A Laboratory Miscible Displacement Study For the Recovery of Saskatchewan's Crude Oil

Abstract

Abstract Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limits of production under waterflooding, leaving 70 to 80 per cent of the initial oil-in-place (IOIP) in the reservoirs. The miscible displacement process using carbon dioxide, hydrocarbon, and other gases as solvents appears to be one of the most promising methods for these reservoirs. A laboratory study was conducted to evaluate the applicability of various solvents for the recovery of oil from a southeast Saskatchewan reservoir. The physical, chemical, and phase behavior (PVT) properties of wellhead and reservoir fluids were determined. Slim tube displacement tests were conducted between the reconstituted Pinto reservoir fluid and three solvents: carbon dioxide, ethane, and wellhead gas from the Steelman gas plant. Tests were carried out at various pressures and the reservoir temperature of 56 °C. The minimum miscibility pressures (MMPs) determined for the above three systems demonstrated that the MMP for ethane was much lower than that for carbon dioxide, which was lower than that for Steelman gas. The MMPs for ethane and carbon dioxide were below the reservoir fracture pressure. Introduction Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limit of production under current technology. Primary and secondary methods together recover about 21 % of the initial oil-in-place (IOIP). Although the remaining light and medium oil-in-place in Saskatchewan is 1135 million cubic metres(1), the remaining producible reserves under primary and secondary recovery are only 65 million cubic metres. Medium oil in southwest Saskatchewan has not been considered here, but if it is included then up to 17 million cubic metres of additional oil (through primary and secondary recovery) may be recovered(1). The development of tertiary recovery techniques, therefore, is essential to increase the production life of these reservoirs and to maintain current production. Application of the miscible oil displacement process using CO2, hydrocarbon, or other gases as injection fluids is expected to increase the LMO reserves threefold and extend the production life of these pools by two decades(2). Miscible flooding with carbon dioxide or hydrocarbon solvents is considered to be one of the most effective enhanced oil recovery (EOR) processes applicable to LMO reservoirs. Current industry interest in CO2 miscible flooding is high, as evidenced by the level of activity in field testing(3,4) and CO2 source development in the United States. CO2 has a viscosity similar to hydrocarbon miscible solvents. Both types of solvent affect the volumetric sweep out because of unfavorable viscosity ratio. However, CO2 density is similar to that of oil. Therefore, CO2 floods minimize gravity segregation compared with the hydrocarbon solvents. In addition, the supply cost of CO2 is generally more favorable than hydrocarbon miscible solvents in the United States(3). However, there is no cheap source of CO2 in Canada yet. It should be noted that, since the world oil price collapse in 1986, the number of EOR projects in the United States has decreased markedly from 512 in 1986 to 366 in 1988, whereas the number of CO2 miscible projects has increased from 38 in 1986 to 49 in 1988, a 29% rise during these two years(4).