Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing
Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing
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DOI：
10.1016/j.jngse.2020.103722
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发表时间：
2020-11
影响因子：
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通讯作者：
Veerle Vandeginste;Andris Siska;G. Belshaw;A. Kilpatrick
中科院分区：
文献类型：
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作者：
Veerle Vandeginste;Andris Siska;G. Belshaw;A. Kilpatrick
页岩水力压裂涉及注入氧化的酸性流体，这些流体会引发氧化还原和酸驱动的反应，特别是与黄铁矿和碳酸盐的反应。这些反应改变了页岩的孔隙和裂缝网络，并调动了潜在的有害元素。因此，了解页岩储层条件下的黄铁矿氧化动力学是至关重要的。在这里，我们研究了盐度对pH为1的60℃，0-3.4mol的L−1NaC溶液中黄铁矿氧化溶解速度的影响。考虑到活化能为57kJ/−·−_2s·−_1，我们测定了60℃时黄铁矿在无盐含氧酸性流体中的溶解速率约为1.2kJ×10~(-10)−·m·−_2s·−_1，与先前建立的速率方程一致。我们的结果表明，60℃时黄铁矿的溶解速率(以摩尔·−_2s·−_1为单位)与Eh(0.580-0.700−)，log(R)_1=44.47Eh-12.03.这一反应级数低于之前有关黄铁矿在溶解O2存在下与富含Fe3+的流体反应的研究中建立的级数。此外，根据盐度测定的溶解氧含量，我们计算出在25℃时，黄铁矿的氧化速率从0.8M×10−10到1.3M×10−10mol/m−2S−1。混流反应器实验结果表明，在60℃下，当流体中含有0.6-3.4mol.6-3.4mol.L−-−，Eh为612±9.8 mV时，黄铁矿氧化速率与Cl-−活度aCl(单位：摩尔·公斤·氯化钠)的关系如下：−(R)=0.58log(Acl)-9.42.在测试的盐度范围内，黄铁矿的氧化速率变化不到半个数量级，因此，盐度的影响没有Eh和温度那么重要。提高对控制黄铁矿氧化动力学因素的认识，可能有助于预测页岩中注入水力压裂液的环境和工程影响。
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 °C, and 0–3.4 mol L−1NaCl. We determine a pyrite dissolution rate of about 1.2 × 10−9mol m−2s−1in non-saline oxygenated acidic fluids at 60 °C, 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 rater(in mol m−2s−1) at 60 °C and Eh (0.580–0.700 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 × 10−10to 1.3 × 10−10mol m−2s−1at 25 °C, based on salinity-determined dissolved oxygen content. The mixed flow reactor experimental results suggest a dependence of pyrite oxidation rate on Cl−activityaCl(in mol kg−1) at 60 °C for fluids with 0.6–3.4 mol L−1NaCl 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.