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.