Unconventional liquid reservoirs are characterized by small matrix permeability that is several orders of magnitude lower than conventional oil reservoirs. The combination of multi-stage hydraulic fracturing and horizontal drilling has improved the overall profitability of these tight-oil reservoirs by enhancing the wellbore - matrix connectivity. Under primary production, however, the recovery factor remains in the range of only 5% to 10% of original oil in place (OOIP). Considering such a large resource base, even small improvements in productivity could lead to millions of barrels of additional oil. Therefore, the need to develop a viable enhanced oil recovery technique for unconventional oil reservoirs is evident.
This study investigates technical feasibility of carbon dioxide as an enhanced oil recovery agent for tight-oil reservoirs. Above minimum miscibility pressure (MMP), CO2 and oil are miscible leading to reduction in capillary forces and therefore high local displacement efficiency. The miscibility pressure of CO2 is also significantly lower than the pressure required for other gases, which makes CO2 miscible injection attainable under a broad spectrum of reservoir pressures.
The coreflood experiments recovered more than 70% of the OOIP from a Bakken core sample with an average porosity of 7.5% and permeability of 1.8 μd. CT scans at dual energies were used as an additional tool to visualize fluid flow and distribution at core level. We discovered that the impact of CO2 penetration is better captured at a lower energy level where the X-ray attenuation mechanism of photoelectric absorption becomes dominant. To decipher the oil recovery mechanisms in the coreflood experiment, a numerical compositional model was constructed to reproduce the laboratory results. Vaporization of light hydrocarbon components into CO2 is shown as a major recovery mechanism. Other controlling factors include re-pressurization, oil swelling, viscosity and interfacial tension reduction. History matching with the laboratory experiment introduces additional complexities such as rock heterogeneities and presence of a fracture that promotes flow perpendicular to the core length. The above issues need to be addressed to match the displacement process exactly.