Simulation of Light Hydrocarbon Migration in a Stacked Petroleum Reservoir at Teapot Dome, Wyoming, with Pressurization During Carbon Dioxide Sequestration

Abstract

The Teapot Dome oil field in the Powder River Basin of Wyoming has been proposed as a test site for experimentation with CO 2sequestration as part of enhanced oil recovery. Because of the solubility and reactivity of CO 2with formation waters, its migration will be attenuated. The increased pressure necessary for appreciable rates of gas injection will result in considerable potential for migration of CH 4and light paraffins.

Baseline measurements were made of gas microseepage at the Teapot Dome oil field with CH 4and light paraffin seepage being found, which is generally associated with faults (Klusman, 2005, 2006). Microbial oxidation of the hydrocarbons to CO 2in the unsaturated zone is occurring, which mostly prevents leakage of hydrocarbons into the atmosphere. The interpretation of the processes operating was supported by isotopic measurements.

A two-phase displacement model was used to simulate gas seepage to the surface at various pressures from the Tensleep, Second Wall Creek, and the Shannon formations to the surface. Hydrocarbon concentrations and fluxes at the top of the saturated zone were estimated, with fluxes increasing by a factor of seven for CH 4and a factor of three for n-C 4H 10by increasing reservoir pressure from hydrostatic to 1.4 × hydrostatic. Concentrations at the top of the saturated zone increased only slightly with increasing pressure.

Smaller changes in flux and hydrocarbon concentrations resulted in the pressurization of the shallower Second Wall Creek formation. The current underpressured condition of the Second Wall Creek allows for the significant attenuation of microseepage if the Tensleep formation was pressurized and used for CO 2sequestration. The system is dynamic, responding to rates of change in reservoir pressures, and responding to the periods of time that pressures are held. The very shallow Shannon reservoir is also currently underpressured, but modeling indicates that it cannot withstand repressurization without significant gas seepage.