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

Abstract

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.

pH-responsive viscoelastic supramolecular viscosifiers based on dynamic complexation of zwitterionic octadecylamidopropyl betaine and triamine for hydraulic fracturing applications

“…67 In addition, shale reservoir stimulated by hydraulic fracturing uids tend to contain connate water with relatively high salinity. 68 Prior research indicated some of viscosifying agents such as wormlike micelles lose their viscous characteristics signicantly in the presence of salts. 69,70 Similarly, polymer viscosiers can degrade in the presence of salts.…”

Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing

“…Flowback waters from shale hydraulic fracturing have a high salinity. Fluid-shale interaction can increase the salinity of the fluid to some extent; however, the high salinity of flowback water from hydraulic fracturing of shale is more likely caused by fluid-fluid and fluid-salt interactions (Zeng et al, 2020). Mixing of the injected hydraulic fracturing fluid with saline formation water suggests that pyrite oxidation occurs in fluids that may have varying degrees of salinity.…”

Experimental Investigation of the Impacts of Fracturing Fluid on the Evolution of Fluid Composition and Shale Characteristics: A Case Study of the Niutitang Shale in Hunan Province, South China

“…In addition, most of the research did not explain what caused the high salinity of flowback water. The geochemical interaction between shale and fracturing fluid, which affects not only physical properties of the shale but also the composition of the waters produced during shale gas production, is an important controlling factor [13] but rarely has been discussed [30]. Therefore, more studies about the geochemical processes between fracturing fluids and the shale are required to evaluate and minimize the potential environmental impacts.…”