Thermo-hydro-mechanical-chemical couplings controlling CH4 production and CO2 sequestration in enhanced coalbed methane recovery

Abstract

We explore the fully coupled thermo-hydro-mechanical-chemical (THMC) response of CO2 enhanced CBM recovery (CO2-ECBM) considering the coupling relationships of competitive sorption of binary gas and dissolved gas in water (C), gas and water transport in two phase flow (H), thermal expansion and non-isothermal gas sorption (T), and coal deformation (M). The THMC model is developed, validated then applied to simulate CO2 enhanced recovery. Parametric studies are completed, systematically switching-off components of the thermal (T) and hydraulic (H) coupling, to provide insights into key processes controlling ECBM recovery and key factors. The evolution of permeability is strongly dependent on coal matrix swelling/shrinkage induced by gas adsorption/desorption, expansion by thermal effects, and compaction by effective stress. Reservoir permeability first decreases, then rebounds before continuously decreasing to low magnitude. Ignoring the impact of water migration overestimates CH4 production, and ignoring heat transfer underestimates. The high injection pressure and initial permeability will promote fluid mixture transport, resulting in an increase in production and sequestration; conversely, high injection temperature and water saturation will result in a decrease. Delaying injection start time is shown to counter the low average production rate and early CO2 breakthrough resulting from early injection (beginning at ∼2500 days for this case).