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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