Enhancing the Spontaneous Imbibition Process in Naturally Fractured Reservoirs through Wettability Alteration Using Surfactants: Mechanistic Study and Feasibility of Using Biosurfactants Produced from Agriculture Waste Streams

Abstract

Naturally fractured reservoirs are characterized by having low-permeability matrix blocks surrounded with fractures of high hydraulic conductivity. Waterflooding process in such reservoirs is successful if the matrix blocks holding the dominant fraction of the reservoir porosity are able to imbibe the injected water (water-wet) and expel the oil into the fracture system and finally to the production well. This mechanism referred to as spontaneous imbibition of water into the matrix blocks is an efficient method to increase oil recovery from fractured reservoirs. However, most naturally fractured reservoir rocks are mixed- to oil-wet and do not imbibe the injected water, which translates into low-efficiency waterflood recovery. To enhance the spontaneous imbibition process, low concentration of surfactants is dissolved into the injected water to induce wettability alteration of the reservoir rock by changing the wettability of the rock toward a more water-wet state. This is the main subject of this research study. The first part of this study was devoted to evaluating and comparing the effectiveness of using a biosurfactant (surfactin) produced from agriculture waste streams against a benchmark chemical surfactant in mediating the wettability of oil-wet rocks. The ability of surfactants to enhance the spontaneous imbibition process in cleaned and crude oil-aged reservoir core plugs was tested. One of the other factors that needs to be considered is the estimation of the loss of surfactants due to adsorption onto the reservoir rock, because for the process to be effective, the injected surfactant solution should be able to penetrate deep into the reservoir. So, in this work, static and dynamic adsorption isotherms for both biosurfactant and benchmark chemical surfactants on crushed reservoir rocks and reservoir core plugs were generated and compared. The second part of the study focused on the mechanistic study of wettability alteration by surfactants. There are two main mechanisms proposed for the wettability alteration process, but none have been verified. This study is focused on investigating the wettability alteration mechanisms though experimental observations. The results of the work will provide better guidelines in designing and improving water flood performance in naturally fractured reservoirs. It was verified that wettability alteration is caused by either surfactant adsorption or the ion-pair formation between the surfactant monomer and the material adsorbed on the reservoir rock from exposure to crude oil. It was further demonstrated that the ion-pair process can be improved by increasing the charge density of the head-group in the surfactant molecule.