Surfactant selection for enhanced oil recovery based on surfactant molecular structure in unconventional liquid reservoirs

Experimental observations and field applications have revealed that surfactants have significant potential to enhance oil recovery in unconventional liquid reservoirs (ULR). This study is designed to determine the effect of surfactant molecular structure on wettability alteration as measured via the contact angle (CA). The relation between contact angle and spontaneous imbibition in enhanced oil recovery (EOR) is determined. A systematic workflow is proposed for surfactant selection based on maximizing wettability alteration. The mechanism of wettability alteration is investigated as a function of the hydrophobic tail group and the hydrophilic head group of various surfactant structures. Contact angle and interfacial tension (IFT) were measured for crude oil and oil-saturated samples from the Eagle Ford and Wolfcamp formations using various surfactant formulations. A novel workflow to test surfactant stability defined as retention of surfactant layers on the rock surface was proposed to evaluate the duration of wettability alteration. Surfactants exhibiting maximum wettability alteration were utilized to perform Surfactant Assisted Spontaneous Imbibition (SASI) to assess their performance on improving oil recovery. A comprehensive analysis was performed to demonstrate the correlation between surfactant performance and molecular structure. The primary production mechanism of surfactant EOR in ULR is highly influenced by wettability alteration, IFT reduction, and the retention of surfactant on the rock surface. CA and IFT measurements were performed with anionic, cationic, nonionic, and zwitterionic surfactants to evaluate the effectiveness of these surfactants. Results indicate that both the hydrophilic head and hydrophobic tail of surfactants have significant effects on wettability alteration. Surfactants with longer tails could effectively alter the wettability of rock from oil-wet to more water-wet and reduce the IFT to a lower range. Surfactant stability tests demonstrate that surfactants with greater stability are more favorable for spontaneous imbibition. In this study, nonionic surfactants generally show better performance than ionic surfactants. The performance of nonionic surfactant is governed by the number of Ethylene Oxide (EO) groups. It is attributed to the fact that nonionic surfactants have greater adsorption density and a more stable adsorption layer than ionic surfactants. At reservoir temperature, some nonionic surfactants reach cloud points that obscure CA measurements and reduce the efficacy of surfactants. The stability test presented in this work provides a method to assess the duration of wettability alteration of rock surface using those nonionic surfactants. The proposed selection method is validated by the results of surfactant screening, stability tests, and SASI experiments. This study addresses the importance of surfactant structure and stability during EOR operations in ULR. This work provides a more definitive interpretation of the surfactant molecular structure on rock-fluid interactions in ULR.