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

Abstract

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, CO2, 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 CO2 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 CO2, although both produced gas and CO2 could be effective at higher pressures.

Volumetric Swelling of Bakken Crude Oil with Carbon Dioxide and Hydrocarbon Gases at 110 °C and Pressures of up to 34.5 Megapascals

“…Factors with MMP, Oil Solubility, and the Recovery of Residual Oil from Middle Bakken Rock Samples. The same five gases and Bakken crude oil have previously been used to determine MMP 39 as well as crude oil hydrocarbon solubility 40 and the recovery of residual oil from Bakken rock samples, 41 which was performed by exposing rock cores to each gas at reservoir conditions of 110 °C and 34.5 MPa and collecting the produced oil hydrocarbons for gas chromatographic analysis, as previously described. 22,23 In general, the performance of the five test gases is similar to the results for crude oil swelling shown in Figure 6, that is, propane is superior to ethane, ethane is superior to carbon dioxide and the produced gas, and methane is the least effective.…”

“…70/20/10 mole fractions) to determine the MMP for each gas with a typical produced oil from the Middle Bakken formation, 39 measure the ability of these gases at different pressures to dissolve Bakken crude oil hydrocarbons, 40 and determine the ability of these gases (again at different pressures) to recover residual oil from both Middle Bakken siltstone and Bakken shale rock samples. 41 These laboratory studies have shown that propane is the superior gas in each of these tests, followed by ethane, then carbon dioxide and the produced gas (which have similar performance), and finally methane as the poorest EOR gas. Propane achieves MMP with Bakken crude oil at the lowest pressure, followed by ethane, carbon dioxide and the produced gas, and finally methane (which requires the highest pressure).…”

“…40 Similar trends occur for the recovery of residual crude oil hydrocarbons from Bakken siltstone and source shale samples. 41 While these previous investigations have addressed the relative abilities of carbon dioxide and produced hydrocarbon gases to achieve MMP, dissolve crude oil hydrocarbons, and penetrate and recover residual hydrocarbons from both reservoir and source shale rock samples, the contribution of crude oil swelling with injected gas is also likely to be an important mechanism for successful EOR projects. However, crude oil swelling has not been investigated with carbon dioxide and produced hydrocarbon gases at temperature and pressures relevant to the Bakken Petroleum System.…”

Dissolving Nonionic Surfactants in CO2 to Improve Oil Recovery in Unconventional Reservoirs via Wettability Alteration

“…One strategy for producing oil from already-fractured shale wells involves injecting oil-miscible gases such as CO 2 , ethane, propane, nitrogen, or natural gas to extract the remaining oil. − Of all the gases tested, CO 2 has demonstrated several advantages. CO 2 increases oil extraction by a variety of mechanisms including diffusion, vaporization, oil swelling, oil viscosity reduction, pressure support, CO 2 –oil interfacial tension (IFT) reduction, solution gas drive, and relative permeability hysteresis .…”

Enhanced Oil Recovery Method Selection for Shale Oil Based on Numerical Simulations

“…One of the most convenient methods of utilizing this gas is to reinject it back into the reservoir as an EOR agent . Using hydrocarbon gas as an EOR agent in huff-and-puff mode could be effective in shale deposits ,, given the presence of natural fractures. ,, The efficiency of this method depends entirely on the minimum miscibility pressure (MMP) at which the associated gas achieves miscibility with the reservoir oil. Miscibility causes oil swelling and a decrease in oil viscosity.…”