Evaluating an Area of Review for CO2 Sequestration and Storage: A Subsurface Modelling Workflow Tied to Regulatory Guidelines

Abstract

Abstract In United States, decarbonization of Carbon Dioxide (CO2) intensive industries is becoming a priority to meet the goals of the ‘Paris agreement’ which is to limit global warming to 1.5 °C. CO2 capture and sequestration (CCS) is an efficient technique to reduce anthropogenic carbon emissions and curtail the impact of climate change. CCS involves capture and permanent underground storage of CO2 which would otherwise be released into the atmosphere into deep saline or depleted hydrocarbon reservoirs. Selected Oil & Gas Exploration and Production companies with upstream expertise are exploring CCS to reduce the broad industrial CO2 footprint. Sequestration of CO2 in the subsurface requires robust techniques to be implemented to monitor the progression of the CO2 plume and the consequent increase in subsurface pressure. The regulatory authorities therefore require evaluation of an Area of Review (AoR) which is a project area that is under the influence of CO2 injection activities. AoR is defined as the project area in which elevated injection zone pressure may cause upward migration of formation fluids into the closest underground source of drinking water (USDW) through a potential geological or mechanical leakage pathway. Pursuant to the Underground Injection Control (UIC) program, US Environmental Protection Agency (EPA) regulates CO2 sequestration wells by the enforcing Class VI permits. Only five Class VI permit applications have currently been reviewed and approved by EPA and state regulatory authorities (USEPA 2023). In this fast-evolving CCS space, understanding key terminologies of the Class VI permit such as AoR is one of the challenges faced by the industry. The aim of this paper therefore is to explore the AoR concept as stated in the US Code of Federal regulations, provide an interpretation of key terminologies involved and demonstrate how a reservoir modelling workflow can be utilized to delineate an AoR. The study summarized by this paper employs a conceptual saline reservoir model developed with geocellular property and facies modelling that is representative of a certain US Gulf Coast area. This model is dynamically simulated by incorporating CO2 – rock interaction flow physics. Reservoir simulation provides predictions of subsurface movement of the CO2. These results are coupled with current guidelines provided by the regulatory authorities to evaluate the AoR as the CCS project progresses over time. Continuous evaluation of AoR provides a robust level of due diligence to prevent the CO2 plume from encountering geologic and mechanical leakage pathways to the surface or overlying sources of drinking water. This technique can also be utilized to prevent the CO2 saturation front from exceeding acreage boundaries that have been prescribed for CCS.