The large quantities of wastewater produced throughout the lifetime of a shale gas well can contain heavy metals and other regulated potentially toxic elements. Their release from the target formation can be enhanced by some of the additives (e.g. ammonium persulfate, EDTA) present in the hydraulic fracturing fluids. High levels of inorganic geogenic chemicals may pose a hazard to the environment through accidental releases such as spills of untreated wastewater. The concentration of mobilised elements and the hazard they pose is uncertain and is likely dependent on the chemical agents used in fracturing fluids, composition of formation waters and the trace element content of targeted shale gas formation.
This study aimed to investigate the release of potential inorganic contaminants of concern (e.g. As, Co, Cu, Ni) from of shale gas formations from around the world. In systematic batch experiments at elevated temperature (80°C) and a range of pressures (1-200 bar), powdered samples were leached for up to 500 hours with synthetic hydraulic fracturing fluid (SHFF) and synthetic groundwater (SGW). Elemental concentrations released into solution were generally much higher in the SHFF leachates than in the SGW treatments, indicating that the chemical additives in the SHFF influenced element mobilisation.
Electron probe microanalyser (EPMA) and energy-dispersive X-ray (EDX) detector images showed mineral etching and precipitation of secondary phases on shale chips leached for 360 h with SHFF at 80°C and ~180 bar when compared to the SGW experiment. Time-series data also showed evidence of mineral dissolution and subsequent precipitation, which resulted in the sequestration of a number of trace elements that were initially mobilised into the solution. Additionally, carbonate content of the unreacted shale sample was the primary control on the final pH of the SHFF leachates.
This study shows that additives can enhance the release of geogenic chemicals, but also that subsequent precipitation within the fracture system could limit ultimate release to surface. Monitoring during field-operations is recommended to understand the systemspecific environmental implications and hazards.
|Date of Award||15 Sept 2021|
- University Of Strathclyde
|Sponsors||University of Strathclyde|
|Supervisor||Zoe Shipton (Supervisor) & Joanna Renshaw (Supervisor)|