As interest increases in the potential storage of CO2 in depleted hydrocarbon-bearing shales, the US-DOE-NETL developed a screening-level methodology for estimating the prospective storage resource of these formations. The majority of shale formations are considered to be impermeable layers that act as caprocks for oil and gas or CO2 storage reservoirs; however, a minority of shales could be considered for CO2 storage. The method is designed to effectively estimate a range of prospective CO2 storage resource given the uncertainties common to regional or basin-scale assessments used for site-screening. These estimates can be applied to inform high-level decision making related to carbon storage initiatives. Three criteria were used to identify shale formations for CO2 storage: 1) hydrocarbons must have been produced from the shale by horizontal drilling and high-volume hydraulic fracturing, 2) the portion of the shale being assessed must be at depths sufficient for CO2 to exist in a dense, typically supercritical or liquid, state, and 3) a suitable seal system must be present above the assessed shale to prevent unwanted upward migration of CO2 to surface or ground water. As with all resource assessments, an uncertainty in the estimate of the prospective storage resource in shale is a consequence of the lack of appropriate quantitative geologic data. This work describes the method that was developed for CO2 storage, discusses approaches to estimate storage efficiency, and identifies data gaps that need to be addressed to improve the geological and petrophysical characterization of shale suitable for carbon storage. Uncertainties include geologic variability in formation thickness, porosity, existing fluid content, organic content, and mineralogy. Knowing how these parameters are related will aid in improving the understanding of pore-to-reservoir scale behavior and provide improved estimates of prospective CO2 storage.
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