U.S. Geological Survey Probabilistic Methodology for Assessment of Co2 Storage Capacity in Oil and Gas Fields and Saline Formations
The U.S. Geological Survey (USGS) has constructed a methodology to assess the CO2 storage capacity of geologic strata. This storage capacity methodology is based on a framework similar to the USGS probabilistic assessments of undiscovered oil and gas for basins of the United States. The storage capacity assessment methodology is primarily a volumetric calculation of pore space coupled with an estimate of ‘storage efficiency’, the percentage of pore space that would be occupied by free-phase CO2. In the storage capacity methodology, basins are divided into individual Storage Assessment Units (SAU) comprised of one or more storage formations, and a regional seal formation. Storage formations are a combination of known traps (KTs) and saline formations (SFs). KTs are primarily oil and gas reservoirs, in which the buoyant CO2 would be held in place by a structural or stratigraphic trap. SFs are the portion of the SAU between these known traps.
Aspects of this capacity assessment methodology which make it distinct include utilization of distribution functions for the best available data as input values. The data available for KTs allow for several types of storage capacity values to be determined, such as: (1) the amount of CO2 stored via enhanced oil recovery (EOR); (2) the amount of CO2 that can be stored relative to the net volume of fluid removed from the trap; and (3) the volumetric estimation of pore space using distribution of the areal extents, thicknesses, and porosities of known traps and storage efficiency values. The ultimate goal of the storage capacity assessment methodology is to determine the total trap volume beyond the petroleum-bearing volume, i.e. the “fill-to-spill” storage capacity. SFs are assessed using a cell-based method, where a cell size is fixed based on the specifics of the SAU. Distributions of the thickness, porosity, and storage efficiency values are used to determine the potential storage capacity. The storage distributions assessed in both KTs and SFs are reported as raw capacity and as conditional capacity, thus taking into account the probability (based on both geologic and anthropogenic factors) that the CO2 can be injected and contained within the SAU. The conditional storage capacity values represent the results with the greatest degree of geologic certainty based on current knowledge and available data.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009