Characterization and evaluation of a natural surfactant extracted from Soapwort plant for alkali-surfactant-polymer (ASP) slug injection into sandstone oil reservoirs

Abstract

Different types of surfactants are used in the chemical water flooding process in oil reservoirs with the aim of water-oil interfacial tension (IFT) reduction. Recently, much attention is paid to the use of natural surfactants such as plant extracts in enhanced oil recovery (EOR) process. However, some characteristics of these surfactants, such as their efficiencies in achieving the appropriate values of IFT, adaptability to different salinities and their temperature stability at reservoir conditions, must be acceptable. In the current work, a non-ionic surfactant was extracted from Soapwort plant under ultrasonic extraction and saponin purification processes. Proton Nuclear Magnetic Resonance (1HNMR), Fourier Transform Infrared Spectroscopy (FTIR) and Thermal Gravimetric Analysis (TGA) techniques were used to investigate the natural surfactant characteristics. The pendant drop surface tension tests were used to estimate the critical micelle concentration (CMC) of this natural surfactant. Finally, the application of the surfactant in the EOR process was demonstrated using the experiments of water-oil IFT, contact angle and alkali-surfactant-polymer (ASP) slug injection. The effects of different salinities on surfactant performance in reducing interfacial tension and contact angle were also investigated. Based on the results, the CMC of the surfactant was obtained at 2250 ppm and at 80 °C. The water-oil IFT at surfactant CMC was 0.832 mN/m, which decreased by 0.541, 0.714 and 0.775 mN/m at the optimal salinity of the diluted formation water, MgCl2 and NaCl solutions, respectively. The IFT was decreased by 0.047, 0.078 and 0.096 mN/m at the optimal concentration of NaOH, Na2CO3 and NaHCO3 alkalis, respectively. Sandstone wettability was altered from hydrophobicity to hydrophilicity by surfactant solution at CMC and the contact angle was decreased from 124.42° to 45.94°. The contact angle at CMC was decreased by 35.12°, 38.44° and 43.62° at the optimal concentration of FW, MgCl2 and NaCl, respectively and eventually, oil recovery was achieved by 32.1% by tertiary ASP slug injection containing surfactant at CMC, optimal saline, optimal NaOH alkali concentration and 1000 ppm of partially hydrolyzed polyacrylamide (PHPA). Surfactants are used in various applications in the oil industry. The focus of this study was on the use of the plant surfactant in EOR by ASP injection process. However, other applications of this surfactant for injection in various scenarios based on chemical water in EOR process can be developed.