Numerical simulation of CO2 storage with enhanced gas recovery in depleted highly heterogeneous carbonate gas reservoir

Injecting CO2 into depleted gas reservoirs facilitates CO2 storage and enhances natural gas recovery rate (CSEGR), offering significant environmental benefits. Carbonate gas reservoirs are widely distributed globally and are characterized by large reservoir thicknesses, abundant reserves, poor reservoir physical properties, and high heterogeneity. In the depletion state, considerable amounts of natural gas remain in the matrix pores, and numerous natural cavities provide ample space for CO2 storage. Consequently, carbonate gas reservoirs hold great potential for implementing the CSEGR program. To accurately model the flow behavior of CO2 in highly heterogeneous carbonate gas reservoirs and assess the feasibility of implementation the CSEGR program, a three-dimensional numerical model was developed at the reservoir scale using geological data from the fourth section of the Dengying Formation (Deng-4) in the Anyue Gas Field. The study investigates the CH4 production and CO2 storage characteristics of four typical reservoir bodies, tracks the spatial migration of CO2, and explores the effects of CO2 injection rate and interlayer permeability on the CSEGR process. Numerical results show that the cavity-type reservoir is ideal for the CSEGR scheme, with an enhanced gas recovery (EGR) rate exceeding 20%, whereas the fracture-cavity reservoir is unsuitable due to premature CO2 breakthrough caused by natural fractures. The cavity-type reservoir serves as the primary site for CO2 injection and storage, with some CO2 migrating into the pore-type reservoir through interlayer crossflow. For both pore-type and cavity-type reservoirs, the migration distance of the CO2 front is proportional to the square root of the injection time. Lower CO2 injection rates and smaller interlayer permeabilities delay CO2 breakthrough, leading to higher EGR rates. However, reduced injection rates also lower CH4 production rates and extend CO2 injection duration.