Geomechanical risk and mechanism analysis of CO2 sequestration in unconventional coal seams and shale gas reservoirs

With global greenhouse gas emissions hitting record highs in 2021, CO₂ geological sequestration (CGS) is the most realistic and feasible technology to ensure large-scale carbon reduction to achieve global carbon capping and carbon neutrality goals. Both coalbed methane and shale gas have the characteristics of self-generation and self-storage, which is considered to be a valuable target reservoir for geological sequestration of CO₂. After a high volume of CO₂ is injected into unconventional coal seams and shale gas reservoirs, many geomechanical issues may be induced, resulting in leakage. Therefore, it is crucial to evaluate the geomechanical risks of CO₂ geological sequestration. In this article, global CO₂ emissions and geological resources available for sequestration are teased out. The effects of coupled CO₂-water-rock-driven geomechanical, geophysical, and geochemical interactions on the evolution of rock physical properties and pore characteristics, as well as caprock sealing, are discussed. The caprock failure and its inducing mechanism are analyzed, and the criteria for predicting the occurrence of risk are summarized, which is necessary for pressure management and risk prevention. To serve as a benchmark for CO₂ sequestration in unconventional coal seams and shale gas reservoirs.