Hydrocarbon Mobilization Mechanisms Using CO2 in an Unconventional Oil Play

Abstract

Enhanced oil recovery (EOR) processes using CO2 in tight unconventional plays like the Bakken formation are expected to be very different from the processes which control EOR in conventional permeable reservoirs. During CO2 EOR in conventional reservoirs, CO2 flows through the permeable rock, while in the Bakken, CO2 flow will be dominated by fracture flow, and not significantly through the rock matrix. Fracture-dominated CO2 flow could essentially eliminate the "flushing" mechanisms responsible for increased recovery in conventional reservoirs. As a result, other mechanisms must be optimized in reservoirs such as the Bakken. To investigate this concept, rock samples from the middle Bakken (low permeability), lower Bakken (very low permeability), and a conventional reservoir (high permeability) were exposed to CO2 at Bakken conditions of 110 °C and 5000 psi (230 °F, 34.5 MPa) to determine the effects of CO2 exposure time on hydrocarbon production. Varying geometries of each rock ranging from small (mm) "chips" to 1 cm-diameter rods were exposed for up to one week, and mobilized hydrocarbons were collected for analysis. Nearly complete (>95%) hydrocarbon recovery occurs in hours with the more permeable middle Bakken and conventional reservoir rocks, while several days of exposure is required to achieve high recoveries from the lower and upper Bakken shales (1-cm diameter rods). Hydrocarbon recovery rates are greatly enhanced by higher rock surface areas, which supports the proposed conceptual model that CO2 EOR is dominated by fracture flow, followed by permeation into the Bakken rock with subsequent mobilization of the oil based on lowered viscosity, swelling and solubilization of oil hydrocarbons. These results also demonstrate that the micropores in even the very tight Bakken shales are accessible to CO2, and indicate that the shales may have substantial CO2 storage capacity.