Numerical simulation of CO2-ECBM based on 3D heterogeneous geological model

CO2 geological sequestration-enhanced coalbed methane (CO2-ECBM) strengthens the development of coalbed methane (CBM) and realizes the stimulation of coalbed methane wells. At the same time, CO2 is stored in deep coal seams. This technology has some great development prospects. Relying on the demonstration project, numerical simulation based on geological modeling is still an important path for its technological development. Based on the analysis of the characteristics for the corner grid geological model and the geological modeling ability of COMSOL Multiphysics, a geological model that can clearly display the three-dimensional spatial morphology and heterogeneity distribution of gas content, adsorption time, porosity, permeability, elastic modulus and Poisson’s ratio of the No. 3 coal in the Shizhuang South Block of Qinshui Basin was established in COMSOL Multiphysics using the split-reconstruction method. On this basis, the numerical simulation of CO2-ECBM for controlling gas injection pressure was carried out in combination with the adsorption-hydraulic-thermal-mechanical-chemical (AHTMC) multi-field coupling mathematical model, and the impact of CO2 injection pressure on CH4 stimulation, CO2 storage and the geochemical environment of coal reservoir were analyzed. Moreover, two kinds of shut-in modes, namely, simultaneous shut-in and sequential shut-in, and four CO2-ECBM engineering objectives, namely, time saving, CH4 stimulation, CO2 storage and comprehensive consideration of the three, were proposed. In addition, the CO2 injection pressures were optimized according to the two shut-in modes with the four objectives. The results show that: ① Under the limit of 15-year and the injection pressure of 5.50−9.50 MPa for the CO2-ECBM project, with the increase of CO2 injection pressure, the CH4 production rate of CBM wells and the CO2 storage capacity of coal reservoir increase, with the maximum of 14.39% and 8.31×107 m3 respectively, the area of coal reservoir containing acid fissure water expands. The CO2 storage rate decreases due to the large output of CO2 from CBM wells with the increase of gas injection pressure, but remains above 95%. ② The heterogeneity of coal reservoirs has a great impact on the production of CBM wells. The higher the CH4 content and the greater the porosity and permeability of the coal reservoirs around the CBM wells, the more significant the CH4 stimulation effect. The stronger the CO2 adsorption capacity of the coal reservoirs surrounding CBM wells, the lower their CO2 production rate. ③ Under the simultaneous and sequential shut-in, CO2 injection pressure has a significant impact on the CO2 concentration and CH4 production rate of gas produced by well group, and the CH4 recovery of coal reservoir decreases with the increase of injection pressure at the end of CO2-ECBM project with 33.28% and 41.63% for 3.50 MPa, 18.90% and 28.03% for 6.50 MPa and 12.81% and 23.01% for 9.50 MPa, respectively. ④ The optimal injection pressure of CO2 for the engineering objectives of time saving, CH4 stimulation, CO2 storage and comprehensive consideration of above three are 8.05, 3.50, 6.35 and 6.25 MPa for the simultaneous shut-in, respectively, and 9.50, 9.50, 3.50 and 9.50 MPa for the sequential shut-in, respectively. ⑤ Compared with simultaneous shut-in, the sequential shut-in can achieve better CH4 stimulation and CO2 storage effects by extending the engineering time when using the optimal CO2 injection pressure.