Interactions between hydraulic fracturing fluid (frac fluid) and shale oil & gas reservoirs produce distinctive and measurable geochemical signatures that constrain in-situ processes. We evaluate these interactions using geochemical experiments that react shale with proppant (bauxite), frac fluid, ± formation water at elevated pressure (35 MPa) but room temperature (25°C) and at reservoir conditions (35 MPa and 115°C). The duration of most experiments ranges from 28 to 43 days, time frames consistent with historic field operations in the Rocky Mountain West. The duration of one experiment, 215 days, is consistent with time frames for extended well “shut in”. Frac fluids used in the experiments possess a range of ionic strengths (I = 0.015, 0.15, and 1.5 molal) and initial pH values (~2.5 and 7.3). The pH of acidic frac fluid reacted with shale increased from 2.5 to 6.2 within 0.5 hrs in an experiment at reservoir pressure (35 MPa) and room temperature (25°C). The pH of near-neutral frac fluid reacted with shale decreased from 7.3 to 6.9 at these same pressure-temperature conditions. A subsequent experiment injected acidic frac fluid (pH 2.31) into an ongoing reaction of shale + formation water (pH 7.34) at reservoir pressure and temperature. The pH of this experiment decreased to 6.81 after 0.5 hrs and 6.61 after 3 hours; pH subsequently increased to 6.8 after 24 hours. In all experiments, carbonate dissolution released Ca and Sr into solution and feldspar dissolution released SiO<sub>2</sub> and Li; concentrations of these solutes remained elevated for the duration of the experiments. Concentrations of other metals (e.g., V and Ni) initially increased due to reaction of frac fluid and shale but subsequently decreased during the course of the experiments. Reaction progress was greater in the experiments containing acidic frac fluid. Preliminary analysis of shale reacted with frac fluid suggests that feldspar reacts to form kaolinite. In the experiment with a mixture of formation water and frac fluid, the fluid is saturated with respect to Mg-rich smectite. Reaction of shale with frac fluid initially releases ions into solution, but formation of secondary minerals provides sinks for some of these ions.

Fluid-rock interactions between hydraulic fracturing fluid and shale oil & gas reservoirs: Experimental insights

251st National Meeting (2016)

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