Hydraulic fracturing and the environment: risk assessment for groundwater contamination from well casing failure

A system approach is used to investigate the potential risk of groundwater contamination from a failure associated with hydraulic fracturing. The focus is on the role of permeability anisotropy, initial saturation of the medium, leakage depth and leakage rate in controlling the contamination risk at environmentally sensitive locations. We numerically simulate the fluid flow and chemical transport in the geological formations, and use the Monte Carlo algorithm to quantify uncertainty. Geological and operational parameters are selected as random variables. We develop a risk framework to assess three environmental performance metrics: the solute concentration, the arrival times from source to receptor, and the ingestion hazard of the contaminated aquifer. We define risk as the probability of exceeding a certain threshold level for each metric. The effect of parametric uncertainty in risk is also analyzed. The results show that risk strongly depends on water saturation and the anisotropy of the permeability distribution. Furthermore, the measured risk value is more sensitive to leakage depth and leakage rate when compared to the hydrogeological properties. Findings of this study may be applied to situations with more stringent well integrity requirements to ensure that hydraulic fracturing is practiced in an environmentally safe and sound manner, with minimal risk to water contamination.