Numerical modelling of single- and multi-phase flow and transport processes in porous media for assessing hydraulic fracturing impacts on groundwater resources

Abstract

The environmental footprint of hydraulic fracturing has gained substantial attention in the water – energy nexus as the technology is relatively new and its long-term impacts are not thoroughly understood. Of primary concerns are the injections of large volumes of chemical-laced water into underground and possible upward migration of chemicals and hydrocarbons to groundwater systems. The current active debates within the scientific community are the environmental risks versus reward ratio of the operation. To date, different modelling and observational studies have explored the contamination of drinking water resources from fracturing operations. This dissertation investigates migration of fracturing and formation fluids from gas formations towards shallow groundwater by means of numerical modelling. First, a generic features, events and processes (FEP) database is used to identify the most relevant factors to define failure scenarios for safety and risk assessment. Out of various scenarios, the focus is on (i) fracturing fluid and brine migration along a conductive fault over the injection and shut-in periods, (ii) fracturing fluid and brine migration along a leaky abandoned well over the lifetime of a typical horizontal well and (iii) methane migration from a natural gas well through overburden sediments. Single and two-phase two-component flow and transport models are employed for the numerical modelling purposes. The spatial and temporal behavior of the contaminant plume in the subsurface, the solute concentration and the arrival times to the aquifer are assessed. Sensitivity analysis are performed to understand the relative importance of key parameters (e.g. hydrogeological parameters) on the flow and transport of contaminants to the shallow aquifer. The results showed that the contamination probability of shallow aquifers by the upward migration of fracturing fluid and brine from a deep gas formation is low. It was observed that only a limited amount of fracturing fluid could reach the aquifer in a long-term period under specific conditions, such as the presence of a permeable pathway. The hydrodynamic properties of the permeable pathway and its distance from the operation were the most important factors controlling the flow of fracturing fluid to the aquifer. Moreover, well production and dilution of fracturing fluid during the transport reduce the rise of fluids to the aquifer in the long-term. Methane is more likely to migrate upward to shallower strata compared to fracturing fluid and deep brine due to the strong buoyancy. Time to breakthrough and flow rates of methane to groundwater monitoring wells strongly depend on the integrity and distribution of low-permeability rocks with respect to the leaky natural gas well. Methane can be manifested in groundwater monitoring wells even at distances of larger than 1 km from the source of leakage because of the flow deviation along low-permeability rocks. The presence of tilted features could further explain fast-developing methane contamination and large lateral spreading reported in field studies. The shape of the contaminant plume in the subsurface, the arrival time to groundwater (if at all) can vary based on hydrogeological characteristics of formations intercalated between the aquifer and gas reservoir