Experimental Determinations of Minimum Miscibility Pressures Using Hydrocarbon Gases and CO_2 for Crude Oils from the Bakken and Cut Bank Oil Reservoirs

摘要

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 degrees C) and Three Forks crudes (127 degrees 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 degrees 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 1.31.4 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.