(217e) The Role of Mass Transfer in CO2 - Oil Recovery

THE ROLE OF MASS TRANSFER IN CO2-OIL REC OVERY              Conventional oil recovery leaves behind about 67% of the original oil and enhanced oil recovery methods are aimed at recovering those.  Of these methods CO₂ flooding is the cheapest.  Since some oil evaporates into the CO₂ phase and some CO₂ dissolves in oil, miscibility is reached leading to very good displacement.  There are additional benefits of dissolution of CO₂ in oil is that it reduces the oil viscosity and makes displacement easier.  In addition, it swells the oil when it dissolves in it, thus squeezing it out of narrow pores and crevices.   However, heavy crude does not evaporate and miscibility with CO₂ is not reached.  More important is the fact that as the viscosity of crude is much higher than the viscosity of CO₂, the displacement process becomes unstable leading to fingering and failure of the displacement process.  Of course, the instability increases for heavy oils because of increased oil viscosity.  In considering the effect of mass transfer on stability, it is expected that fingers with small wavelengths will dissolve out leading to stability to disturbances of small wavelengths.  It is also expected that the decrease in volume due to the dissolution of CO₂ in oil will add to the stabilizing effect depending on the amount of mass transferred.  Linear stability analysis of a simplified displacement process in presence of mass transfer is presented where the above issues and the overall impact of mass transfer will be discussed.