Geomechanical Assessment of Potential CO2 Storage Sites in the US Shallow Water Gulf Coast

Abstract

Abstract Carbon capture and storage (CCS) is an essential technology that will play a major role in transitioning toward net-zero carbon emissions. CCS is the only group of technologies that reduces emissions in key energy and industrial sectors directly but also reduces CO2 to balance emissions in sectors difficult to abate. Offshore CCS offers most of the CO2 storage opportunities to achieve the growth required in storage capacity beyond the feasible onshore storage. Geomechanical screening of potential CCS sites for safe and efficient CO2 injection in deep geological formations is still a substantial challenge, especially over large areas with hundreds of drilled structures and fields, such as the shallow water Gulf of Mexico (GOM). In this study, we collect existing geophysical and petrophysical logs, drilling data (e.g., mud weight, leak-off test), pore pressure (MDT) and temperature data from 123 wells to evaluate potential CO2 leakage via the caprock due to gas injection-induced fault reactivation or fracturing. For this, we develop pore pressure and 1D mechanical Earth models for six wells, strategically distributed throughout the study area with all the required data, and map Shear Stress Levels (SSL) and Pressure Rooms (PR) for all potential storage formations. We further develop stress polygons based on friction equilibrium and poroelastic failure criteria for gas injection-induced shear failure. We characterize the contemporary state of stress in GOM by normal faulting (NF) stress, which is consistent with the predominantly extensional regime in the western intraplate North America. Using the World Stress Map (WSM) database, we find a mean SH orientation of N98° (±47°) based on 79 borehole breakouts with an overall length of 1241 m in 23 offshore wells in GOM. Stress orientations are locally affected by salt bodies and faults. We observe a pore pressure transition from hydrostatic at a shallow depth (i.e., wide PR) to an extreme overpressure zone (i.e., narrow PR), which makes deep reservoirs more sensitive to tensile fracturing during CO2 injection. We show that SSL is less than 0.4 in all reservoirs and seals in the GOM area, and PR decreases northwest of the study area. Furthermore, we reveal that the critical pressure and temperature limits for shear failure are far beyond the PR limit. Hence, following the PR limit, gas injection-induced reservoir failure and fault reactivation is unlikely in the study area, providing confidence that caprock mechanical leakage is a low-risk issue for long-term CO2 storage.