Mechanisms of surfactant enhanced oil recovery in oil-wet fractured carbonate reservoirs

Oil recovery by conventional water-flooding is not effective for fractured carbonate systems which are usually oil-wet in nature. The water bypasses through the fractures and due to negative capillary pressure is not imbibed into the oil-wet matrix. Alkaline surfactant solutions are used to recover oil from these reservoirs by enhancing the imbibition of water between fracture and the matrix by both wettability alteration and interfacial tension (IFT) reduction.; Surfactants have been identified which can lower the IFT between the oil and aqueous phase. The reduction in IFT leads to mobilization of the oil by buoyancy forces. Surfactants have also been identified which can alter the wettability of the porous medium from an initially oil-wet to a final intermediate-wet to water-wet regime. This change in wettability leads to positive capillary pressure and provides a driving force for spontaneous counter-current imbibition. Anionic surfactants adsorb on the calcite mineral as calcite mineral is positively charged at neutral pH. Alkaline solution of Na2CO 3 is used to increase the pH of the solution above the point of zero charge for calcite. Preparation of surfactant solutions in Na2CO 3 is found to significantly reduce the adsorption of surfactant solutions on calcite mineral.; Surfactants which give low IFT and alter the wettability to intermediate-wet/water-wet regime were used for imbibition studies on an initially oil-wet carbonate rock. The imbibition results show that surfactant brine imbibition can lead to 40%-62% recoveries of oil in terms of original oil in place (OOIP). No oil recovery was observed when the rock was surrounded by field brine alone. Different parameters were varied to study the sensitivity of the process. The process of surfactant brine imbibition for oil recovery was found to be robust. The rates of recoveries increase with increase in permeability, decrease of rock height, and decrease in oil viscosity. The rates of recoveries were found to overlap each other when scaled with gravitational dimensionless group indicating that gravity is the major driving force for oil recovery.; A numerical simulator was developed which simulates the process of surfactant brine imbibition. It is a two phase, four component (oil, water, surfactant and salt) implicit finite volume simulator with flexible grid structure and flexible boundary conditions. It incorporates surfactant diffusion into the porous medium, which alters the IFT and wettability of the rock. The changes in IFT and wettability lead to changes in residual saturations, relative permeabilities of both phases and also alter the capillary pressure. The numerical simulator accounts for these changes.; The simulator is used to model water imbibition into water-wet medium with no surfactant present. The results obtained from these simulations show consistency between the simulated results and those obtained from theoretical dimensionless groups used in literature. It was found that even for a strongly water-wet medium, gravitational forces play an important role for oil-recovery. The effect of gravity is found to become more prominent as the fracture block dimensions were increased.; The simulation of lab-scale surfactant brine imbibition experiments showed a match between the experimental results and the numerical simulation. The simulator was then used to analyze the mechanisms involved in oil recovery for fractured carbonates using surfactant-brine process. It was found that when the surfactants alter the wettability to a water-wet regime, surfactants which do not lower the IFT to low values (>0.1 mN/m) recover more oil and the rate of recovery is almost independent of the permeability of the porous medium (between 0.1 mD and 150 mD). So cationic surfactants recovery rates are higher if they are able to alter the wettability to water-wet regime. If the surfactants do not alter wettability to water-wet regime, then gravitational forces could still be used...