Toxic Metals in Shales: Questions and Methods for a Better Management of Flow-Back Waters

Toxic metals and in some case radionuclides can be concentrated in shales by comparison with others types of sedimentary rocks. These radionuclides and toxic metals (RTM) can be released by the rock in the flow-back waters and can cause environmental concerns. Their concentrations can also increase the cost of the water treatment and the complexity of the management of residual salts if they are potentially radioactive. A better anticipation of this problem can be made by knowing the minerals or organic phases bearing these elements. A modeling of their behavior in function of the components used in the fracturing fluid is also needed. Knowing that their bearing phases are mostly sulfide minerals (Pyrite…), it becomes obvious that acidic additives and above all oxidizing additives in the fracking fluids can strongly increase their concentrations in the flow-back waters. However, it is not always the case and for example Radium is not sensitive to the dissolution of minerals caused by acids or oxidants. To reduce the release of RTM, our approach is based on the idea that if there is less dissolution of minerals, the RTM will stay in the rock and will not concentrate in the flow-back waters. This can strongly reduce the cost of treatment of the flow-back waters and reduce the eventual environmental impact of the operations. Our method is firstly to identify the concentration of RTM in minerals by a precise mineralogical characterization. When the bearing phases are known, it is possible to model their dissolution in function of the nature of the fracking fluids. Then the challenge is to remove or replace the more aggressive additives. The idea is to maintain the fracking properties without the inconvenient of the dissolution of RTM bearing minerals. For this last point, leaching test of the rock by the new fracking fluids is required.