Effective, considerate shale play water management supports operations and protects the environment. A parameter often overlooked is total dissolved solids (TDS) of produced water from the formation. Knowledge of TDS is important to meet these dual goals. Subsurface TDS typically increases with depth. However, produced-water samples from the Eagle Ford Shale show a strong TDS decrease by a factor of ∼10 with increasing well depth (∼200 000 ppm at ∼2.5 km to 18 000 ppm at ∼3.6 km). Water stable isotopes strongly suggest that the low TDS is not due to dilution by meteoric water. Rather, we attribute the change to smectite-to-illite conversion, in which the smectite interlayer water is released into the pore space. Depth, temperature, and other related indicators (source for K, excess silica) support such a mechanism. In addition, water-isotope patterns and ⁸⁷Sr/⁸⁶Sr ratios suggest a conversion operating with limited contributions external to the shale. Order-of-magnitude calculations show that the 8% of mixed-layer clay present on average in the Lower Eagle Ford Shale is sufficient to bring formation water TDS to observed levels when some of the resident water is expelled. Understanding that the low salinity is an intrinsic property of the formation water rather than due to short-term mixing allows stakeholders to have a more optimistic outlook on water recycling and on using produced water for other uses (irrigation, municipal).

中文翻译：

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美国德克萨斯州Eagle Ford页岩中的盐度逆转和水清新

有效，周到的页岩游水管理可支持运营并保护环境。一个经常被忽略的参数是来自地层的采出水的总溶解固体（TDS）。TDS的知识对于实现这些双重目标很重要。地下TDS通常随深度增加。但是，随着井深的增加，鹰福特页岩的采出水样品显示TDS下降了约10倍（在约2.5 km时约200 000 ppm至在3.6 km时约18 000 ppm）。具有水稳定性的同位素强烈表明，低TDS并非归因于大气水的稀释。相反，我们将变化归因于蒙脱石到伊利石的转化，其中蒙脱石层间水被释放到孔隙空间中。深度，温度和其他相关指标（K的来源，过量的二氧化硅）支持这种机制。此外，⁸⁷ Sr / ⁸⁶ Sr比率表明，在页岩外部贡献有限的情况下进行了转换。数量级计算显示，当排出一些居民水时，下鹰福特页岩中平均存在的8％混合层粘土足以使地层水TDS达到观测水平。理解低盐度是地层水的固有属性，而不是由于短期混合所致，这使利益相关者对水的循环利用以及将生产的水用于其他用途（灌溉，市政）的看法更为乐观。