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)过程中，煤储层中发育的构造裂隙在注入压力诱导作用下可能成为气−水渗流的优势通道，进而影响煤层气增产与CO2封存效果。基于理论分析建立了煤储层CO2驱替运移剖面，构建了CO2驱煤层气地质封存数值模型，探讨了构造裂隙优势渗流通道对CH4增产和CO2封存的影响规律，评价了试验井组注CO2驱煤层气地质封存效果，揭示了注入试验中CO2快速突破机制，剖析了CO2快速突破对注采工艺优化的启示内涵，并提出了构造裂隙发育煤储层注采工艺建议。研究结果表明，注−采井间可依次形成CO2富集区、CH4−CO2混合区、CH4富集区和裂隙水富集区。随注入进行，各富集区形成、演化并接续影响采气井，其中裂隙水运移难易决定了产能抑制强度，进而控制了采气井增产效果和储层封存效果。沿构造裂隙采气井能实现CH4高效增产，但裂隙渗透率偏大往往导致CO2窜流进而引起CH4累计产气量与CO2累计封存量降低。项目期内CO2注入对强突破井具有显著的增产效果，实际累计产气量提高10.4%，流压等效累计产气量提高92.3%。构造裂隙为CO2快速突破提供了潜在优势通道，注压是诱导其成为优势渗流通道的主要原因，储层压裂改造区的存在和液态CO2相变膨胀增能可能加速CO2运移突破，而煤基质膨胀降渗效应有望延缓突破。为达到更好的驱替封存效果，需要在前期快速提升井底压力，在产能抑制阶段微调注压或增加采气井排采强度，全阶段维持注入井井底压力低于储层最小主应力。突破后，通过不定期关启采气井或降低注压可增加CO2与煤基质接触时间，有望进一步提高CH4采收率。

In the process of CO2-ECBM, the tectonic fractures developed in the coal may become the preferential channels of gas-water seepage under the impact of injection pressure, and then influence the recovery of coalbed methane (CBM) and CO2 storage. Based on the theoretical analysis, the displacing profile of CO2-ECBM is established. A numerical model is constructed referring to the TS-634 field trial, and the influence of the tectonic fractures on CH4 recovery and CO2 storage is investigated. The CBM recovery and CO2 storage in the TS-634 trial are evaluated, the mechanism of the rapid breakthrough of CO2 is revealed, the enlightenment of the rapid breakthrough on the optimization of the injection-drainage process is dissected, and the proposals for the injection-drainage process in reservoirs with tectonic fractures are put forward. The results show that CO2 enriched zone, CH4-CO2 mixed zone, CH4 enriched zone, and water enriched zone can be formed among injection-production wells successively. With an injection, the zones affect the producers after their formation and evolution. The difficulty of water seepage determines the level of CH4 output inhibition and then controls the CBM recovery and CO2 storage. The producer along tectonic fracture can achieve an efficient recovery of CH4, but a high permeability often leads to apparent CO2 channeling which causes the loss of cumulative CH4 production and CO2 storage. During the trial period, the strong-breakthrough producer attains a remarkable increase in gas output, with the actual cumulative gas production increasing by 10.4% and bottomhole-pressure-equivalent cumulative gas output increasing by 92.3%. Tectonic fractures provide potential preferential channels for the CO2 rapid breakthrough and become real preferential channels mainly induced by injection pressure. The existence of fracturing affected areas and liquid CO2 phase transformation-induced expansion and energization accelerate the CO2 breakthrough, but the coal matrix expansion-induced permeability decrease is expected to delay the breakthrough. To achieve a better displacement, enhancement, and sequestration, it is necessary to boost the bottomhole pressure in the early injection stage, fine-tune the pressure of the injector or increase the bottomhole pressure drop of producers in the inhibition stage, and maintain the pressure lower than the minimum principal stress of the reservoir in the whole stage. After the breakthrough, the exposure time between CO2 and coal matrix can be increased by irregularly shutting down/restarting the producers or reducing the pressure of the injector, which is expected to further improve the CH4 recovery.

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)

王梓良，桑树勋，周效志，等. 构造裂隙对CO2驱煤层气地质封存的影响[J]. 煤炭学报，2023，48(8)：3151−3161.

WANG Ziliang，SANG Shuxun，ZHOU Xiaozhi，et al. Influence of tectonic fractures on CO2 storage and enhanced CH4 recovery[J]. Journal of China Coal Society，2023，48(8)：3151−3161.