Water flooding typically recovers about 50% of the original oil in place leaving much oil in the reservoir. Recovery efficiency in fractured reservoirs can be dramatically lower in comparison to conventional reservoirs because water channels selectively from injector to producer leaving considerable oil within the matrix and uncontacted by injected water. An enhanced recovery process is needed to access such oil held in the reservoir matrix. Addition of aqueous surfactants to injection water dramatically reduces oil/water interfacial tension and surfactant may adsorb to oil-wet rock surfaces inducing a shift in wettability that improves the imbibition of water. At the pore level, capillary forces are responsible for oil trapping and generally dominate over viscous and gravitational forces. Because of the reduction in interfacial tension between oil and water with the addition of surfactant, the role of capillary forces on fluid flow can be minimized. When gravity parameters are large enough to give a Bond number (ratio of gravity to capillary forces) greater than 10, gravitational forces become more dominant and oil held with rock matrix by capillarity may be released as a result of buoyancy. In this work, we use experiments conducted in two-dimensional micromodels to investigate the effect of gravity at low interfacial tension. The micromodels have the geometrical and topological characteristics of sandstone and the network is etched into silicon. Porelevel mechanics are observed directly via a reflected-light microscope. A screening study of sulfonate and sulfate surfactants was conducted to choose an appropriate system compatible with the light crude oil (27°API). A variety of flow behavior through the microscope is investigated including forced and spontaneous imbibition. Results are illustrated via pore-level photo and image analysis of microscopic pictures of the micromodel. Forced displacements are conducted at realistic flow rates to maintain a 1 m/day Darcy velocity and at surfactant concentrations of 0.9% to 1.25%. Forced displacement with a horizontal or vertical positioning of the micromodel yields dramatic improvement of recovery for surfactant injection cases. Most of the oil retained after a waterflood was recovered by tertiary injection of surfactant solution. In comparison, about 25% oil saturation remained after a waterflood.

The technique of injecting water is used for about 50% of the world's oil production. However, recovery efficiency of water injection in fractured reservoirs can be very low because, guided by capillary forces, the injected fluid will preferentially flow in the fractures rather than through the matrix. As a result, most of the oil in the matrix is not contacted by the injected water and remains within the matrix. To access this oil, the role of capillary forces on fluid flow can be minimized by adding surfactants and surface-active agents to injected water. Surfactants dramatically change the interfacial properties by reducing the surface tension.In this study, micromodels were used in a series of experiments to compare the results of forced and spontaneous imbibition tests in the presence of aqueous surfactant solutions. These micromodels contain a repeated flow pattern that is etched onto silicon and roughly offer the pore-size distribution and pore sizes of sandstone. Two-dimensional movement of fluids is studied under a Nikon Optiphot-M reflected light microscope and 10X pictures are taken. Matlab is used to run image analysis on these pictures to interpret quantitatively the residual oil saturation, oil recovery and the porosity values.A screening study of 6 surfactants at different concentrations was run to choose the system that was most compatible with low-viscosity crude oil (~4.95cP). Solutions that were 2 % (by volume) were used in forced imbibition at 1 m/Day Darcy velocity. The same surfactant solution was also used in spontaneous imbibition Darcy velocity. Forced displacement compared to spontaneous displacement improves recovery efficiency. If most of the oil was removed in secondary injection of surfactant solution in forced displacement, only about 1% of oil was removed in secondary injection of surfactant solution in spontaneous displacement.