A Laboratory Study on Near-Miscible CO2 Injection in Steelman Reservoir

Abstract Miscible flooding is considered unsuitable for some reservoirs in southeast Saskatchewan because of high C)2 minimum miscibility pressure or operating pressure constraints. Therefore, the effectiveness of near-miscible C)2 injection was assessed for the Steelman reservoir in a laboratory study. The minimum miscibility pressures (MMP) were estimated for Steelman reservoir fluids with pure C)2 and C)2-hydrocarbon gas mixtures, the partially flashed reservoir fluids and the dead oils with pure C)2. The results of MMP studies demonstrated (1) addition of ethane or propane can reduce C)2 MMP greatly; and (2) achieving a miscible C)2 flood in the Steelman reservoir could be possible at a lower operating pressure than the measured C)2 MMP, by partially depleting the reservoir. Asphaltene flocculation tests showed that, after the onset point, flocculation increased linearly with gas concentration, but that the presence of brine had a negligible effect. Three tertiary C)2 coreflood tests were conducted with Steelman reservoir fluids at the reservoir temperature. These results showed that the microscopic displacement efficiency during the C)2 injection stage improved with the operating pressure in the near-miscible region, but no dramatic change in oil recovery was observed with a change in operating pressure. Introduction The Steelman pool, located about 200 km southeast of Regina, Saskatchewan, was discovered in 1954. Covering approximately 370 km2, the field was estimated to contain over 134 million m3 of light oil in the Frobisher and Midale beds located at a depth of about 1,400 m. Most of the reservoirs in the Midale beds, which contain about 83﹪ of the reserves of the pool, have been under waterflood for over 20 years and have nearly reached the estimated production limit by primary and waterflood(1). For achieving additional oil recovery and consequent financial benefits, the development of tertiary enhanced oil recovery (EOR) techniques, such as C)2 flooding, is essential. Miscible C)2 flooding is a proven enhanced oil recovery technique(2). Over the last decade, C)2 injection has become the leading EOR process for light oil(3). In Canada, the industry interest in C)2 flooding is evidenced by Shell Canada's C)2 pilot tests(4, 5), a miscible C)2 flood at Joffre field in Alberta(6), and the implementation of PanCanadian's Weyburn C)2 injection project(7). Additional field applications are expected to be initiated in southeast Saskatchewan and other regions when C)2 sources are available. Miscible C)2 displacement offers the greatest oil recovery potential, but it can only be achieved at a pressure greater than a certain minimum referred to as the minimum miscibility pressure (MMP). The effect of solution gas on the development of miscibility is not well understood, as is reflected by the lack of a consensus in the literature. Rathmell et al.(8) found that C)2 MMP is related to the volatile and intermediate fractions of the oil. Yelling and Metcalfe(9) found from their experimental study that, for a saturated reservoir oil, areal variation in gas-oil ratio (GOR) will result in an areal variation in the C)2 MMP. They pointed out that the C)2 MMP determined for a pilot area may not apply to the rest of the field.