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Travertine deposits present above the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (>400 kyr) history of surface leakage of CO2 from a subsurface reservoir. These deposits are concentrated along surface traces of faults, implying that there has been a structural control on the migration pathway of CO2 -rich fluids. Here, we combine slip tendency and fracture stability to analyse the geomechanical stability of the reservoir-bounding Coyote Wash Fault for three different stress fields and two interpreted fault rock types to predict areas with high leakage risks. We find that these areas coincide with the travertine deposits on the surface, indicating that high-permeability pathways as a result of critically stressed fracture networks exist in both a fault damage zone and around a fault tip. We conclude that these structural features control leakage. Importantly, we find that even without in situ stress field data, the known leakage points can be predicted using geomechanical analyses, despite the unconstrained tectonic setting. Whilst acquiring high-quality stress field data for secure subsurface CO2 or energy storage remains critical, we shown that a first-order assessment of leakage risks during site selection can be made with limited stress field knowledge. [ABSTRACT FROM AUTHOR]

Travertine deposits at the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (>400ka) history of surface leakage of CO2 from a subsurface reservoir. Travertine deposits are concentrated along surface traces of faults implying that there has been a structural control on the migration pathway of CO2 rich fluids. Here, for the first time, we combine slip tendency and fracture stability to analyse the geomechanical stability of the reservoir-bounding Coyote Wash Fault for three different stress fields and predict areas with high leakage risks. We find that these areas coincide with the travertine deposits on the surface indicating that high permeability pathways as a result of critically stressed fracture networks exist in both fault damage zones and around a fault tip. We conclude that these structural features control leakage. Importantly, we find that even without in-situ stress field data the known leakage points can be predicted using geomechanical analyses, despite the complex tectonic setting. Thus, even though acquiring high quality stress field data for secure subsurface CO2 or energy storage is a must, a first order assessment of leakage risks during site selection can be made with limited stress field knowledge. [ABSTRACT FROM AUTHOR]

Faults can act as barriers to fluid flow in sedimentary basins, hindering the migration of buoyant fluids in the subsurface, trapping them in reservoirs, and facilitating the build-up of vertical fluid columns. The maximum height of these columns is reliant on the retention potential of the sealing fault with regards to the trapped fluid. Several different approaches for the calculation of maximum supported column height exist for hydrocarbon systems. Here, we translate these approaches to the trapping of carbon dioxide by faults and assess the impact of uncertainties in (i) the wettability properties of the fault rock, (ii) fault rock composition, and (iii) reservoir depth on retention potential. As with hydrocarbon systems, uncertainties associated with the wettability of a CO2 –brine–fault rock system for a given reservoir have less of an impact on column heights than uncertainties of fault rock composition. In contrast to hydrocarbon systems, higher phyllosilicate entrainment into the fault rock may reduce the amount of carbon dioxide that can be securely retained due a preferred CO2 wettability of clay minerals. The wettability of the carbon dioxide system is highly sensitive to depth, with a large variation in possible column height predicted at 1000 and 2000 m of depth, which is the likely depth range for carbon storage sites. Our results show that if approaches developed for fault seals in hydrocarbon systems are translated, without modification, to carbon dioxide systems the capacity of carbon storage sites will be inaccurate and the predicted security of storage sites erroneous. [ABSTRACT FROM AUTHOR]