School of Resources and Geosciences, China University of Mining and Technology
Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining and Technology
Carbon Neutrality Institute, China University of Mining and Technology
Jiangsu Key Laboratory of Coal-based Greenhouse Gas Control and Utilization, China University of Mining and Technology
China United Coalbed Methane Corporation Ltd.

CO2驱煤层气地质封存(CO2-ECBM)强化煤层气开发、实现煤层气井增产的同时，将CO2封存到深部煤层中，该技术极具发展前景。依托示范工程，基于地质建模开展数值模拟研究，仍是技术研发的重要路径。基于对角点地质模型和COMSOL Multiphysics地质建模能力的分析，利用拆分−重构法在COMSOL Multiphysics内建立了能有效描述沁水盆地柿庄南区块3号煤储层的三维空间展布和含气量、吸附时间、孔隙度、渗透率、弹性模量、泊松比非均质分布的地质模型。在此基础上，结合吸附−渗流−温度−应力−化学(AHTMC)多场耦合数学模型开展了控压注气方式的CO2-ECBM数值模拟工作，分析了CO2注入压力对CH4增产、CO2封存及煤储层地球化学环境的影响，提出了同时关井、序贯关井2种关井方式和省时提效、CH4增产、CO2封存以及3者综合兼顾4种CO2-ECBM工程目标，优化了2种关井方式、分别实现4种工程目标场景下的CO2注入压力。研究结果表明：① 在15 a工期、5.5～9.5 MPa注气压力情况下，随着注入井CO2注气压力的增加，生产井CH4增产率、煤储层CO2封存量均增加，最大可分别达14.39%和8.31×107 m3，含酸性裂隙水的煤储层区域扩大，受CO2从生产井大量产出的影响，储层CO2封存率降低，但保持在95%以上；② 煤储层非均质性对煤层气井生产影响很大，煤层气井周围煤储层CH4含气量越高、孔隙度和渗透率越大，CH4增产效果越显著，周围煤储层吸附CO2能力越强，CO2产出速度越小；③ 同时关井和序贯关井方式下，CO2注气压力对生产井组产出气CO2体积分数和CH4产气速度变化有显著影响，CO2-ECBM工程结束时煤储层CH4采收率随注气压力增加而降低，3.50、6.50、9.50 MPa的注气压力下分别为33.28%、18.90%、12.81%和41.63%、29.03%、23.01%；④ 以省时提效、CH4增产、CO2封存及3者综合兼顾为工程目标的CO2最优注入压力在同时关井时分别为8.05、3.50、6.35和6.25 MPa，在序贯关井时分别9.50、9.50、3.50和9.50 MPa；⑤ 相较于同时关井，序贯关井在采用最优CO2注气压力时能够通过延长工程时间获得较好的CH4增产和CO2封存效果。

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.

国家自然科学基金碳中和专项资助项目(42141012)；国家重点研发计划资助项目(2018YFB0605600)

刘旭东，桑树勋，周效志，等. 基于煤储层三维非均质地质模型的CO2-ECBM数值模拟研究[J]. 煤炭学报，2023，48(7)：2773−2790.

LIU Xudong，SANG Shuxun，ZHOU Xiaozhi，et al. Numerical simulation of CO2-ECBM based on 3D heterogeneous geological model[J]. Journal of China Coal Society，2023，48(7)：2773−2790.