Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States.
Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University, Stanford, California 94305-2115, United States.
Environ Sci Technol. 2020 Jun 16;54(12):7320-7329. doi: 10.1021/acs.est.0c01022. Epub 2020 May 28.
Uranium and other radionuclides are prominent in many unconventional oil/gas shales and is a potential contaminant in flowback/produced waters due to the large volumes/types of chemicals injected into the subsurface during stimulation. To understand the stability of U before and after stimulation, a geochemical study of U speciation was carried out on three shales (Marcellus, Green River, and Barnett). Two types of samples for each shale were subjected to sequential chemical extractions: unreacted and shale-reacted with a synthetic hydraulic fracture fluid. A significant proportion of the total U (20-57%) was released from these three shales after reaction with fracture fluid, indicating that U is readily leachable. The total U released exceeds labile water-soluble and exchangeable fractions in unreacted samples, indicating that fluids leach more recalcitrant phases in the shale. Radiographic analysis of unreacted Marcellus shale thin sections shows U associated with detrital quartz and the clay matrix in the shale. Detrital zircon and TiO identified by an electron microprobe could account for the hot spots. This study shows that significant proportions of U in three shales are mobile upon stimulation. In addition, the extent of mobilization of U depends on the U species in these rocks.
铀和其他放射性核素在许多非常规油气页岩中很突出，由于在刺激过程中向地下注入了大量不同类型的化学物质，它们是回流/产出水中的潜在污染物。为了了解刺激前后铀的稳定性，对三种页岩（马塞勒斯页岩、绿河页岩和巴尼特页岩）进行了铀形态的地球化学研究。对每种页岩的两种类型的样品进行了连续化学提取：未反应的和与合成水力压裂液反应的页岩。这三种页岩与压裂液反应后，有相当大比例的总铀（20-57%）被释放出来，这表明铀很容易浸出。与未反应样品中的可溶水相和可交换相相比，释放的总铀超过了实验室，这表明流体浸出了页岩中更顽固的相。对未反应的马塞勒斯页岩薄片的射线照相分析表明，铀与碎屑石英和页岩中的粘土基质有关。电子探针鉴定的碎屑锆石和 TiO 可以解释这些热点。这项研究表明，在三种页岩中，相当大比例的铀在刺激后是可移动的。此外，铀的迁移程度取决于这些岩石中的铀物种。
