Gas Injection for Enhanced Oil Recovery in Organic Rich Shale Reservoirs

Oil production from organic rich shale has grown significantly in the last decade due to the combination of hydraulic fracturing and horizontal drilling. Yet, this primary recovery exploitation scheme exhibits fast production rate decline and low ultimate recovery. Gas injection, mainly COv2 injection, has been successfully applied for EOR in conventional reservoirs, making it attractive for implementation in shales. However, organic rich shale exhibits poor fluid transport, and contains kerogen rich in micropores and mesopores. This causes fundamental differences in the storage, transport and phase behavior mechanisms. This work investigates how these differences affect the implementation of gas injection in shale reservoirs, and the benefits we can obtain from it, in terms of oil recovery. Laboratory equipment simulating gas injection through a hydraulic fracture was designed, and coupled to a CT-scanner to track compositional changes with time. We performed 23 core-flooding experiments using shale sidewall cores that were either used as received, or re-saturated in the laboratory; and Berea sandstone. Continuous injection and huff-and-puff were compared at different pressures. The injection gases were COv2 or nitrogen. We provide a comprehensive procedure for sample preparation involving the measurement of fluid and rock properties. CO2 injection in organic rich shale resulted in a maximum recovery factor of 40%. Most of the oil production occurred in the first 24 hours. The main production mechanism is a peripheral slow-kinetics vaporizing gas drive. Oil is not displaced, instead, it is vaporized by the gas contained in the fracture, and transported outside the reservoir where it is condensed. Vaporization occurs from the periphery of the matrix, where the injection gas is stored. Mass transfer is slow due to the poor transport through the shale matrix. Therefore, recovery depends on the fraction of hydrocarbon components the injection gas can vaporize at the prevailing conditions of pressure and temperature, the concentration of those hydrocarbons in the reservoir oil, and the time allowed for mass transfer. COv2 was better than Nv2. Increasing pressure beyond MMP continued to increase recovery, and at a constant pressure, a huff-and-puff injection scheme was more effective than continuous gas injection