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

Abstract

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.

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

“…Decreasing solvent strengths were shown by CO 2 and produced gas (which showed similar results) and finally methane, which was a poor hydrocarbon solvent at even the highest pressure. 64 Even though earlier reports have shown the ability of an injected gas to dissolve residual hydrocarbons will be important for successful EOR in unconventional reservoirs, 12,13,44,45,54−57 the factors ultimately controlling oil recoveries will certainly be more complex than can be understood based on experimental MMP and solvent strength data reported for these five candidate injection gases. A variety of additional factors will likely influence oil recoveries including (but not limited to) injected gas penetration into the Bakken rock nanopores, dissolution of the gas into the remaining trapped crude oil, associated oil swelling and reduced viscosity, diffusion of dissolved crude oil hydrocarbons from interstitial pores into the bulk injected gas that dominates fracture spaces, and transport of the gas/oil mixture to production wells.…”

“…62,63 A recent experimental study reported the ability of CO 2 , methane, ethane, propane, and produced gas to dissolve crude oil hydrocarbons at reservoir pressures and temperatures relevant to the Bakken System and showed that Bakken crude oil solvation is (as might be expected) inversely related to the individual gases' MMP values with the same crude oil sample. 46,64 Gases with low crude oil MMPs (e.g., propane and ethane) were superior solvents for crude oil hydrocarbons, while gases with high crude oil MMPs (e.g., methane) showed very poor ability to dissolve crude oil hydrocarbons, especially the heavier hydrocarbons. 64 Propane dissolved the most total oil and the highest fractions of heavier hydrocarbons (e.g., C20− C36) at pressures from 10.3 and 34.5 MPa.…”

“…46,64 Gases with low crude oil MMPs (e.g., propane and ethane) were superior solvents for crude oil hydrocarbons, while gases with high crude oil MMPs (e.g., methane) showed very poor ability to dissolve crude oil hydrocarbons, especially the heavier hydrocarbons. 64 Propane dissolved the most total oil and the highest fractions of heavier hydrocarbons (e.g., C20− C36) at pressures from 10.3 and 34.5 MPa. Ethane was the second-best solvent and approached propane's effectiveness at the highest pressure tested (34.5 MPa).…”

CO2-Enhanced Oil Recovery Mechanism in Canadian Bakken Shale

“…The authors' analysis of the mineralogical composition of the samples indicated that these parameters were not relevant for predicting the recovery factor. In a later study, Hawthorne and Miller [8] investigated the impacts of the injection of various gases under different pressures on the produced hydrocarbon. CO 2 , produced gas (natural gas), and several pure hydrocarbons were tested as potential EOR injections.…”

“…In conventional systems, CO 2 penetrates the matrix and causes swelling, viscosity reduction or sweeping of the in-place oil [15]. In shales, the concentration gradientdriven diffusion of hydrocarbons from the interstitial pore spaces in the rock to the surface and into the bulk-injected fluid in the fractures is considered to be the main recovery mechanism [7,8,16,17]. The minimum miscibility pressure (MMP) may not be as important in CO 2 -EOR in shales as it is in conventional systems.…”