Recalibration of CO₂ storage in shale: prospective and contingent storage resources, and capacity

Storing CO₂ in shale formations is an essential complement to the resources of geological CO₂ sequestration and the achievement of carbon neutrality. However, previous estimations have reported significant discrepancies in estimating CO₂ storage resources. To rationalize and address the broad range of these resource estimates, we introduce a categorization framework inspired by the SRMS (CO₂ Storage Resources Management System). Thus, we reclassify estimates according to the categories of prospective storage resources, contingent storage resources, and capacity estimates for CO₂ storage in shales, in rank order considering the decreasing challenge associated with their attainment (such as energy/pressure requirements and time-consumed in injection). Classical volumetric and production-based methods are employed for assessing prospective and contingent storage resources, respectively. The capacity is estimated by analyzing the historical records of hydraulic fracturing, focusing on the dominant role of fractures in the storage process. A significant disparity (two to three orders of magnitude) is revealed between capacity and contingent or prospective storage resources, which aligns with known challenges encountered during field injections. This disparity highlights the importance of further efforts and advanced techniques to secure CO₂ injection in fields and recalibrate the geological CO₂ inventories to achieve carbon neutrality.