CO2 flooding has been successfully implemented in conventional reservoirs for enhanced oil recovery for more than 40 years (Brock and Bryan, 1989). The main advantage of using CO2 flooding is the injected CO2 can mix with the oil to swell it, make it lighter, detach it from the rock surfaces, and cause the oil to flow more freely within the reservoir so that it can be ‘swept up’ in the flow paths from injector to production well. Depending on the reservoir pressure, the injected CO2 can be fully or partially miscible with oil. Laboratory test showed that CO2 flooding can achieve an oil recovery of 90% OOIP if fully miscible condition is approached. However, its application in tight reservoir (k<0.1 md) is very challenged because of a poor CO2 injectivity which reduces viscous driving force significantly. In this paper, we will present a renovation CO2 EOR technology which focuses on using huff and puff CO2 injection at high pressure to improve oil recovery from fractured tight reservoir. We attempted to understand if the oil recovery is mainly relied on CO2 diffusion from fractures into matrix.
A special designed high pressure CO2 injection system is developed which allows us to conduct CO2 injection test at a pressure up to 5000 psia and a temperature up to 300°F. This system is equipped with a high resolution (∼50 (μm) industrial CT-scanner and titanium coreholder so we can image the core to capture in-situ oil saturation change versus time during an CO2 injection test. Core samples from candidate reservoirs with a permeability of 0.01 to 1 md were used for the tight rock sample. Dead reservoir crude (API=46, viscosity=2.4 cp at room temperature) and mineral oil (API=42 and viscosity=0.9–1.5 cp at room temperature) were used as oil phase. Supercritical CO2 at a pressure of 2400 psia and temperature of 160°F was used as the gas phase. An artificial fracture was induced for injection of supercritical CO2. Supercritical CO2 can only enter the matrix (tight core) by diffusion since it is only injected through fracture which is at the top of the tight core and the bottom end of the core is closed (no viscous force is applied to the tight rock).
Our experimental results revealed that huff and puff or cyclic injection of supercritical CO2 through the fracture of tight rock is an effective method for improved oil recovery (IOR). Within less than 10 days, the oil recovery can be as high as 90% OOIP. We also found that the oil production rate increases with increase in rock permeability, from 0.01 to 1 md, indicating the CO2 diffusion rate from fracture to matrix is function of permeability at a constant pressure and temperature. Besides, oil production rate also increases with reduction of oil viscosity. Comparing to continuous injection of CO2 through the fracture, huff and puff injection process provides a better oil recovery than a continuous injection process. CT scan images also validated that CO2 diffusion from fracture into matrix is the main recovery mechanism. CT images indicate that there is no clear CO2 injection front and oil and diffusion style of CO2 injection is observed.