Nationally there are 81 projects under regulatory review with a total of 246 wells. These projects and wells were located by owners and operators to make use of valuable pore space for long-term storage and to locate near large-scale emissions sources. As a result, projects are not evenly distributed across the United States; and this has focused attention on Class VI project interference.

Class VI projects are regulated under the Safe Drinking Water Act which is designed to protect underground sources of drinking water (USDWs). There are two mechanisms by which USDWs can be endangered by Class VI operations. The first is the upward migration of buoyant supercritical carbon dioxide into an overlying USDW. This upward migration could occur through improperly plugged artificial penetrations or flaws in the cap rock. The introduction of carbon dioxide into USDWs could result in changes in aquifer pH, precipitation or dissolution of solid mineral phases in USDW, and changes in the geochemistry which could mobilize naturally occurring contaminants, such as arsenic. The second mechanism is the upward migration of formation fluids induced by pressurization of the reservoir during injection. Formation fluids could migrate into overlying USDWs through flaws in the cap rock or improperly plugged artificial penetrations. Formation fluids are typically saline and can contain trace elements with concentrations greater than safe drinking water maximum contaminant levels.

The size of a Class VI project is characterized by the Area of Review (AoR). The AoR is the superposition of the extent of the supercritical plume (designed to guard against the first mechanism of contamination) and the extent of the pressure front sufficient to elevate native formation fluids into the USDW (designed to guard against the second mechanism). Because of the relative incompressibility of formation fluids, the pressure front associated with a Class VI sequestration project can propagate over several townships.

What will happen to your sequestration project if another project is located close enough to raise the potential for project interference? USEPA will make a preliminary technical determination of the potential for project interference. If this determination suggests that supercritical carbon dioxide plumes or pressure fronts from nearby projects have the potential to interfere, they will issue a Request for Additional Information (RAI) regarding project interference.

USEPA guidance (https://www.epa.gov/sites/default/files/2015-07/documents/epa816r13005.pdf) states that “In all cases, EPA recommends that AoR delineation models account for all wells injecting into (including any injection wells associated with other UIC well classes or other Class VI operations) or pumping from the injection zone or any other zones that are hydraulically connected to the injection zone.” The applicants will be provided well locations and operational details of the potentially interfering projects (even if they were originally claimed as confidential business information). Each applicant will be asked to revise their AoR models to determine whether their AoR is affected by nearby projects. This process can be difficult to navigate if the original AoR was not designed with a sufficiently sized domain or a suitably refined mesh. SCS Engineers use the GEOS computational model and advanced meshing software packages that have proven useful in expediting the response to RAIs involving project interference.

Monte Markley, P.G. is the SCS Engineers’ Practice Lead for Carbon Sequestration and Charles Hostetler (Ph.D.) is the SCS Engineers’ Subject Matter Expert for dynamic AoR modeling.

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