A Geomechanical Approach to Pressure Front Delineation for Class VI Carbon Storage Projects in the Absence of an Overlying Underground Source of Drinking Water

The delineation of the Area of Review (AoR) is a fundamental requirement for Class VI carbon storage permits in the United States. The regulatory definition of the pressure front relies on the potential for injected fluids or formation brine to migrate into an Underground Source of Drinking Water (USDW). However, in deep sedimentary basins such as the Texas Gulf Coast and offshore regions, targeted saline formations often lack overlying USDWs. In these scenarios, traditional methods for calculating the critical pressure threshold become mathematically undefined or yield infinite AoR boundaries. This paper proposes a practical, geomechanics-based methodology for defining the pressure front in the absence of a USDW, framed as an alternative site-specific approach under the authority of the UIC Program Director (40 CFR 146.84). By leveraging existing regulatory limits on injection pressure, the proposed framework establishes a threshold based on the minimum horizontal stress, caprock fracture pressure, and fault reactivation limits via Mohr–Coulomb failure analysis. The framework further incorporates capillary breakthrough pressure as a third containment threshold, ensuring that the most restrictive condition governs the AoR boundary. A synthetic case study of a deep Gulf Coast saline formation demonstrates that this approach produces a finite, physically meaningful AoR that scales appropriately with injection operations (evaluated at 1.0 and 2.0 Mt/yr) and captures post-injection pressure evolution during the Post-Injection Site Care (PISC) period. Sensitivity analyses on permeability and fracture gradients confirm the robustness of the method. The study also examines model limitations, injection feasibility boundaries, and extensions toward a probabilistic framework. This framework provides operators and regulators with a defensible, regulatory-consistent pathway for advancing carbon storage projects in deep sedimentary basins, complete with a standardized reviewer checklist and an example AoR delineation report template.