Experimental investigation of high-temperature brine-shale interactions

Anna Atasha Hoffmann, Missouri University of Science and Technology

Abstract

Hydrofracturing (fracking), a common practice in the Petroleum Industry to induce or improve fluid flow in tight formations, creates chemical disequilibrium that further alters the porosity and permeability of host rocks and results in the production of saline and contaminated produced waters (PW). The PW of the Tuscaloosa Marine Shale (TMS) are Na-Ca-Mg-K-Cl brines with mean concentrations of approximately 16% Total Dissolved Solids (TDS) and circumneutral pH. Analysis of composition suggests the PW result from a 20 to 80% dilution of formation waters (relict brines of the Louann Salt) by fracking fluid. Trace element concentrations generally show moderate to strong correlations with overall salinity. Low concentrations of Pb and Zn suggest the presence of high amounts of H2S in the TMS, limiting the solubility of sulfide phases. Fracking is also used in wastewater disposal and enhanced geothermal system (EGS) energy production, where chemical variations of injection fluid affect the disequilibrium reactions in the host rocks. To improve our understanding of fluid-shale interactions over a broad range of fluid chemistries, we performed 56-day batch reaction experiments between TMS shales and solutions with combinations of the following initial values; (1) pH of 2, 4, or 6 (2) salinities of <0.1%, 3.2%, or 12% TDS, and (3) NaCl or Na-Ca-Mg-K-Cl compositions. Findings show that while the timing and extent of reactions varied with the initial fluid chemistry, resulting solutions converged to similar end chemistries, indicating that wastewater disposed or reused as fracking fluid into parent formations and the cycling of water-based EGS fluids should not have a significant secondary impact on host formation porosities.