--> --> Geochemical interactions between stimulation fluid, formation water, and a gas shale from the Green River Basin, Wyoming

AAPG Rocky Mountain Section Meeting

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Geochemical interactions between stimulation fluid, formation water, and a gas shale from the Green River Basin, Wyoming

Abstract

Hydraulic fracturing fluids may induce mineral-fluid reactions in unconventional reservoirs. This laboratory study evaluates the potential for mineral-fluid reactions between injected stimulation fluid, formation water, and gas shale. The Baxter Shale of the Green River Basin, Wyoming was selected for this study 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 Baxter Shale with formation water at 45 MPa (6,525 psi) and 125 °C (255 °F) to simulate the reservoir. One experiment was terminated after ~4 weeks of reaction between shale and formation water; it served as the baseline experiment. In the second experiment, stimulation fluid was injected after ~4 weeks of reaction between shale and formation water. This injection 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 the composition of fluids used in hydraulic fracking in the Rocky Mountain region. The fluid was an acidic (pH = 1.8), NaCl water (5400 ppm TDS) containing Ca, Mg, HCO3 -, and SO4 2- (100’s ppm). Two chemicals commonly used in stimulation fluids were added, KCl (540 ppm) as a clay stabilizer and acetic acid (200 ppm) to inhibit precipitation of Fe oxyhydroxides. BET surface area, BJH pore size distribution, and MICP permeability measured for reacted shale from both experiments were approximately the same. Water samples collected from the injection experiment suggest that the shale began to react with the mixture of formation water and stimulation fluid on the time scale of the experiment, but reaction was limited to changes in water chemistry and no mineral alteration was observed. Dissolved O2 in the stimulation fluid was consumed by reaction with rock within a few hours after injection, suggesting that O2 loading from injected fluids will not lead to Fe oxyhydroxide scale. However, the sulfate content of the stimulation fluid, combined with Ba present in the formation water, generated barite scale.