Ionic Strength and pH Effects on Water–Rock Interaction in an Unconventional Siliceous Reservoir: On the Use of Formation Water in Hydraulic Fracturing

Hydraulic fracturing employs a mixture
of freshwater and chemical
additives. Substituting produced formation water for freshwater conserves
freshwater resources, but formation water possessing a high ionic
strength may damage formations and inhibit production. Hydrothermal
experiments were conducted at reservoir conditions (115 °C, 35
MPa) to evaluate geochemical interactions between reservoir rock and
hydraulic fracturing fluid synthesized using formation water. The
specific hypotheses tested are (1) progressively greater ionic strength
and acidity enhance the solubility of carbonate and aluminosilicate
minerals and (2) acidity is more important than ionic strength for
carbonate solubility but ionic strength is more important for aluminosilicate
mineral solubility. Experiments replicated a shut-in well in the Wall
Creek Member of the Cretaceous Frontier Formation, Powder River Basin,
Wyoming , an important unconventional reservoir composed of low permeability
sandstones interbedded with organic-rich mudstones. The core was reacted
for ∼650 h (27 days) with (1) acidic hydraulic fracturing fluid
(pH ∼ 2.3) mixed from formation water spanning two orders of
magnitude ionic strength (0.016, 0.16, and 1.13 mol/kg) and (2) hydraulic
fracturing fluid (ionic strength 0.11 mol/kg) of circumneutral pH
(7.3). Reaction with hydraulic fracturing fluid of progressively greater
ionic strength increased calcite solubility, but acidity was more
important to calcite solubility than ionic strength. Trends of aqueous
silica, potassium, and magnesium are consistent with the dissolution
of feldspar and/or quartz and the precipitation of clay. Ionic strength
was more important than acidity for feldspar–clay equilibrium,
the predominant aluminosilicate mineral reaction. Hydraulic fracturing
fluid with a progressively greater ionic strength contained larger
amounts of aqueous silica, enhancing the potential importance of secondary
clay mineralization. Mineralogic evidence for dissolution was limited
to calcite and feldspar; no secondary clay was observed. Formation
water may be used for hydraulic fracturing fluids, provided that the
potential benefits and deleterious effects of calcite and feldspar
dissolution and secondary clay mineralization are assessed on a case-by-case
basis.