(457d) Illinois Basin Experiences Estimating the Area-of-Review for Industrial-Scale Carbon Capture and Storage Projects

One of the key requirements for obtaining a US Environmental Protection Agency’s (EPA) Underground Injection Control (UIC) Class VI permit to construct CO₂ injection wells is to estimate the Area-of-Review (AoR), which is “delineated using computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream and displaced fluids, and is based on available site characterization, monitoring, and operational data.” AoR delineation is required to ensure the project protects Underground Sources of Drinking Water (USDW) during injection operations. The AoR of a carbon dioxide (CO₂) storage project is either the maximum areal extent of the CO₂ plume, a threshold pressure front during the lifetime of a storage project, or a combination of these. The CO₂ plume extent is estimated based on CO₂ saturation within the injection reservoir using a predefined saturation cutoff. The threshold pressure front is defined by the minimum pressure increase required to initiate fluid flow from the injection zone into the lowermost USDW through a notional conduit. The CO₂ plume extent and threshold pressure front in storage projects are estimated from dynamic reservoir simulation of the injection and post-injection phases, which uses the conceptual and static reservoir models developed during site characterization as inputs.

Determining the threshold pressure front requires the calculation of a threshold differential pressure using measured (1) depths, (2) pressures, and (3) fluid densities at the lowermost USDW and the injection zone. The threshold differential pressure, the difference between the threshold pressure and the initial pressure in the injection zone, can be positive, zero, or negative, corresponding to an injection reservoir that is under-pressurized, at hydrostatic equilibrium, or over-pressurized relative to the USDW. An under-pressurized injection reservoir can accommodate a pressure increase equivalent to the positive threshold differential pressure without the potential migration of fluids to the USDW. For an injection reservoir at hydrostatic equilibrium, migration of fluids to the USDW is likely during CO₂ injection. On the other hand, the migration of fluids to the USDW from an over-pressurized reservoir is likely even without CO₂ injection.

In this study, we use US Department of Energy-sponsored CarbonSAFE project experiences involving the characterization of several storage projects in the Illinois Basin to demonstrate the challenges in determining the AoR. Uncertainty of in-situ fluid density propagates to AoR delineation: the accuracy of in-situ fluid density measurements can be low due to (1) challenges in obtaining representative fluid samples or (2) the use of a salinity-density correlation from the technical literature when a fluid sample is unavailable. The threshold pressure calculation is highly sensitive to fluid density; therefore, these challenges can have a substantial impact on the calculated threshold pressure. Because the areal extent of the pressure front decreases exponentially with the threshold differential pressure, the pressure front extent is sensitive to changes in threshold differential pressure when threshold differential pressure is low.