Adding a surfactant to completion fluids has been widely applied to improve the initial oil production from unconventional liquids reservoirs (ULR). However, very limited research has been conducted to understand the interactions between surfactants and salt in ULR, and to identify the impact of Zeta Potential (ZP), Interfacial Tension (IFT), and Contact Angle (CA) on oil recovery through Surfactant-Salt Assisted Spontaneous Imbibition (SSASI). This study investigates the interactions of surfactant and salinity in different lithology of ULR in West Texas and determines the relationship between the experimented parameters to oil recovery to provide potential screening criteria for completion fluid design.

With the combination of numerous chemicals at different concentration and nine salinity levels, more than fifty variations of aqueous phase solution were blended. Also, heterogeneous ULR rock samples of West Texas formation with two different dominant lithologies, quartz-rich and carbonate-rich, were selected. All combinations of fluid were used for ZP, IFT, and CA measurements, and some were selected for SSASI experiments with timely CT-scans. Then, all experimented results were plotted against oil recovery factor to determine the most impactful parameter on oil recovery and to obtain the knowledge of desired conditions of salinity for surfactant-added completion fluid.

The impact of salinity on ZP, IFT, and CA had the similar trend for all fluid cases. With increasing salinity, the magnitude of zeta potential decreased close to 0 mV, meaning thin or no electric double layer (EDL) surrounding the rock particles. Surfactant and salt reduced IFT strongly until it reached the critical salt concentration (CSC) of 30,000 ppm, then reduction occurred gradually and slowly. Surface wettability was different for two rock types, and the ability to alter wettability varied by the condition of fluids. Wettability alteration occurred with the presence of both surfactant and salt, but most effectively at the salinity between 20,000 and 30,000 ppm. Also, the wettability of quartz-rich rock type showed more water-wetting surface compared to carbonate-rich rock type. Out of all experimented parameters, CA showed the strongest impact on oil recovery. Only 5 to 10% of the oil was produced when rock surface is oil-wet, but the oil recovery increased up to 25 % when the surface became intermediate-wet. Finally, when the rock surface became water-wet, 25 to 40% of the oil was produced from SSASI. Consequently, based on this investigation of surfactant and salt interactions, the salinity of 20,000 ~ 33,000 ppm was determined to be the most favorable condition for salt-surfactant added completion fluids, which effectively reduces IFT and yields the strongest wettability alteration.

Understanding the interactions between salt and surfactant, and their behaviors in different types of reservoirs are essential when to analyze the reusability or dilution of high salinity produced water and to successfully design completion fluids in ULR. The significance of this study is the ability to determine the most synergetic condition of salt-surfactant blended solutions and identify the most impactful parameter on oil recovery in ULR to provide surfactant screening criteria, and, furthermore, economic and environmental benefits by utilizing produced water and fresh water for completion activities.

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