3D seismic-based structural and slip tendency analysis of a depleted reservoir offshore South Africa: Implications for safe CO2 storage

Abstract

Carbon capture and storage (CCS) technology is increasingly recognised as a key enabler of a low-carbon energy future, with growing importance for reducing anthropogenic CO2 emissions in resource-rich countries such as South Africa. The long-term success of CCS depends on the availability of secure storage capacity and a robust geomechanical understanding of injection sites, particularly in structurally complex, depleted hydrocarbon reservoirs. This study presents one of the first reservoir-scale integrated deterministic and probabilistic assessments of fault slip potential (FSP) applied to a depleted offshore gas reservoir in the Bredasdorp Basin. A high-resolution 3D seismic dataset was interpreted to characterise the fault network and construct a structural framework comprising fifty (50) selected faults within a syn-rift sandstone reservoir. This framework underpins a combined deterministic geomechanical and probabilistic sensitivity-modelling workflow to evaluate fault reactivation risk during CO₂ injection, using a simplified radial pressure-diffusion formulation.Results show that faults proximal to injection wells exhibit elevated slip potential due to their orientation relative to the regional stress field and their spatial association with pressure build-up. Sensitivity analyses indicate that pore-pressure increase is the dominant control on slip potential, followed by fault friction coefficient (µ), fault strike, and stress-field uncertainty. Variations in permeability primarily affect the magnitude and persistence of pressure build-up, while variations in μ govern slip-onset thresholds. Faults within ∼5 km of injection wells are most susceptible to reactivation. Time-dependent modelling highlights elevated risk during late-stage injection and early post-injection pressure redistribution, underscoring the need for continuous monitoring and adaptive pressure management. Overall, the study provides a rigorous, fault-specific geomechanical framework for CCS operations, and emphasises the importance of explicitly incorporating uncertainty into CCS risk assessments.