Effect of Surfactant Adsorption on the Wettability Alteration of Gas-Bearing Shales

Water loss in low permeability reservoirs during hydraulic fracturing well completions typically results in a decrease in natural gas production due to capillary trapping near the fractures. Shale gas reservoirs, however, have shown a trend of improved gas production with increased loss of completion fluids to the shale. This nonintuitive relationship between water imbibition and enhanced gas production in shale gas reservoirs is explored here through investigation of shale wettability alteration after exposure to two surfactants, one cationic and one anionic, commonly used in hydraulic fracturing fluids. Wettability alteration of samples from two unconventional natural gas reservoirs, the Marcellus and Collingwood shales, was examined in this study. In addition to individual surfactant solutions, 1:1 mixtures of cationic and anionic surfactants were examined at concentrations above and below critical micelle concentration levels. This study provides deeper understanding of adsorption mechanisms of cationic, anionic, and mixed ionic surfactants on the Marcellus and Collingwood reservoirs. Mixed surfactants were observed to alter wettability of shale from intermediate water-wet to more oil-wet and lower the capillary pressure and interfacial tension between gas and liquid phases at very low concentration (<0.45 mM). Such a reduction in capillary pressure may reduce capillary trapping of natural gas by imbibed treatment water and may help explain why natural gas production in the Marcellus and Collingwood has been observed to increase even after water is lost to the reservoir during shut-in periods.