Improving oil recovery of Eagle Ford shale samples using cryogenic and cyclic gas injection methods - An experimental study

Despite containing massive volumes of hydrocarbons, unconventional reservoirs have low recovery factors, <10% in oil reservoirs. Production from such ultra-low-permeable formations mainly depends on the flow conductivity. Any method enhances flow conductivity improves hydrocarbon-production from these reservoirs. In this study, implementing cryogenic and cyclic gas injection to improve oil recovery was investigated. Liquid nitrogen (LN₂) was injected into saturated core samples from Eagle Ford formation at 82 °C reservoir temperature. Then, three cycles of nitrogen (N₂) injection at four pressures (6.89, 13.79, 20.68, 27.58 MPa) were conducted, and oil recovery factor (RF) was calculated for up to 24 h of production period after each cycle. LN₂ and cyclic N₂ injection effects on core samples were assessed using computerized-tomography scan, porosity, permeability, and ultrasonic-velocity measurements. The results revealed that implementing LN₂ and cyclic N₂ techniques improves shale oil RF by up to 78%. Implementing LN₂ and cycling N₂ resulted in initiating new cracks and extending them inside the samples providing high-flow channels for the injected gas, promoting the oil and gas exchange through injection and production stages and utilizing pressure near minimum miscibility pressure yielding higher oil RF. Samples’ porosity, permeability, and velocities were altered after completing the treatments. Porosity increased by up to 60% and permeability significantly enhanced up to 2.4 orders of magnitude. Whilst P- and S-wave velocities reduced by up to 10% and 8%, respectively. Implementing LN₂ and cyclic N₂ could potentially be employed in shale oil fields to improve the oil recovery.