Understanding the water uptake of gas shales is critical for designing and optimizing hydraulic fracturing operations during which a large volume of fracturing water containing dissolved oxygen is injected into tight reservoirs. Recent studies show that the dissolved oxygen may promote oxidation reactions which can affect salinity and pH value of flowback water; however, the effects of dissolved oxygen and oxidation reactions on water imbibition into the shale matrix and on the concentration of individual ions in flowback water are still poorly understood. In this study, we conduct water imbibition experiments under degassed and oxic conditions, and measure the imbibed water mass and concentrations of different ions in water. The results show that the initial rate and final amount of water imbibition are higher under degassed conditions compared with that under oxic conditions. These differences are mainly due to the enhanced dissolution of air in the shale pore network into the imbibing water under degassed conditions and the consequent increase in relative permeability of water. The results also suggest that oxidation of pyrite by dissolved oxygen produces sulfate and iron ions. The abundance of pores in the vicinity of pyrite minerals as observed in the SEM/EDS images provides a pyrite, water, and oxygen rich environment which favors pyrite oxidation.

中文翻译：

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溶解氧对页岩水吸水的影响

了解气页岩的吸水率对于设计和优化水力压裂作业至关重要，在该过程中，将大量含有溶解氧的压裂水注入致密储层中。最近的研究表明，溶解氧可能促进氧化反应，从而影响返水的盐度和pH值。然而，对溶解氧和氧化反应对水吸收到页岩基质中以及对返排水中单个离子浓度的影响仍然知之甚少。在这项研究中，我们在脱气和有氧条件下进行了吸水实验，并测量了吸水量和水中不同离子的浓度。结果表明，与在有氧条件下相比，在脱气条件下吸水的初始速率和最终吸水量更高。这些差异主要是由于页岩孔隙网络中的空气在脱气条件下增加了向吸入水中的溶解度，并因此增加了水的相对渗透率。结果还表明，溶解氧将黄铁矿氧化会生成硫酸根和铁离子。在SEM / EDS图像中观察到的黄铁矿矿物附近的大量孔隙提供了有利于黄铁矿氧化的黄铁矿，水和氧气丰富的环境。这些差异主要是由于页岩孔隙网络中的空气在脱气条件下增加了向吸入水中的溶解度，并因此增加了水的相对渗透率。结果还表明，溶解氧将黄铁矿氧化会生成硫酸根和铁离子。在SEM / EDS图像中观察到的黄铁矿矿物附近的大量孔隙提供了有利于黄铁矿氧化的黄铁矿，水和氧气丰富的环境。这些差异主要是由于页岩孔隙网络中的空气在脱气条件下增加了向吸入水中的溶解度，并因此增加了水的相对渗透率。结果还表明，溶解氧将黄铁矿氧化会生成硫酸根和铁离子。在SEM / EDS图像中观察到的黄铁矿矿物附近的大量孔隙提供了有利于黄铁矿氧化的黄铁矿，水和氧气丰富的环境。