Regional storage capacity estimates: Prospectivity not statistics

AbstractOver the last decade, there has been significant scrutiny and criticism regarding the reliability and efficacy of values put forward as CO2geological storage capacity estimates. Initial estimates were unsophisticated, with little or no geological or technical components used in the assessments. Enormous numeric ranges were quoted, and reliance was placed on gross oversimplifications of both complex geological settings, as well as the physical limitations of the geological strata to accept and retain any CO2that might be injected and stored. More recent efforts have focused upon the need to determine better standards for making storage capacity estimates and to establish some uniformity in the estimation methods. As more emphasis has been placed on the approaches (formulas and algorithms) that various authors have utilized, less effort is being documented on the actual prospectivity of the rocks i.e. the geology. Unless the rocks at any given site are understood well enough then the level of uncertainty regarding their geological suitability for storage will never be low enough to allow financial investment and consequently geological storage will be unable to prove-up and deliver the outcomes that it promised a decade ago.The Queensland CO2Geological Storage Atlas assessed 36 sedimentary basins across the state of Queensland in Australia and during the assessment a methodology was developed (“CGSS methodology”) for regional storage capacity estimations. The CGSS methodology produces conservative regional storage volumetric estimates, that can be relied upon by policy decision makers to be highly likely to be available in a given sedimentary province, and which can be duplicated and revisited by other scientists and engineers, whilst also preserving the assumptions and decision processes. The CGSS methodology contrasts starkly with some existing approaches which deal with prospectivity by applying a Storage Efficiency (SE) factor at a whole of basin scale with limited specific depth, temperature, pressure, geological and geophysical information to guide the estimate or by use of generic assumptions (e.g. for CO2 density). If such an approach had been adopted in the Queensland Atlas assessment, it would have generated storage capacity values several orders of magnitude higher than the CGSS methodology. Back calculating a Storage Efficiency (SE) factor for the Queensland Atlas assessment, so as to get the same final numerical estimate produced with the CGSS methodology, results in a SE factor of ∼0.10-0.15% of the total basin pore volume. This contrasts with SE factors of ∼4% commonly applied when using the storage efficiency factor methodologies. Given the high level of technical detail that was used in the Queensland CO2 Geological Storage Atlas to arrive at a regional storage capacity estimate using the CGSS methodology, and the substantial disparity that a storage efficiency approach generates, it raises the question of how reliable the storage efficiency approach may actually be, especially where site specific or regionally representative geological parameters have not been used as a guide or constraint in an assessment