Area of Review in CO2 Sequestration Projects Defined for Class VI Compliance

Abstract This paper describes a comprehensive subsurface work guide to define the Area of Review (AoR), an important component for Carbon Capture and Storage (CCS) projects to obtain the US Environmental Protection Agency (EPA) Class VI permit. Accurately understanding and assessing the AoR helps to avoid risking the underground sources of drinking water (USDW) from potential contamination with CO2. Delineating the AoR also prevents CO2 lateral leakage outside the allocated land lease, resulting in secure regulatory compliance. In this study, a 3D numerical model that represents saline aquifer in the Texas Gulf Coast area is developed using available geological geophysical and well logs in the area. We examined different development scenarios for sequestrating CO2 for 20 years, including multiple injections and pressure management strategies through aquifer water production, to determine the extent to which the CO2 and pressure plumes circulate over time. We conducted a sensitivity analysis to assess the impact of various subsurface conditions and design parameters on the movement of the plume and, thus, AoR determination. Results showed that although the four scenarios considered could achieve the target of storing up to 33 million metric tonnes (MMT) of CO2, the AoR definition varied depending on the development plan. The work aimed to minimize the AoR as soon as possible, with the completion design used in addition to brine production from the aquifer. To avoid the potential hazard of the AoR extending to the risky legacy well, we proposed several mitigation plans, including dividing injection into two injectors at different locations within the project site and discovering opportunities to produce brine to manage the pressure buildup. The best scenario was selected, and it reflected the advantage of combining various strategies optimized well placement, water production through two producers and injecting using the upper hole recompletion method. Beyond pressure and plume control, the study investigated the impact of the trapping mechanisms on the redefinition of the AoR and thus enhanced long-term containment and security. This presented work, which incorporates analytical and numerical approaches, guides the evaluation of CCS projects established in challenging sites with (1) a high density of existing wellbores, (2) geological uncertainty, and (3) potential environmental risk. To the best of our knowledge, this is the first comprehensive work that targets AoR definition while considering strategies of pressure management and optimized completion design to balance containment and operational feasibility, potentially leading to approved Class VI permits and ultimately successful CCS projects.