Coupled adsorption-hydro-thermo-mechanical-chemical modeling for CO2 sequestration and well production during CO2-ECBM

Injecting CO2 into the methane-bearing coal seams can not only store CO2 but improve coalbed methane recovery. In this paper, a fully coupled adsorption-hydro-thermo-mechanical-chemical (AHTMC) model for CO2 enhanced coalbed methane (CO2-ECBM) is constructed, including the multiple fields with the coupling relationships of competitive adsorption of CO2 and CH4 in coal matrix (A), gas-water two-phase flow and gas diffusion in formation water (H), heat transfer and conversion (T), coal deformation (M), geochemical reactions of various anions and cations and the mineral dissolution in formation water (C). The model is verified by cored coal sample experiments in the laboratory and applied to simulate a CO2-ECBM pilot project in Qinshui Basin with different CO2 injection rates. The simulation results show that CH4 adsorption capacity decreases and CH4 production rate increases significantly with CO2 injection. When CO2 breaks through production well, the CO2 storage rate decreases and the pH in drainage decreases significantly. The CO2 injection rate has a significant influence on CO2-ECBM. Importantly, 25 t/d of CO2 injection is available for the pilot project with a CH4 recovery growth rate of 14.89% and a CO2 storage ratio exceeding 96% in a CO2 injection duration of 8000d.