Hydrothermal experiments were conducted to evaluate mineral dissolution and precipitation that may occur during hydraulic fracturing of unconventional reservoirs. The B Bench of the Niobrara Formation and the Wall Creek Member of the Frontier Formation (Powder River Basin, WY) were reacted with a hydraulic fracturing fluid (pH = 2.5, ionic strength = 0.05) at 115 °C and 35 MPa for one month. Data collected from both experiments indicate that calcite began to dissolve in less than 50 h, Al-bearing solids began to dissolve immediately upon contact with hydraulic fracturing fluid, and an aluminosilicate precipitate began to form in less than 27 h. Data from the Wall Creek experiment suggest potential dissolution of chert. We estimate that calcite dissolution increased porosity of the B Bench from 1.7% to 2.8% and of the Wall Creek from 3.5% to 4.8%. Dissolution of calcite increases porosity and storage space for injected fluids. However, removal of calcite may weaken the mechanical integrity of the rock, leading to proppant embedment and fracture closure. Precipitation of an Al-bearing phase may decrease rock and fracture permeability. Aqueous Al in our experiments behaves similarly to aqueous Al collected during flowback from hydraulically fractured wells, suggesting that Al-bearing phases precipitate during time scales of hydraulic fracturing.

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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.

Hydraulic fracturing of low-permeability rocks significantly enhances hydrocarbon production from unconventional reservoirs. However, fluid transport through low-permeability rocks and the influence of geochemical transformations on pore networks are poorly constrained. Mineral reactivity during interactions with injected water may alter the physical nature of the rock, which may affect hydrocarbon mobility. To assess alterations to the rock, we have previously conducted a hydrothermal experiment that reacted cubed rock samples (1 cm 3 ) with synthetic hydraulic fracturing fluid (HFF) to simulate physicochemical reactivity during hydraulic fracturing. Here, we analyze unreacted and reacted rocks by small-angle neutron scattering and high-pressure mercury intrusion to determine how the pore networks of unconventional reservoir rocks are influenced by the reaction with hydraulic fracturing injectates. Our results suggest that fluid−rock interactions exhibit a two-fold influence on hydrocarbon recovery, promoting both hydrocarbon mobilization and transport. Pore−matrix interfaces smooth via the removal of clay mineral surface asperities, reducing the available surface area for hydrocarbon adsorption by 12−75%. Additionally, HFF-induced dissolution creates new pores with diameters ranging from 800−1400 nm, increasing the permeability of the rocks by a factor of 5−10. These two consequences of mineral dissolution likely act in concert to release hydrocarbons from the host rock and facilitate transport through the rock during unconventional reservoir production.

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A strategy for sustainable energy development couples recovery of critical metals to development of unconventional hydrocarbon reservoirs. Hydrothermal experiments were conducted at reservoir conditions (115°C, 35 MPa) to evaluate potential lithium mobility due to geochemical interactions between reservoir rock and hydraulic fracturing fluid spanning a range of chemical composition. Experiments replicated a shut-in well in the Wall Creek Member of the Cretaceous Frontier Formation, Powder River Basin, Wyoming, USA, an important unconventional reservoir composed of low permeability sandstones interbedded with organic-rich mudstones. Six samples representing the full extent of hydraulic fracture propagation across the Wall Creek Member (110 ft, 33.5 m) were collected from core and homogenized for use in the experiments. This rock mixture consisted of 33% quartz, 33% chert, 20% calcite, 6% illite, 5% albite, 1% kaolinite, 1% pyrite, 1% chlorite, and <1% TOC. Lithium was slightly above crustal abundance (20 ppm) in five samples (range 29-57 ppm); one sample contained lithium below crustal abundance. The rock mixture was reacted for ~650 hours (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). The fracturing fluid initially contained no Li(aq). Trends of aqueous SiO 2 , K, and Mg are consistent with dissolution of feldspar±quartz and precipitation of clay; mineralogic evidence for dissolution was limited to calcite and feldspar, no secondary clay was observed. Ionic strength was more important than acidity for feldspar-clay equilibrium. Progressively greater ionic strength of the hydraulic fracturing fluid, from 0.016 to 0.16 to 1.13 mol/kg, increased Li(aq) concentrations, from 132 to 201 to 250 ppb, at the time the final sample was collected from the experiments. Hydraulic fracturing fluid of circumneutral pH contained 167 ppb Li(aq) at the time the final sample was collected. Hydraulic fracturing fluids extracted lithium from the reservoir, but little harvestable lithium was produced in flowback and produced water.