Changes to pore architecture due to geochemical interactions between stimulation fluid, formation water, and an unconventional gas shale

Hydraulic fracturing (stimulation) fluids may induce mineral-fluid reactions that change the pore architecture of unconventional reservoirs. This laboratory study evaluates potential mineral-fluid reactions between injected stimulation fluid, formation water, and gas shale. The Baxter Shale of the Green River Basin, Wyoming, USA was selected due to its TOC content (1.8-2%), thermal maturity in the gas window (VR=1.28-1.34), and relatively large proportion of carbonate (34 wt%). The shale was cut into 1-cm³ cubes; the faces of these cubes emulate a fracture wall in the reservoir. Two experiments reacted cubes of shale with formation water at 45 MPa and 125 °C to simulate reservoir conditions. One experiment was terminated after 4 weeks; it served as the control experiment. In the second experiment, stimulation fluid was injected after 4 weeks of reaction between shale and formation water. This experiment continued for an additional 4 weeks to simulate the shut in period of a hydraulically fractured reservoir. The chemistry of the stimulation fluid was based on stimulation fluids used in in the Rocky Mountain region. The stimulation fluid was an acidic (pH 1.8), NaCl water (I = 85 mmol/kg) containing Ca, Mg, HCO₃^-, and SO₄^2- (5-50 mmol/kg). Two chemicals commonly used in stimulation fluids were added, KCl (14 mmol/kg) as a clay stabilizer and acetic acid (3 mmol/kg) to inhibit precipitation of Fe oxyhydroxides. Increases in aqueous Ca and SiO₂ in the injection experiment suggest that shale reacted with the formation water-stimulation fluid mixture. Dissolved O₂ in the stimulation fluid was consumed by reaction a few hours after injection, suggesting that O₂ loading from injected fluid does not yield Fe oxyhydroxide scale. SO₄^2- in the stimulation fluid combined with Ba in the formation water to precipitate barite scale. No mineral alteration was observed in SEM micrographs. N₂ adsorption data were interpreted via BET surface area as well as BJH and DFT pore-size distribution. Results indicate that geochemical reactions did not alter mesopores (2-50 nm). Macropores (>50 nm) appear affected for control and reacted samples relative to unreacted samples, suggesting redistribution of material. Alteration of macropore architecture doesn't necessarily alter flow properties; permeability measurement is a key next step.