects of different controlling parameters on gas CO₂injec-tivity and methane productivity.Finally,a coupled flow and geomechanical approach is performed to analyze fault reactivation and seismicity associated during CO₂-cnhanced coalbed methane.The experimental and numerical results indicate that an increasing pore pressure promotes the enhancement of permeability.The deformation of caprock keeps increas-ing during the physical similar simulation and decreases considerably all the way to the outer boundary,though larges decrease occurs close to the wellbore.The new subrou-tine SOLIT improves the linear algebra and overall convergence.The capacity of the updated simulator for modeling methane production is verified by a code-to-code com-parison with the general-purpose finite-element solver COMSOL.The non-isothermal simulation shows that the permeability evolution,mechanical stress,and deformation are all affected by changes in pressure,temperature and adsorption swelling,with ad-sorption swelling having the largest effect.The calculated stress changes do not induce any mechanical failure in the coal seam,except near the injection well in one case of a very unfavorable stress field.The hydrofracturing and refracturing treatments con-tribute to increase a significant increase the injection and production.The reduction of pore pressure near the injection well decreases the damage potential possibility of well areas.After 250 days of injection,the fault slip occurs.Numerous micorseimic events,as well as aseismic deformations are induced during CO₂injection.The max slip dis-tance and magnitude are 0.35 m and 3.3796,repectively.The magnitude and timing of microrseimic the event reduces with deceasing fault angle and stress ratio between z-and x-directions,and increasing permeability of fault.No slips occurs when stress ratio is over than 0.9.