Evaluation of the Potential of Monovalent/Divalent Salts and Surfactants for Improved Oil Recovery from Unconventional Liquid Reservoirs

Abstract

Wettability alteration to water-wet and interfacial tension (IFT) reduction were found to play a major role in determining the rate of penetration of imbibing fluids into low permeability and porosity rocks where capillary imbibition dominates and is favored by a water wet rock nature and moderate to high IFT values. This study aims to investigate the effect of brines containing monovalent sodium and divalent calcium ions as well as of three types of surfactants; anionic, nonionic and a complex nanofluid, on alteration of the wettability and oil-water IFT of an unconventional liquid reservoir (ULR) with ultra-low permeability and porosity values and improvement of the resulting oil recovery from imbibition.

Contact angle and IFT measurements were conducted on sidewall cored rock samples obtained from a liquid rich shale play in South Texas which were followed by zeta potential measurements to analyze the stability of aqueous films on the ULR rock surface and understand the observed contact angle trends. IFT was measured using the pendant drop technique using the oil obtained from the same well. Spontaneous imbibition experiments were conducted and monitored periodically for fluid penetration and oil recovery. Salt concentrations varied from 1 wt% to 15 wt% while surfactant concentrations examined were 0.2 gpt, 1 gpt and 2 gpt, where gpt stands for gallons per thousand gallons.

Experimental results suggest that both salts and surfactants were able to alter wettability of samples from intermediate-wet to water-wet. Zeta potential results for salts suggest an optimum salinity where maximum water wetness is observed. Zeta potential results for surfactants validated the observed contact angle results as all the surfactants produced stable wetting films. Increasing surfactant concentration resulted in significant reduction of IFT, while increase in salt concentration lowered the crude oil/brine IFT marginally. Spontaneous imbibition suggests that wettability alteration is a more important factor than IFT reduction, as moderate to high IFT values favored imbibition in the ultra-low permeability rocks examined. This could be verified from the higher recoveries observed for sodium chloride and nonionic surfactant, both of which were able to alter wettability to water wet while producing high to moderate IFT values.