Pore scale mechanisms of carbonated water injection in oil reservoirs
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Concerns over the environmental impact of carbon dioxide (CO2) have led to a resurgence of interest in CO2 injection (CO2I) in oil reservoirs, which can enhance oil recovery from these reservoirs and store large quantities of CO2 for a long period of time. Oil displacement and recovery by CO2I has been studied and applied in the field extensively. However, CO2I lacks acceptable sweep efficiency, due to the large viscosity contrast between CO2 and resident reservoir fluids. Various CO2I strategies e.g. alternating (WAG) or simultaneous injection of CO2 and water have been suggested to alleviate this problem. An effective alternative strategy is carbonated (CO2-enriched) water injection. In carbonated water, CO2 exists as a dissolved as opposed to a free phase, hence eliminating the problems of gravity segregation and poor sweep efficiency. In this thesis, the results of an integrated experimental and theoretical investigation of the process of carbonated water injection (CWI) as an injection strategy for enhanced oil recovery (EOR) with the added value of CO2 storage are described. High-pressure micromodel technology was used to physically simulate the process of CWI and visually investigate its EOR potential, at typical reservoir conditions. Using the results of these flow visualisation experiments, the underlying physical processes and the pore-scale mechanisms of fluid-fluid and fluid-solid interactions during CWI were demonstrated to be oil swelling, coalescence of the isolated oil ganglia, wettability alteration, oil viscosity reduction and flow diversion due to flow restriction in some of the pores as a result of oil swelling and the resultant fluid redistribution. A mathematical model was developed that accounts for the pore-scale mechanisms observed during the micromodel experiments. In this study, some of the micromodel experimental observations were interpreted and the impact of some of the pertinent parameters on CWI and CO2I processes was studied. The results predicted by the model were linked to the results obtained using a new relationship developed based on the dimensional analysis technique. To examine and investigate the effect of CWI on wettability, micromodel experiments, designed only to observe possible variation of contact angles and spontaneous imbibition displacement mechanisms due to CW, were performed. Contact angle measurements were also conducted to quantify different tendencies of CW and water to wet solid surfaces, using three different solid plates with different salinity of the aqueous phase, under different pressure and temperature conditions. Two other important parameters affecting the performance of CWI, i.e. CO2 solubility in water and its CO2 diffusion coefficient, were also experimentally studied and estimated. A mathematical model was developed to estimate CO2 diffusion coefficient from the corresponding experimental results. The results of this research show that CWI is an effective and efficient injection strategy that offers great potential for enhanced oil recovery and at the same time a unique solution to the problem of reducing CO2 emission.