An insight into enhanced oil recovery process by chemical water injection into oil reservoirs.

Abstract

Water injection into oil reservoirs in the tertiary stage of production is one of the most common and in some reservoirs the most effective method of enhanced oil recovery (EOR). Additives to injected water are usually engineered and adjusted for better performance and improvement of parameters affecting oil production. This type of water injection is known as chemical water injection. In one division, smart water injection, carbonated water injection, injection of surfactant solutions, polymers, alkalis and mutual solvents can be considered as various methods of chemical water injection. In addition, sometimes to increase the performance of chemical water, a combination of several types of additives and foam injection is recommended according to the structure of the reservoirs. Chemical water injection is sometimes used with special chemicals such as fluorinated surfactants to treat reservoir rock around gas condensate wellbores and remove the liquid blockage in this area. In this project, the effects of different types of additives on the performance of chemical water in different types of reservoirs have been investigated. Chemical and natural additives including mineral salts, dissolved carbon dioxide, natural surfactants such as saponin extracted from Anabasis Setifera plant in pure and improved samples, saponin extracted from Soapwort plant, surfactant synthesized from natural oils and fats such as anionic surfactant synthesized from Rapeseed oil and anionic surfactant synthesized from waste chicken fat, mucilage extracted from Hollyhocks plant as a natural polymer, methanol and acetone as mutual solvents and an anionic fluorinated surfactant were investigated. Some of them were used in combination with each other or other materials such as conventional polymers and alkalis. New materials were extracted, synthesized and characterized. Various experiments such as surface and interfacial tension, wettability and contact angle, foam analysis and emulsion stability, surfactant adsorption on rock and flooding under different scenarios were performed according to the methodology of each additive. The results of this project, considering the materials used for chemical enhancement of injected water, are summarized as follows: - Mutual solvents: Both methanol and acetone reduced water-oil interfacial tension more than diluted saline water. These solvents had a great effect on reducing the contact angle and wettability towards hydrophilicity. Besides, the addition of acetone to injected water increased oil swelling. - Saponin extracted from Anabasis Setifera plant: This non-ionic surfactant had a critical micelle concentration (CMC) equivalent to 3000 ppm at 75 °C. The surfactant solution in CMC reduced the interfacial tension of water and oil to 1.066 mN/m. The interfacial tension values in the optimal salinities resulting from the dissolution of different mineral salts were again reduced. This surfactant changed the wettability of carbonate rock to hydrophilicity by recording a contact angle of 56.5° and finally, a 15.4% increase in oil recovery was achieved by surfactant flooding in CMC and optimal salinity into a carbonate plug. Injection of pre-generated foam from the surfactant solution at the optimum concentration resulted in 66% oil recovery from a fractured carbonate plug. - Modified saponin extracted from Anabasis Setifera plant: This non-ionic surfactant performed better than its prototype. A CMC of 4000 ppm at 75 °C was obtained for it. Interfacial tension of 3.6×10−2 mN/m and contact angle of 86.1° were obtained in CMC. Finally, an increase in oil recovery of 19.1% was achieved by injecting surfactant-alkali slug into a carbonate plug. - The anionic surfactant synthesized from Rapeseed oil had a CMC of 4500 ppm at 80 °C. The interfacial tension of water-oil at this concentration was equal to 3.4×10−2 mN/m. The wettability of the sandstone/carbonate composite changed to hydrophilicity. An increase in oil recovery of 14.6−25.7% was achieved under different injection scenarios into sandstone/carbonate composite plugs. The combination of dissolved carbon dioxide with surfactant solution at different concentrations improved the EOR parameters such as interfacial tension, wettability and oil swelling. - Anionic surfactant synthesized from waste chicken fat recorded a CMC equivalent to 5500 ppm at 75 °C. This surfactant reduced the interfacial tension to 4.3×10−2 mN/m and altered the wettability of carbonate rock to hydrophilicity. A 17.8% increase in oil recovery was achieved by injecting an alkali-surfactant-polymer (ASP) slug into a carbonate plug. - Saponin extracted from Soapwort plant: This nonionic surfactant had a CMC of 2250 ppm at 80 °C. The interfacial tension at this concentration decreased to 0.834 mN/m and the sandstone wettability shifted to hydrophilicity. Finally, a 32.1% increase in oil recovery was achieved by injecting ASP-slug into a sandstone plug. - Polymer extracted from Hollyhocks plant: This polymer increased the viscosity of the injected fluid to suitable values for EOR and its non-Newtonian behavior was confirmed due to changes in the polymer solution viscosity against increasing shear rate. Finally, injection of the optimal solution containing this polymer and anionic surfactant synthesized from waste chicken fat and alkali in the volume of 0.5 PV into a sandstone plug increased the oil recovery by 27.9%. - Synthesized anionic fluorinated surfactant: This surfactant had a CMC of 3500 ppm at ambient temperature and changed the carbonate rock wettability to gasophilic proportion to the surfactant concentration.