Stress field orientation controls fault leakage at a natural CO<sub>2</sub> reservoir

Abstract

Abstract. Travertine deposits at the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (> 400 ka) 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.

Resolved stress analysis, failure mode, and fault-controlled fluid conduits in low-permeability strata

“…The clear relationships displayed in the dilation tendency versus slip tendency pattern, the failure and reactivation modes, and the mechanical layering, demonstrates the importance of understanding this interplay when investigating faultrelated permeability development. Unconventional hydrocarbon reservoirs and low-permeability seal strata for aquifers, oil and gas reservoirs, and CO 2 reservoirs or sequestration sites are commonly not lithologically homogeneous, but instead are 210 heterolithic and mechanically layered (e.g., Petrie et al, 2014;Miocic et al, 2020). Consequently, failure modes and failure orientations are likely to vary bed to bed and result in refracted fault shapes and fluid pathways similar to those discussed here.…”