Simulation of CO2 Exsolution for Enhanced Oil Recovery and CO2 Storage

Advisor

Sally M. Benson

Abstract

Exsolution of CO₂ from carbonated brine occurs when pore pressures decline and CO2 solubility decreases, forming a free gas phase. The presence of exsolved CO₂ has been shown experimentally to significantly reduce wetting phase mobility. Numerical simulations of CO₂ exsolution were performed to assess the practicality of using CO₂ exsolution as a novel oil recovery method. In general, the approach involves carbonated water injection followed by CO₂ liberation through depressurization, which reduces the relative permeability of water where CO₂ has exsolved such that the sweep efficiency of subsequent water flooding is improved. Three-phase flow at the core-scale was simulated using ECLIPSE 300 with the CO2SOL option in order to compare results to laboratory experiments. The simulation results match reasonably well with experimental results, showing an incremental recovery factor of 15% for exsolution compared to carbonated water flood alone, which is at the upper end of the range for experimental results. Simulations of two-phase flow using TOUGH2-ECO2N were performed to evaluate whether fluid production could achieve exsolved gas phase saturations high enough to significantly reduce wetting phase mobility at the reservoir scale. These results show that sufficient exsolution can be achieved provided a confined reservoir or confining well pattern such that reservoir pressure can be reduced throughout the reservoir. Simulation of three-phase flow in a heterogeneous reservoir was also performed to evaluate the effect of CO₂ exsolution on oil recovery for several different types of oil under immiscible conditions. For each case investigated, carbonated water injection led to an enhancement in oil recovery compared to water flooding alone and carbonated water injection with CO₂ exsolution provided a further improvement in recovery. Depending on the oil type, CO₂ exsolution EOR led to an incremental recovery factor of 4 to 12% compared to water flood alone and an incremental recovery factor of 2 to 8% compared to carbonated water flood without depressurization under the conditions simulated. The results also show that particularly for heavier oil the immobile disconnected exsolved CO₂ phase provides a useful wetting phase mobility control mechanism. Overall the simulation results indicate that exsolution EOR can be an effective recovery technique that provides concurrent CO₂ storage.