Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 ºC and implications for hydraulic fracturing

Abstract Shale hydraulic fracturing involves the injection of oxygenated acidic fluids which trigger redox driven and acid driven reactions, in particular with pyrite and carbonates. These reactions alter the shale pore and fracture network and mobilize potentially harmful elements. Hence, understanding the kinetics of pyrite oxidation at shale reservoir conditions is critical. Here, we investigate the effect of salinity on the rate of pyrite oxidative dissolution by fluids of pH 1 at 60 oC, and 0 to 3.4 mol L−1 NaCl. We determine a pyrite dissolution rate of about 1.2 x 10−9 mol m−2 s−1 in non-saline oxygenated acidic fluids at 60 oC, consistent with previously established rate equations, considering an activation energy of 57 kJ mol−1. Our results suggest the following correlation between the pyrite dissolution rate r (in mol m−2 s−1) at 60 oC and Eh (0.580 to 0.700 V), log(r) = 4.47 Eh – 12.03. This reaction order is lower than that established in previous studies involving pyrite reaction with Fe3+ enriched fluids in the presence of dissolved O2. Furthermore, we calculate that the pyrite oxidation rate varies from 0.8 x 10−10 to 1.3 x 10−10 mol m−2 s−1 at 25 oC, based on salinity-determined dissolved oxygen content. The mixed flow reactor experimental results suggest a dependence of pyrite oxidation rate on Cl− activity aCl (in mol kg−1) at 60 oC for fluids with 0.6 to 3.4 mol L−1 NaCl and Eh of 612 ± 8 mV as follows, log(r) = –0.58 log(aCl) – 9.42. The pyrite oxidation rate varies within less than half an order of magnitude for the salinity range tested, and hence, the impact of salinity is less important than the Eh and temperature. The improved understanding of factors controlling pyrite oxidation kinetics may help in predicting environmental and engineering impacts of injection of hydraulic fracturing fluids in shale.