UNDERSTANDING IN-SITU FLUID-ROCK REACTIONS IN SHALES USING HIGH TEMPERATURE-PRESSURE REACTORS

Over the last decade, there has been a tremendous growth in hydraulic fracturing (HF) operations to produce natural gas from shale reservoirs. More than 300,000 shale gas wells have been drilled in the U.S., and each well at an average consumes approximately 1 million gallons of water for hydraulic fracturing. A fraction of this water returns to the surface as flowback or produced water and can be reused and recycled for other HF operations. However, lack of understanding of shale-fracturing fluid interactions taking place at reservoir limit reuse and recycling of produced water. In addition, variations in mineralogy and maturity of the reservoir can significantly affect the fluid-rock reactions and the chemistry of produced water. In this study, experiments were conducted using high P-T autoclave reactors to better understand the chemical (inorganic and organic) changes that take place in the fracturing fluid and rock during the shut-in phase of HF operations in Marcellus Shale. Our results indicate a decrease in Ba²⁺ ions, an increase of SO4^2- and PO4^3- ions, and barite precipitation in all fluid-shale reaction set-ups. However, the concentrations of these ions are primarily controlled by variations in shale mineralogy. Further, we also note a decrease in the release of organic compounds such as BTEX (benzene, toluene, ethylene, and xylene) with increasing maturity. Our observations suggest that maturity (of organic matter) and mineralogy play a key role in the adsorption/release of organic and inorganic compounds during HF operations. Therefore, a comprehensive understanding of mineralogy and maturity in different parts of the basin is necessary to design water reuse and recycling strategies.