Methane Leakage From Hydrocarbon Wellbores into Overlying Groundwater: Numerical Investigation of the Multiphase Flow Processes Governing Migration

Abstract

Methane leakage due to compromised hydrocarbon well integrity can lead to impaired groundwater quality. Here we use a three‐dimensional, multiphase (vapor and aqueous), multicomponent (methane, water, salt), numerical model (TOUGH2 EOS7C) to investigate hydrogeological conditions that could result in groundwater contamination from natural gas wellbore leakage that migrates upward toward a freshwater aquifer. The conceptual model used for the simulations assumes methane leakage at 20–30 m below groundwater. We perform 180 simulations for a sensitivity analysis, examining (1) multiphase flow parameters related to storage, capillarity, and relative permeability, including porosity (ϕ), initial fluid‐phase saturation (SL), and van Genuchten n and α, (2) geostatistical variations in intrinsic permeability (ki), and (3) methane source‐zone pressure. Simulated mean ki values are 10−18 and 10−13 m2 with variances of 1 and 5 m4. Simulated source‐zone pressures range from just over ambient hydrostatic pressure at the depth of leakage (100 kPa) to the maximum pressure that steel casings are commonly rated to withstand (20,340 kPa). ki, initial SL, ϕ, and van Genuchten's n and α were the most important parameters in determining the volume of methane reaching groundwater during a given time period. Multiphase parameterization of formations underlying freshwater aquifers and overlying hydrocarbon production zones is fundamental to assessing aquifer vulnerability to methane leakage.

A Critical Review of State-of-the-Art and Emerging Approaches to Identify Fracking-Derived Gases and Associated Contaminants in Aquifers

“…14,22,28,33 Upward leakage along well bores is, therefore, a likely pathway for the migration of fugitive fluids into shallow aquifers ( Figure 1). 11,13,14,[30][31][32]34 Stray formation waters, HVHF fluids, and natural gas can also migrate into potable aquifers via vertical leakage along faults or zones of intense fracturing, via imperfectly sealed abandoned wells, or from underground gas storage facilities (Figure 1). 2,11,14,16,24,35−38 Accidental surface spills of HVHF chemicals and produced fluids can contaminate local environments, including surface waters and shallow aquifers.…”

Modeling of methane migration from gas wellbores into shallow groundwater at basin scale

“…The dominant physical and chemical processes for fluid migration are advection (viscous, buoyant, and capillary), dissolution, and diffusion (Nowamooz et al 2015). Methane sorption is assumed to be negligible (Rice et al 2018b). The flow and transport of fluids are simulated using both a miscible flow model and an immiscible flow model.…”

“…The domain is initially water-saturated. The residual water saturation is set to 20% (Nowamooz et al 2015;Rice et al 2018b). In all simulations, we modeled a drainage-like process, because the gas saturation increases continuously with time.…”

“…In all simulations, we modeled a drainage-like process, because the gas saturation increases continuously with time. As gas cannot be trapped during the drainage, the residual gas saturation is assumed to be 0.01% (Rice et al 2018b).…”

“…The major focus of previous numerical studies was on the rapid migration of methane to groundwater through leaky wellbores, faults, and fractures (e.g., Kissinger et al 2013;Reagan et al 2015;Schwartz 2015;Roy et al 2016;Rice et al 2018b;Moortgat et al 2018;Soltanian et al 2018;Nowamooz et al 2018;D'Aniello et al 2019), while less effort was directed at understanding the slow migration through overburden rocks (Nowamooz et al 2015) as well as the influence of methane solubility on transport (Soltanian et al 2018;Schout et al 2020). In this study, we investigate the migration of free-and dissolved-phase methane from a leaky natural gas well to groundwater through overburden sediments at the basin scale.…”