Tetra Tech’s engineer, Grant Billings, explores why the size of the Area of Review is so important when planning and permitting a Class VI well for carbon capture and storage.
In Grant’s white paper, he explains the regulatory and technical context for Class VI wells—EPA’s permitting class for geological sequestration of CO2—and defines the AoR as the geographic zone around a CCS project where underground sources of drinking water (USDWs) could be affected by injection activities. It outlines why the AoR is a foundational element of the permitting process and summarizes the primary contamination pathways and water-quality risks (e.g., vertical leakage, pressure-driven brine migration, pH change and trace-constituent mobilization) that make robust AoR delineation critical for protecting USDWs.
The paper describes methods used to delineate the AoR, emphasizing the role of site characterization, geology and hydrogeology, and numerical modeling to simulate CO2 and displaced-fluid behavior across different phases and timescales; it emphasizes that AoR size materially affects the scope and cost of site characterization, corrective action, monitoring, and long-term stewardship. The document also interrogates the EPA-prescribed equations for AoR, showing how embedded assumptions (for example, freely transmissive pathways to USDWs) can produce widely varying AoR sizes and why selecting the smallest calculated AoR can increase operational, contractual, and reputational risk.
Finally, the white paper presents practical guidance for “right-sizing” the AoR: balancing regulatory expectations with project-specific geology, legacy infrastructure (e.g., orphan or abandoned wells), and lifecycle risk management. It outlines the implications of AoR decisions for corrective-action planning (including evaluation of artificial penetrations), monitoring strategies (4D seismic and monitoring wells), emergency and remedial response planning, and potential long-term liabilities—highlighting that alternative, site-specific approaches to AoR delineation have been accepted in permitting when well-supported by data and expert analysis.
Explore the AoR further. Learn about technical challenges and solutions in right-sizing the AoR.
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About the author
Grant Billings
Grant Billings is a professional engineer based in Texas specializing in water and carbon management.
Grant began his career as a field engineer for an independent oil and gas exploration and production company. He has many years of experience in facilities and production engineering and as a pipeline integrity engineer. He has successfully held both technical and operational roles.
Grant has been an active member of organizations such as the Society of Petroleum Engineers, Energy Water Initiative, and the SCOOP/STACK Water Consortium.