Reservoir Pressure and Temperature Regimes in Sedimentary Basins and the Geologic Carbon-Dioxide Sequestration Resource

The resource potential of geologic carbon-dioxide (CO₂) sequestration in a basin is impacted by its subsurface pressure and temperature (P/T) regime. As part of its probabilistic assessment of domestic CO₂ sequestration, the U.S. Geological Survey is estimating residual and buoyant trapping volumes in large sedimentary basins. Residual trapping of super-critical CO₂ is a larger resource whereas buoyant trapping is a resource more easily quantified based structural and stratigraphic closures. When buoyantly trapped in brine formations, CO₂ should be sequestered by top and lateral seals with the resource size dependent on column height. Column height is a complex function of (1) the density difference between the reservoir brine and super-critical CO₂, (2) the contact angle between the brine and CO₂, (3) the interfacial tension, and (4) the pore throat size of the seal rock. Our prior investigations have shown that column height is largely sensitive to the brine-CO₂ density difference when compared to other parameters. Additionally, brine and CO₂ density are unique functions of subsurface P/T variation. Previous studies suggest that temperature affects CO₂ density more directly than pressure; and in cool basins the optimum P/T regime for super-critical CO₂ sequestration occurs shallower than in warm basins. Consequently, to predict brine and CO₂ density changes in our assessed sedimentary basins, we cross-plot the P/T regimes based on oil and gas operator data. The reported reservoir P/T data is grouped regionally and thus helps to characterize a basin as either warm or cool. This basin distribution agrees with a published geothermal gradient map of North America. Based on their tectonic environment and reservoir data, example cool basins for sequestration are located in west Texas, and include the Illinois and Michigan Basins; example warm basins are located along the California coast and in the northern Rocky Mountains. This investigation shows that for higher reservoir P/T regimes, the density difference between brine and CO₂ decreases. A smaller density difference results in higher columns, and thus allows for a larger buoyant storage resource. Ultimately, reported oil and gas reservoir P/T data should be invaluable to improving our assessment of CO₂ storage potential in geologic basins.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California