Comparison of CO2 and Produced Gas Hydrocarbons to Dissolve and Mobilize Bakken Crude Oil at 10.3, 20.7, and 34.5 MPa and 110 °C

The ability of an injected gas to dissolve crude oil hydrocarbons may be a major factor controlling the success of enhanced oil recovery (EOR) projects in unconventional reservoirs. Multiple laboratory gas injection tests were conducted using Bakken crude oil in which the gas-dominated upper phase in equilibrium with the bulk crude oil was collected and analyzed to determine dissolved crude oil concentrations and their molecular weight distributions. Dissolved concentrations varied greatly among the test gases, as well as at different pressures for all of the fluids except propane. The ranges of total dissolved hydrocarbons at 10.3, 20.7, and 34.5 MPa were methane (8-67 mg/mL), ethane (40-228 mg/mL), propane (230-278 mg/mL), and CO2 (13-254 mg/mL). The oil solubility in a representative produced gas (69.5/21/9.5 methane/ethane/propane) ranged from 9 to 145 mg/mL. The gases and pressures that mobilized the lowest total hydrocarbon concentrations were also the least effective at mobilizing heavier hydrocarbons. For example, the C20-C36 fraction of the crude oil that was mobilized with each gas exposure at 10.3-34.5 MPa was methane (<1-6%), ethane (12-95%), propane (58-99%), produced gas (<1-33%), and CO2 (<1-40%), indicating concentration of the heavier hydrocarbons in the residual crude oil after exposure to gases (and pressures) that are less effective. Residual crude oil viscosities after gas exposure showed significant increases (with the notable exception of propane at all three pressures), also consistent with each test gas's ability to dissolve heavier hydrocarbons. Consideration of each gas's density changes with pressure correlated well with their abilities to dissolve crude oil, and increases in pressure always increased oil solubilities regardless of each gas's minimum miscibility pressure (MMP) value with the crude oil used for these mobilization experiments. Multiple exposures of a crude oil sample to fresh test gas showed declining dissolved hydrocarbon concentrations demonstrating that crude oil solubilities were not controlled by saturation solubility, but were controlled by equilibrium partitioning of hydrocarbons between the gas-dominated and oil-dominated phases.