Nuclear magnetic resonance study on enhanced shale oil recovery by CO₂ injection: A case study

The successful development of shale oil is significant to provide oil steadily. The comparisons of enhanced shale oil recovery by CO2 flooding and huff-n-puff methods for shale cores without and with fractures were investigated by nuclear magnetic resonance (NMR) spectra and physical simulation experiments. The effects of injection pressure and injection amount on enhanced shale oil recovery by CO2 flooding were considered, as well as the displacement mechanism of oil at different sections of the core. With the increases of injection pressure and injected PV, the oil recovery of CO2 flooding increases while the utilization efficiency decreases. The oil recoveries of CO2 with 2PV are 14% and 23% at 9 and 18 MPa, meanwhile, the values of the produced oil amount per mole of CO2 are 7% and 5% at 9 and 18 MPa. The oil content near the inlet of the core decreases greatly with the increasing PV while the oil content near the outlet has a slight decrease. After CO2 injection of 7 PV, the oil content near the outlet is still as high as 80%. Moreover, the factors of pressure, cycles, and soaking time were examined for CO2 huff-n-puff. The oil recovery increases while the utilization efficiency decreases with the increasing pressure and cycles in CO2 huff-n-puff. The total oil recoveries after three cycles are 56% and 69% at 9 and 20 MPa, while the values of the produced oil amount per mole of CO2 are 42% and 20%. With the increase of cycle, the swept area increases and the displacement efficiency of the swept core has a great enhancement. Compared with CO2 flooding, CO2 huff-n-puff method has greater oil recovery and CO2 utilization efficiency because of the sufficient contacting time between oil and CO2 and high swept ability. Therefore, CO2 huff-n-puff is beneficial for shale oil production. This work can provide the optimized method and parameters for the production in Enan Chang 7 zone.