| dc.description.abstract | Experimental observations and field applications have revealed that surfactants have significant potential to enhance oil recovery in unconventional liquid reservoirs (ULR). The objectives of this study are to determine the effect of surfactant molecular structure on wettability alteration and to evaluate the performance of surfactant formulation in enhanced oil recovery (EOR). A comprehensive workflow is proposed for surfactant selection including surfactant screening, a novel surfactant stability test, and spontaneous imbibition experiments. The mechanism of wettability alteration is investigated for different surfactant structures. Contact angle (CA) 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 workflow to test surfactant stability was proposed to evaluate the duration of wettability alteration. Surfactants having great wettability alteration were utilized to perform Surfactant Assisted Spontaneous Imbibition (SASI) to assess their performance in improving oil recovery. Wettability was measured on a glass slide using different surfactants to further understand the mechanism of wettability alteration on a smooth surface. A systematic analysis was performed to demonstrate the correlation between surfactant performance and molecular structure. The primary recovery mechanism of surfactant EOR in ULR is highly influenced by wettability alteration, IFT reduction, and the retention time of surfactant on the rock surface. IFT and contact angle measurements were performed with anionic, cationic, nonionic, and zwitterionic surfactants to evaluate the effectiveness of these surfactants. Results indicate that both hydrophilic head and hydrophobic tail 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 lower range. Surfactant stability tests demonstrate that surfactants with greater stability are more favorable for spontaneous imbibition. The performance of nonionic surfactants is governed by the number of Ethylene Oxide (EO) groups. In this study, nonionic surfactants generally have better performance than ionic surfactant. It is attributed to that nonionic surfactants have higher adsorption density and more stable adsorption layer than ionic surfactants. At reservoir temperature, some nonionic surfactants reach cloud points that obscure CA measurements. The stability test provides a method to assess the wettability of rock using those nonionic surfactants. The proposed selection method is validated by the results of surfactant screening, stability tests, and SASI experiments. This work provides a more definitive interpretation of the surfactant molecular structure on rock-fluid interactions in ULR. | |
