A Laboratory Study Of Heavy Oil Recovery With Carbon Dioxide

Abstract

Abstract Heavy oil reservoirs of Saskatchewan are characterized by thin pay, shaley sand, heterogeneity, low productivity and bottom water. Thermal recovery processes are not expected to be economically feasible for reservoirs containing approximately eighty per cent of the heavy oil in the province. The carbon dioxide immiscible flooding process is considered to be the most promising among the non-thermal recovery methods for these reservoirs. This paper presents a survey of the recent laboratory and field studies a/the heavy oil-carbon dioxide immiscible process and the results obtained from phase behaviour and core displacement experiments. Viscosity, density bubble point pressure, gas-oil ratio (GOR) and swelling factor far the reconstituted reservoir fluids and CO2 saturated wellhead oil and reservoir fluid were measured at 28 °C and pressures lip to 8.27 MPa. A forty-five-fold reduction in viscosity and a 16% increase in the swelling factor were observed in the heavy oil CO2 system. Carbon dioxide once dissolved in the heavy oil at a high pressure remains in solution even when the pressure is reduced. Waterflooding of the oil-saturated core produced 31.5% to 46.2% initial oil-in-place (IOIP). The incremental recovery from the CO2 injection into the waterflooded core was between 12.7% and 38,6% residual oil-in-place (ROIP) in the water-alternating gas (WAG) mode and between 23.1% and 33.2% ROIP for the slug process. The total recovery appeared to level off at 60% IOIP. The CO2 injection into the core at the initial oil saturation recovered 29.9%. IOIP in the WAG mode and 41.6% IOIP for a slug process where CO2 was allowed to soak for 69 h. There appears to be considerable merit in soaking with CO2 prior to water injection. Introduction Saskatchewan has an extensive resource of heavy oil. The proven resource is 1.0 × 109m3 and the probable oil-in-place 3.4 × 109m3 (l). The heavy oil reservoirs of Saskatchewan are characterized by thin pay, shaley sand, heterogeneity, low productivity and bottom water. Fifty-five per cent of the initial heavy oil-in-place is contained in reservoirs less than 3 m thick and ninety-five per cent in less than 7 m thick(2). Primary and secondary methods together recover less than ten per cent of the initial oil-in-place. The development of appropriate recovery techniques specific to Saskatchewan reservoirs is necessary if the full potential of this resource is to be realized. Saskatchewan's heavy oil enhanced recovery projects have centred around steam injection and in-situ combustion. While steam injection has been very successful, it is not expected to be economically feasible for those reservoirs which contain approximately eighty per cent of the heavy oil(3,4). Insitu combustion, although applicable to thin reservoirs, has been only marginally successful thus far. Recovery of heavy oil using carbon dioxide immiscible flooding is considered to be the most promising among the non-thermal EOR processes, particularly for those reservoirs having such undesirable geology as found in Saskatchewan(4–6). Since 1982 the number of carbon dioxide immiscible projects in the United States has grown from zero to eighteen(3).