Naturally Underpressured Compartments and Geologic Sequestration of Carbon Dioxide

James O. Puckette
Oklahoma State University, Stillwater, OK

Reservoir-pressure data from multiple sources were used to characterize the compartmentalized nature of the Pennsylvanian and Permian rocks of the Anadarko basin. Integrated pressure, production and wireline log data were used to delineate abnormally high- and low-pressured reservoirs and identify bounding seals. Abnormal overpressure is restricted to the deeper parts of the basin, whereas natural underpressure occurs across the northern shelf. Compartmentalized reservoirs offer a geologically unique opportunity for the subsurface sequestration of fluids. Abnormally underpressured reservoirs, by virtue of their shallow depth, were identified and mapped using available petroleum industry data. Case study reservoir compartments were selected and analyzed. The results indicate that they contain low pore-fluid pressures and are completely sealed by thick confining units. Pressures in these exemplar reservoirs are further reduced by the production of oil and gas. As a result they have low injection and displacement pressures. Volumetric calculations indicate these depleted oil and gas reservoirs can accept large volumes of injectate without exceeding original pre-production reservoir pressures. Estimated disposal volumes for these selected reservoirs range from approximately 0.5 million to 21 million stock tank barrels of liquid per well.

Compartmentalized reservoirs with abnormally low fluid pressures offer an intriguing alternative for CO2 sequestration. Seal longevity and integrity are evidenced by the intra-stratal isolation of compartments in the Pennsylvanian, which contains underpressured reservoirs in the Oklahoma Panhandle that have not equalized with extreme overpressures in the deep Anadarko basin. These reservoirs, by virtue of their compartmentalized nature, fulfill two critical criteria for CO2 sequestration, (1) non-migration and (2) isolation from the sphere of human activities.