Hydraulic
fracturing of unconventional hydrocarbon resources involves
the sequential injection of a high-pressure, particle-laden fluid
with varying pH’s to make commercial production viable in low
permeability rocks. This process both requires and produces extraordinary
volumes of water. The water used for hydraulic fracturing is typically
fresh, whereas “flowback” water is typically saline
with a variety of additives which complicate safe disposal. As production
operations continue to expand, there is an increasing interest in
treating and reusing this high-salinity produced water for further
fracturing. Here we review the relevant transport and geochemical
properties of shales, and critically analyze the impact of water chemistry
(including produced water) on these properties. We discuss five major
geochemical mechanisms that are prominently involved in the temporal
and spatial evolution of fractures during the stimulation and production
phase: shale softening, mineral dissolution, mineral precipitation,
fines migration, and wettability alteration. A higher salinity fluid
creates both benefits and complications in controlling these mechanisms.
For example, higher salinity fluid inhibits clay dispersion, but simultaneously
requires more additives to achieve appropriate viscosity for proppant
emplacement. In total this review highlights the nuances of enhanced
hydrogeochemical shale stimulation in relation to the choice of fracturing
fluid chemistry.