WATER-ROCK INTERACTION IN A GAS SHALE: EFFECTS OF STIMULATION FLUID ON MINERALOGY AND POROSITY IN THE PRESENCE OF FORMATION WATER

Hydraulic fracturing fluids may induce water-rock interactions in reservoir rocks of the Baxter Shale, a productive formation for gas exploration in south central Wyoming. The primary objectives of this study are to evaluate potential reactions between the injected fracturing fluid and Baxter Shale and to assess the potential impact of these reactions on the porosity and minerology of the shale. The Baxter Shale was selected due to its TOC content (1.8-2%), thermal maturity in the dry gas window (VR=1.28-1.34), and having a higher percentage of calcite. Two batch hydrothermal experiments reacted core samples of the Baxter Shale with formation water for 4 weeks at 45 MPa and 125 °C to simulate the in-situ reservoir. Hydraulic fracturing fluid was then injected into one of the experiments to simulate a hydraulically fractured reservoir. This experiment continued for 4 weeks, the amount of time that fluid may remain within a single stage of the well before production begins. For both experiments, shale was cut into 12-15g cubes subsampled from a 3-inch core section. Scanning electron microscopy, mercury intrusion capillary pressure, and gas adsorption with N₂ were performed before and after the experiments to evaluate potential mineral and porosity changes. Approximately 12-15 aqueous and gas samples were collected from each experiment to record fluid chemistry and redox changes over time. Results suggest an increase in sample surface area by as much as 14% and negligible changes in shale pore size distributions. Geochemical modeling was able to reproduce the behavior of the major aqueous components and of the observed mineral dissolution from calcite and precipitation from clay minerals. Despite the reactions induced by the hydraulic fracturing fluid, change in shale porosity was negligible with a pore volume (pores from 1.7-300nm diameter) increase of only 3%.