Propane-Enriched CO2 Immiscible Flooding For Improved Heavy Oil Recovery

The majority of heavy oil reservoirs in west-central Saskatchewan, Canada, are thin and marginal, such that they are economically unsuitable for thermal recovery methods. Immiscible gas flooding appears to be highly promising for these underexploited reservoirs. The goal of the research described here is to advance development of cost-effective immiscible gas flooding processes applicable to moderately viscous heavy oils in this region, by improving recovery factors through designing an injection solvent mixture with optimized properties. In this study, two solvents were studied and compared: pure CO2 and 81 mol % CO2 + 19 mol % C3H8. The phase behavior (pressure volume temperature (PVT)) and physical properties of a reconstituted live heavy oil with these solvents were measured, respectively. In terms of oil viscosity reduction and swelling, the CO2-C3H8 mixture was found to be substantially more effective than CO2 alone. At about 4 MPa and 21 degrees C, CO2-C3H8 addition reduced the live oil viscosity by 96.5%, compared with 92.6% for CO2. As well, the live oil swelled by 10.6% when saturated with the CO2-C3H8 mixture, much higher than the 6.8% with CO2. Furthermore, a comparative coreflood study of the oil recovery performance using CO2 alone and the CO2-C3H8 mixture at different operating pressures was carried out to assess their respective effectiveness. The obtained oil recovery factor confirmed that the solvent mixture was much more effective than CO2 alone. At the operating pressure of 4 MPa, the CO2-C3H8 mixture recovered 34.2% original oil in place (OOIP) during the water-alternating-gas (WAG) cycles, compared to 22.5% OOIP using CO2 alone at 4 MPa. The gas utilization factor for CO2-C3H8 flooding, however, was only 72% of that for CO2-alone flooding. The comparative advantage of the solvent mixture was also apparent in two specially designed radial corefloods, which were conducted to better represent fluids radial flow in heavy oil reservoirs.