Study on the Evaluation Method of CO2 Storage Suitability in Offshore Depleted Oil and Gas Reservoirs

Excessive carbon dioxide (CO2) emissions have contributed to global climate variability, resulting in a range of environmental challenges. As a pivotal technology for mitigating CO2 emissions, carbon capture, utilization, and storage (CCUS) plays a crucial role in achieving large-scale CO2 emission reductions. Geological storage of CO2 in depleted gas reservoirs offers several advantages, such as high storage capacity, excellent sealing properties, and well-established infrastructure. Consequently, depleted gas reservoirs are considered among the most promising candidates for long-term secure CO2 storage. Currently, the assessment of CO2 storage suitability and potential prediction predominantly focuses on the basin level, with insufficient integration of detailed analysis and evaluation at the block-specific scale. Based on the geological characteristics of the offshore A gas reservoir, a CO2 storage model was developed through numerical simulation. An evaluation index system for assessing the suitability of CO2 storage in depleted gas reservoirs was constructed using the Analytic Hierarchy Process (AHP). This system comprises three evaluation index layers—injectivity capacity, storage capacity, and storage safety—as well as ten specific evaluation indicators. The weights of each evaluation index were determined by employing the coefficient of variation method. The findings indicate that reservoir porosity, permeability, depth, area, thickness, pressure coefficient, geothermal gradient, and permeability aspect ratio are the critical factors influencing CO2 storage efficiency. The target gas reservoir achieved a CO2 storage suitability score of 5.22, which corresponds to an evaluation result of “suitable.” These research outcomes offer scientific guidance for evaluating the feasibility of offshore CO2 storage.