Effect of the Fluid–Shale Interaction on Salinity: Implications for High-Salinity Flowback Water during Hydraulic Fracturing in Shales

Hydraulic fracturing has been widely implemented to enhance hydrocarbon production from shale reservoirs. However, one of the main challenges during hydraulic fracturing is to understand what factor(s) trigger high salinity of flowback water, which sometimes can be up to 300 000 mg/L. While several mechanisms have been proposed to explain the controlling factor behind the high salinity of flowback water, there has been little discussion about the effect of fluid–shale interactions (e.g., mineral dissolution and surface complexation) on the high salinity and far less attention has been paid to quantify the contribution of fluid–shale interactions. We thus conducted spontaneous imbibition experiments using deionized water and outcrops from Marcellus, Barnett, and Eagle Ford shale plays with minor precipitated salts. We also monitored the pH, electrical conductivity, and ion concentrations (Cl^–, K⁺, Ca²⁺, NO₃^–, F^–, Br^–, and NH⁺) of the surrounding water during the spontaneous imbibition process in 4 consecutive weeks. To quantify the impact of fluid–shale interactions on salinity, we performed geochemical modeling to examine the contribution of mineral dissolution (calcite, albite, quartz, chalcopyrite, pyrite, and dolomite) and surface complexation on fluid salinity. Spontaneous imbibition tests show that Barnett shale plugs imbibed more water than Marcellus and Eagle Ford largely as a result of the highest content of calcite and lowest content of organic carbon. The order of sequence of pH is Eagle Ford (8.3) > Barnett (8.0) > Marcellus (7.6), in line with prediction of geochemical modeling, confirming that pyrite oxidation plays a significant role in local pH and, thereby, brine composition at ambient conditions. Geochemical modeling also shows that salinity increment induced by fluid–shale interactions is less than 3% of flowback water salinity, suggesting a minor impact of fluid–shale interactions on the salinity increase. These findings point out that fluid–fluid and fluid–salt interactions likely play a more important role in high salinity of flowback water during hydraulic fracturing in shales.