Chemical Flooding of Fractured Carbonates Using Wettability Modifiers

Abstract

Abstract Waterfloods performed in carbonate and naturally fractured reservoirs frequently suffer from relatively poor sweep efficiency by virtue of geological heterogeneity and preferentially oil-wet rock surface commonly seen in these reservoirs. The application of chemical-based wettability modification in such problematic formations has become one of the most potential enhanced oil recovery techniques for the worldwide abundance of fractured carbonates with significant amount of remaining oil. Chemical stimulation with surfactant or electrolyte to alter the wettability towards more water-wetness has the potential to enhance water imbibition to expel more oil from matrix to the fractures. The countercurrent interaction at matrix-fracture interface involves interplay of capillary, gravitational and viscous forces. A clear understanding of these processes is required for an optimum oil recovery design and field implementation in fractured carbonate reservoirs. To this end, we have developed a systematic experimental and modeling approach on the combined benefit of wettability alteration for enhanced water imbibition and interfacial tension reduction. Both natural and forced imbibition experiments were performed in mixed-wet rocks where oil volume produced was recorded for sequential injection of water, alkali, and surfactant/alkali mixture. The alkali was effective in wettability modification and enhanced water imbibition. Additional oil was recovered by injection of surfactant/alkali mixture following alkali injection due to interfacial tension reduction and oil mobilization. A wettability alteration model based on these mechanisms was developed and implemented in a compositional chemical flooding simulator. The experiments were successfully modeled with the enhanced simulator. A better understanding of mechanisms involved in improved recovery of oil from fractured carbonates using wettability modifier will aid in identifying and implementing future field demonstration projects. Introduction A large quantity of world's oil reserves is contained in carbonate reservoirs (Roehl and Choquette, 1985). Most of these reservoirs are naturally fractured. The fracture network normally has much higher permeability than that of porous matrix, accounting for poor primary oil recovery. Waterflooding would be an effective method for improvement of oil recovery from water-wet fractured reservoirs, where spontaneous imbibition of the injected water is known to be the main recovery mechanism. Unfortunately, about 80% of carbonate rocks are mixed-wet to preferentially oil-wet (Downs and Hoover, 1989) which are unfavorable conditions for spontaneous water imbibition. Hirasaki and Zhang (2004) refer to "Imbibition" as the process of water displacing oil and "Spontaneous Imbibition" as imbibition that takes place under the influence of capillary or buoyancy forces when a core sample or matrix block is surrounded by aqueous phase. At high interfacial tension (IFT) the main driving force for spontaneous imbibition is capillary pressure. Morrow and Mason (2001) studied the rate of capillary pressure driven imbibition for different wettability conditions.