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.

中文翻译：

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盐度对 60 ºC 含氧流体中黄铁矿溶解动力学的影响以及对水力压裂的影响

摘要 页岩水力压裂涉及注入含氧酸性流体，引发氧化还原驱动和酸驱动反应，特别是与黄铁矿和碳酸盐的反应。这些反应改变了页岩孔隙和裂缝网络，并动员了潜在的有害元素。因此，了解页岩储层条件下黄铁矿氧化的动力学至关重要。在这里，我们研究了盐度对 60 oC、pH 1 和 0 至 3.4 mol L-1 NaCl 的流体对黄铁矿氧化溶解速率的影响。考虑到 57 kJ mol-1 的活化能，我们确定在 60 oC 的非盐水含氧酸性流体中黄铁矿的溶解速率约为 1.2 x 10-9 mol m-2 s-1，这与先前建立的速率方程一致。我们的结果表明，60 oC 下的黄铁矿溶解速率 r（以 mol m−2 s−1 为单位）与 Eh（0.580 至 0.700 V）之间存在以下相关性，log(r) = 4.47 Eh – 12.03。该反应级次低于先前研究中建立的反应级次，该研究涉及在溶解的 O2 存在下黄铁矿与富含 Fe3+ 的流体反应。此外，我们计算出黄铁矿氧化速率在 25 oC 时从 0.8 x 10-10 到 1.3 x 10-10 mol m-2 s-1，基于盐度确定的溶解氧含量。混流反应器实验结果表明，对于含有 0.6 至 3.4 mol L-1 NaCl 和 Eh 为 612 ± 8 mV 的流体，黄铁矿氧化速率在 60 oC 时依赖于 Cl- 活性 aCl（以 mol kg-1 为单位），如下所示，log (r) = –0.58 log(aCl) – 9.42。对于所测试的盐度范围，黄铁矿氧化速率在不到半个数量级的范围内变化，因此，盐度的影响不如 Eh 和温度重要。更好地了解控制黄铁矿氧化动力学的因素可能有助于预测在页岩中注入水力压裂液的环境和工程影响。