Theoretical Framework for Coupled Reservoir-Geomechanical Modelling of Fault Seal Integrity During Pressure Cycling in Gas Storage Operations in Nigerian Depleted Reservoirs

Underground natural gas storage in depleted oil and gas reservoirs is a strategic energy security option being evaluated for Nigeria, and the geomechanical integrity of fault seals during repeated pressure cycling associated with storage injection and withdrawal cycles must be theoretically characterised before commercial-scale operations can be safely deployed. This study develops a theoretical framework for coupled reservoir-geomechanical modelling of fault seal integrity during pressure cycling in storage operations in Nigerian depleted reservoirs. The theoretical framework couples Biot's poroelastic constitutive equations for porous reservoir and caprock deformation with a fault mechanics model based on the Mohr-Coulomb failure criterion modified for rate-and-state friction behaviour observed in shale fault zones. An original fault seal integrity index is derived as a function of effective normal stress, shear stress, and fault zone permeability, tracking the progressive damage accumulation across fault planes subjected to cyclic pressure loading. Analytical solutions are derived for a simplified planar fault geometry and validated against finite element solutions. Application of the theoretical framework to a hypothetical Nigerian depleted reservoir storage case calibrated with Niger Delta normal fault orientation data and in-situ stress conditions demonstrates that pressure cycling amplitudes below 1,200 psi maintain fault stability at 95 percent confidence, while amplitudes above 2,000 psi carry non-negligible fault reactivation risk even for optimally oriented faults. Maximum sustainable injection pressure limits are defined for three candidate storage reservoir configurations. Keywords: fault seal integrity, gas storage, geomechanical modelling, pressure cycling, Nigerian depleted reservoir.