In a miscible CO2 flood, multicontact miscibility between the injected CO2 and the reservoir fluid can be achieved at pressures greater than a minimum value. This minimum miscibility pressure (MMP) is a key parameter for assessing the applicability and design of a miscible gas flood for an oil reservoir. The effect on CO2 MMP of reservoir temperature, oil composition (molecular weight of C5+ or C7+ fraction), and the purity of injected CO2 have been investigated extensively. Numerous empirically derived CO2 MMP correlations have been reported in the literature. However, how the solution gas in oil influences the CO2 MMP is not well understood.
In this study, the rising bubble apparatus (RBA) was used to determine the CO2 MMP for various oils. RBA tests permit direct observation of changes in bubble behaviour and thus offered insight into the phase behaviour for the CO2- reservoir fluid system. The CO2 MMPs were estimated for two Steelman reservoir fluids with a high gas-oil ratio, the partially flashed reservoir fluids, and the dead oils. The composition of solution gas of each partially flashed reservoir fluid was determined and the effects of different gas components were analysed. The MMP was also determined and discussed for Weyburn reservoir fluids which had a low gas-oil ratio with pure and impure CO2. The results of this study demonstrated that the effect of solution gas in oil on CO2 MMP could be significant. Furthermore, achieving a miscible CO2 flood (in a reservoir with a Steelman-like reservoir fluid) could be possible at a lower operating pressure than the measured CO2 MMP, by partially depleting the reservoir. This may be the only option for some reservoirs which cannot sustain the relatively high pressure required for achieving miscibility.
Carbon dioxide flooding is a proven oil recovery process.1 Over the last decade, carbon dioxide injection has become the leading enhanced oil recovery (EOR) process for light oils.2 CO2 injection can prolong, by 15 to 20 years, the production life of light oil fields nearing depletion under waterflood. CO2 injection may recover 15 to 25% of the original oil in place. More than 20 years of field experience of CO2 injection has advanced the CO2 technology. CO2 injection can be introduced gradually and use some of the same equipment currently in place for waterflooding.
Saskatchewan's light and medium oil resource, representing nearly 45% of proven reserves, has been on waterflood for over 20 years and is fast approaching its economic limit of production.
3 For the light and medium oil reservoirs in Saskatchewan, carbon dioxide or hydrocarbon injection is considered to be the most effective EOR process.4–6 These gases can be injected into the reservoir to develop miscible or immiscible conditions with the oil depending upon the operating pressure. Carbon dioxide is preferred over hydrocarbon gases (e.g., ethane, propane) because it is cheaper, has higher density, and offers environmental benefits by providing storage for CO2 in the reservoir. The Saskatchewan Research Council (SRC) is conducting a comprehensive research program to assess the suitability of miscible CO2 displacement for reservoirs in southeast Saskatchewan and to optimize the field operating procedures.