Investigation of Shale Wettability Alteration upon Exposure to Surfactants

The development of unconventional resources such as shales has gained great popularity in the past decade. The objective of this study is to investigate the effect of surfactants on the wettability of shale rocks. In particular, we examine the influence of different concentrations of CTAB and SDBS surfactants on Eagle Ford, Wolf Camp and Mancos shale samples to determine their wettability alteration potential at the macro-scale. In this work, macro-scale contact angle (CA) measurements of the three studied shale samples were conducted at ambient conditions as a function of surfactant concentration. Additionally, rock surface imaging was conducted via Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) at the nano- and micro-scale respectively. Surface chemistry was also investigated through zeta potential and Fourier-transform infrared spectroscopy (FTIR) analysis to understand the interactions at the surfactant-mineral interface and its associated impact on wettability alteration. The results indicate that the wettability alteration potential of surfactants on shale surfaces is closely related to rock minerology, while it is a relatively weak function of surfactant concentration. The contact angle results of Eagle Ford and Wolf Camp indicate mixed-wet conditions, while Mancos indicates water-wetness. The contact angle results at high pressure show that the increase in pressure leads to contact angle increase. The results also show that zeta potential results for all shales tends to increase in magnitude as the concentration of both surfactants increase. Additionally, FTIR results indicate the presence of C-O (carboxylate group), Ca-C, and Si-O bonds on the surfactant-treated surfaces at different extent. Finally, surface topography images revealed that Eagles ford and Wolf Camp have a relatively higher surface roughness compared to Mancos. This study aims to develop scientific understanding of the different shale compositions and surfactant wettability alteration of shale rocks from a micro-scale perspective.