The release of organic and inorganic compounds from two different black shales into a lab-made stimulation fluid was studied over time to provide a first idea of the main factors influencing the inorganic and organic composition of flowback water. Two long-term experiments at 100°C / 100 bar, were performed with samples from Alum and Posidonia shale, using a stimulation fluid containing commercially–available biocide, surfactant, friction reducer and clay stabilizer. Our results show that the amount of dissolved inorganic constituents at the end of the experiment is independent of the pH of the stimulation fluid but highly dependent on the composition of the black shale and the buffering capacity of specific shale components, namely pyrite and carbonates. The amount of elements released into the fluid is also dependent on the residence time in as much as half of the measured 23 elements show highest concentrations within four days. We clearly demonstrate that the composition of the experimental fluid is effected by the natural organic matter composition and maturity of the shale and not just by the selected chemicals in the stimulation fluid. Alum and Posidonia shale extracts tend to have their specific composition of DOC. These patterns in DOC composition are also released in presence of DOC-rich stimulation fluids. Due to this observation it can be assumed that even with the application of DOC-rich stimulation fluids, the resulting flowback waters tend to have very shale-specific compositions of DOC and these compositions also may change over time. The compositions of the stimulating fluids have the potential to change the mobilization of organic compounds from shales. The high yields of formate from Posidonia samples differ from earlier studies evaluating the potential of shales to release formate as being low (Olsson et al., 2013). It is not clear whether the formate released from the natural organic matter of the shale may have been enhanced by the chemical additives or whether the organic polymers within the stimulation fluids may have been the subject of decarboxylation and generation of formate at the environmental conditions of the shale formations. These results may also complicate the application of formate and acetate concentration as tracers for stimulation fluids. Reference: Olsson, O., Weichgrebe, D. and Rosenwinkel, K.-H. (2013) Environmental Earth Sciences, 70, 3895-3906.

Autoclave experiments to investigate possible interactions between black shales and stimulation fluid during unconventional gas production

249th National Meeting (2015)

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