Comparison of CO₂ and Produced Gas Hydrocarbons to Recover Crude Oil from Williston Basin Shale and Mudrock Cores at 10.3, 17.2, and 34.5 MPa and 110 °C

Carbon dioxide and hydrocarbon gases including methane, ethane, propane, and produced gas (69.5/21/9.5 mol ratio of methane/ethane/propane) were used to recover oil from rock samples collected in the Middle Bakken (MB) target drilling zone and from a Lower Bakken Shale (LBS) source rock. Experiments were designed to mimic the fracture-dominated flow expected to occur during gas injection into hydraulically fractured unconventional reservoirs such as the Bakken Petroleum System. Higher pressures recovered oil (and heavier hydrocarbons) more efficiently than lower pressures from both rock samples for each of the test gases but recoveries varied dramatically with the gas used. In general, oil recovery efficiencies from both MB and LBS rocks were the highest with propane, followed by ethane, CO 2, and produced gas, and finally methane. Propane and ethane were the most efficient in recovering oil hydrocarbons from the rock samples at all three pressures, although ethane required higher pressures. Recoveries with CO 2 and produced gas were low at 10.3 MPa but increased substantially at higher pressures with the recoveries achieved at 34.5 MPa approximating those achieved at lower pressures using propane and ethane. Finally, methane was only capable of significant oil recoveries from MB samples at the highest (34.5 MPa) test pressure but was not effective in mobilizing hydrocarbons from the tighter LBS samples at any of the pressures tested. Molar densities and the related injected gas solvent strengths at the three test pressures had a strong influence on oil recovery rates. For example, when molar densities of all five gases at the three test pressures were correlated with total hydrocarbon recoveries, linear correlation coefficients ( r 2 ) were significant (0.69 for 1 h oil recoveries from the MB mudrock samples and 0.68 for the 24 h oil recoveries from the LBS shale samples). When oil recoveries at the three test pressures were compared to the molar densities of each individual gas, linear correlation coefficients ( r 2 ) ranged from 0.89 to 0.99 for the recoveries from the MB samples, and 0.95 to >0.99 for the LBS samples for all of the gases except propane (which gave high recoveries at every test pressure, consistent with the fact that propane’s molar density shows little change at the three test pressures). The results of these laboratory studies indicate that ethane, propane, or very rich produced gas should be capable of recovering oil from unconventional reservoirs at significantly lower pressures than leaner produced gas or CO 2, although both produced gas and CO 2 could be effective at higher pressures.