Experimental Determinations of Minimum Miscibility Pressures Using Hydrocarbon Gases and CO₂ for Crude Oils from the Bakken and Cut Bank Oil Reservoirs

Abstract

Minimum miscibility pressures (MMPs) were measured at reservoir temperatures using a capillary-rise vanishing interfacial tension (VIT) technique for four crude oils collected from different formations in the deep/hot Bakken Petroleum System and the shallow/cool Cut Bank field. Potential injection fluids tested were pure CO2, methane, ethane, propane, and hydrocarbon gas mixtures typical of the rich gas produced from tight shale formations like the Bakken Petroleum System (ca. 7/2/1 mol ratios of methane/ethane/propane). Depending on the oil and test temperature, MMPs were achieved with the fluids in the gas, liquid, or supercritical states. Regardless of the physical state of the test fluids at MMP, propane achieved MMP at the lowest pressure with all four crude oils, followed by ethane, then CO2 and produced gas, and finally methane requiring the highest pressures. For the Bakken (110 °C) and Three Forks crudes (127 °C), MMPs dropped from 29 to 31 MPa with methane from 16.2 to 18.7 MPa with CO2 or produced gas, and further lowered from 9.2 to 10 MPa with ethane, and from 3.8 to 4.3 MPa with propane. Changes in the MMPs with the different fluids were even more dramatic for the Madison and Cut Bank crude oils (both at 28 °C) with methane MMPs about 28–29 MPa, produced gas at 10–10.6 MPa, CO2 at 8.3–8.7 MPa, ethane at 4.2–4.5 MPa, and propane only requiring 1.3–1.4 MPa to achieve MMP. Enriching produced gas by adding either ethane or propane showed approximately linear decreases in the MMPs with the Bakken crude oil. For example, increasing propane in produced gas from 6.7 to 25 mol % reduced the Bakken crude oil’s MMP from 18 to 12.7 MPa, while increasing ethane from 13.5 to 68 mol % reduced the MMP from 18.6 to 11.4 MPa. The results of this experimental study show that injecting produced rich gas may be as effective as injecting CO2 for enhancing oil recovery and that enriching produced gas with ethane or propane may be superior to CO2 for EOR in both shallow/cool and deep/hot reservoirs.

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

“…Though both milligrams of oil per milliliter of gas and milligrams of oil per gram of gas are useful comparisons of oil solubilities in the different test gases and pressures, the milligrams per milliliter units more closely reflect EOR processes because they are analogous to common oil-field units of cubic meters of gas per cubic meter of oil (or million standard cubic feet of gas per barrel of oil), injected gases may be purchased on a volume ( 2 shows the measured MMP values previously reported for the same crude oil as used in the present study with each of the test gases, 54 as well as comparing their mass densities (grams per milliliter) and molar densities (moles per liter) at MMP. For methane, only the highest pressure (34.5 MPa) exceeded methane's MMP value of 31.1 MPa and, as might be expected, the solubilities of the crude oil hydrocarbons increased by nearly a factor of 5 when the pressure was increased from 20.7 to 34.5 MPa.…”

“…Table shows the measured MMP values previously reported for the same crude oil as used in the present study with each of the test gases, as well as comparing their mass densities (grams per milliliter) and molar densities (moles per liter) at MMP. For methane, only the highest pressure (34.5 MPa) exceeded methane’s MMP value of 31.1 MPa and, as might be expected, the solubilities of the crude oil hydrocarbons increased by nearly a factor of 5 when the pressure was increased from 20.7 to 34.5 MPa.…”

“…The crude oil sample was from a well producing from the Middle Bakken target drilling zone and was the same oil as previously used in experimental MMP studies. , In brief, it is a typical Bakken crude oil with a viscosity of 2.2 cP, a density of 0.817 g/mL, and an API gravity of 41.7. More detailed characterizations are given in refs and .…”

Effect of Functional Groups on Chemical-Assisted MMP Reduction of a Methane-Oil System

“…However, in actual oil reservoirs, the gas is rarely pure and the presence of some impurities in the gas stream certainly affects the miscibility pressure with oil. For example, the presence of gases like CO 2 , ethane, or propane will enrich the methane and reduce the miscibility pressure. − In other words, pure methane injection could be considered as the worst-case scenario for gas injection in terms of miscibility pressure (i.e., requiring the highest pressure to achieve miscibility).…”

Mini Review of Miscible Condition Evaluation and Experimental Methods of Gas Miscible Injection in Conventional and Fractured Reservoirs

“…Huang et al 114 slim tube displacement Changqing oilfield of China CO 2 Wang et al 115 vanishing interfacial tension Changqing oilfield, western China CO 2 Hawthorne et al 116 vanishing interfacial tension Bakken crude oil in North Dakota (U.S.A.) CO 2 and hydrocarbon gases…”