Understanding wastewater from hydraulic fracturing to minimise environmental risk
O'Donnell, Megan Catherine
Hydraulic fracturing to extract shale gas is widely regarded to be socially, environmentally, and politically controversial due to the perceived negative impacts of the process. One issue of environmental contention is the management and fate of the wastewater generated by the fracturing process when fluids used to generate fractures in the rocks underground are returned to the surface. These waste fluids require appropriate storage, transportation, treatment, and discharge to ensure that overall fluid requirements are minimised, process efficiency is maximised, and risks to the environment are reduced. Prior understanding of the chemistry and volumes of waste that will require management are beneficial to operators, waste treatment professionals, and environmental regulators. The effects of temperature, pressure, shale mineralogy, and injected fluid composition on the chemistry of waste fluids were investigated through a series of experiments. Experiments were designed to replicate the hydraulic, thermal, and chemical interaction between shale rocks and injected fluids in the subsurface to provide insight into the source of contaminants of concern in wastewaters. This work also modelled the economic and energetic cost of treating wastewater with varied composition under current regulatory conditions in the UK. The limited UK waste fluid composition data was compared with a wealth of waste fluid data from the USA to ascertain what lessons could be learned from legacy operations and their environmental impacts. Experiments with injection fluids containing no chemical additives yielded waste fluids with few contaminants of concern. The greatest concentrations of contaminants of concern such as heavy metals, sulphates, and salts occurred during reaction with an injection fluid containing 10 % HCl additive to the base injection fluid. Key factors affecting the release of contaminants into solution include solution pH, mineralogical composition (particularly carbonates and sulphates), and oxidising conditions. Modelling of the volumes and chemistry of wastewater expected from operations in the UK concluded that the economic viability of any shale gas well is impeded by the cost of treatment. The cost of treating the salinity varied from 2 to 26 % of well revenue, with a predicted median of ~ $495,472 per well. Additional costs of up to £163,450 per well will be incurred for disposal of NORM concentrated sludge in permitted landfill sites in line with UK regulations. Based on the findings from this research, it is strongly recommend that this area receives further attention from the emergent unconventional gas industry, the established waste water management industry, and regulatory bodies in the UK, in order to produce a coherent strategy for the future management of wastewaters from hydraulic fracturing. This work has found that no such co-ordinated strategy currently exists, and limited management capacity in the UK will present a significant hurdle to future expansion of the industry unless it is urgently addressed.