A multi-tiered approach for carbon sequestration Area of Review delineation

Abstract:

Geologic carbon sequestration through injection of supercritical carbon dioxide (CO₂) into the subsurface is one option for climate change mitigation through reduction of anthropogenic CO₂ emissions. Deep saline formations are suitable targets for CO₂ sequestration due to their large storage capacity, high injectivity, and favorable pressure and temperature regimes. The injection-induced migration of both CO₂ and resident formation fluids (brine) may pose a risk to overlying underground sources of drinking water (USDW). The US Environmental Protection Agency (EPA) uses the Area of Review (AoR) concept to delineate portions of a USDW that are susceptible to degradation from brine and/or CO₂. The AoR is a combination of the maximum CO₂ footprint and the area where the injection induced over-pressure can lift brine from the injection formation through an open conduit to a USDW. A sequestration operator needs to conduct site characterization, monitoring, and corrective action within the AoR. Although the driving forces for CO₂ and brine leakage are different, the regulatory requirements are the same, regardless if CO₂ or brine leakage is the concern. In addition, the AoR delineation assumes that leakage occurs along an open conduit instead of through fractures along the wellbore. The open-conduit assumption gives un-necessary conservatism and can lead to very large AoRs. In this presentation we discuss a risk-based alternative AoR approach where the AoR is divided into three tiers: the footprint of the CO₂ plume, the area where the elevated pressure is sufficient to cause significant flow through fractures along boreholes, and the area where the elevated pressure can lift brine through an open conduit to a USDW. As each of the three tiers would have a different level of regulatory requirements, the cost of compliancy should be reduced when compared to the current AoR delineation, although the overall size of the AoR does not change.