(51d) Effect of Temperature on Oil Recovery from Oil-Wet Carbonate Reservoirs by Surfactant Brine Imbibition

Little oil is recovered by conventional waterflooding from fractured oil-wet carbonate reservoirs. Water flows through the fractures, but does not imbibe into the matrix because of negative capillary pressure. We have been working on a method to recover oil from these systems is by surfactant imbibition. In our previous studies, the temperature was limited to the room temperature. In this study, the effect of higher reservoir temperature is analyzed by studying the rate of oil recovery from the oil-wet carbonate blocks.

Ethoxy/propoxy sulfates and sulfonates have been studied. Sulfates phase separate at temperatures higher than 650 C and are hence rendered inactive with their ability to alter wettability and lower IFT. Other surfactants, mainly sulphonates are studied for the higher temperature applications. Surfactants are identified with cloud points greater than 900 C. The optimal salinity have been found from phase behavior study where low interfacial tension (~10-2 mN/m) was also found. Some of these surfactants are found to alter the wettability of a calcite slab from an initial oil-wet state to a final intermediate wet / water wet state. The surfactants are then used for studying the rate of oil recovery from an initial oil-wet core by conducting imbibition studies at different temperatures. It was found that increasing the temperature increases the rate of oil recovery for a given set of surfactants. This is primarily because increased rate of surfactant diffusion and hence faster lowering IFT and wettability alteration.

A numerical simulator is developed and utilized for analyses of these imbibition results to find in-situ distribution of IFT, surfactant concentration, wettability and relative permeability. Surfactant molecules from the fracture block diffuse into the matrix and lower the interfacial tension between the oil and the aqueous phase. This reduction in the IFT decreases the entry capillary pressure. Gravitational forces overcome the entry capillary pressure and water invades the matrix and pushes out the oil from the top. The surfactants also alter the wettability of the matrix from an initial water-wet/intermediate wet state to a final water-wet nature. This change in wettability increases the oil phase relative permeability and thus increases the rate of oil recovery.