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.