Numerical investigation of methane and formation fluid leakage along the casing of a decommissioned shale gas well

Abstract

Methane and brine leakage rates and associated time scales along the cemented casing of a hypothetical decommissioned shale gas well have been assessed with a multiphase flow and multicomponent numerical model. The conceptual model used for the simulations assumes that the target shale formation is 200 m thick, overlain by a 750 m thick caprock, which is in turn overlain by a 50 m thick surficial sand aquifer, the 1000 m geological sequence being intersected by a fully penetrating borehole. This succession of geological units is representative of the region targeted for shale gas exploration in the St. Lawrence Lowlands (Qu ebec, Canada). The simulations aimed at assessing the impact of well casing cementation quality on methane and brine leakage at the base of a surficial aquifer. The leakage of fluids can subsequently lead to the contamination of groundwater resources and/or, in the case of methane migration to ground surface, to an increase in greenhouse gas emissions. The minimum reported surface casing vent flow (measured at ground level) for shale gas wells in Quebec (0.01 m 3 /d) is used as a reference to evaluate the impact of well casing cementation quality on methane and brine migration. The simulations suggest that an adequately cemented borehole (with a casing annulus permeability k c 1 mD) can prevent methane and brine leakage over a time scale of up to 100 years. However, a poorly cemented borehole (k c ! 10 mD) could yield methane leakage rates at the base of an aquifer ranging from 0.04 m 3 /d to more than 100 m 3 /d, depending on the permeability of the target shale gas formation after abandonment and on the quantity of mobile gas in the formation. These values are compatible with surface casing vent flows reported for shale gas wells in the St. Lawrence Lowlands (Quebec, Canada). The simulated travel time of methane from the target shale formation to the surficial aquifer is between a few months and 30 years, depending on cementation quality and hydrodynamic properties of the casing annulus. Simulated longterm brine leakage rates after 100 years for poorly cemented boreholes are on the order of 10 25 m 3 /d (10 mL/d) to 10 23 m 3 /d (1 L/d). Based on scoping calculations with a well-mixed aquifer model, these rates are unlikely to have a major impact on groundwater quality in a confined aquifer since they would only increase the chloride concentration in a pristine aquifer to 1 mg/L, which is significantly below the commonly recommended aesthetic objective of 250 mg/L for chloride.

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

“…We simulate methane migration over a time scale of 100 years. Previous modeling studies have considered time scales for groundwater contamination associated with natural gas wellbore leakage ranging from 2 years (Reagan et al, 2015) to 30 years (Kissinger et al, 2013) to 100 years (Kissinger et al, 2013;Nowamooz et al, 2015) to greater than 1,000 years (e.g., Gassiat et al, 2013). The studies with shorter time scales examine methane migration whereas longer duration studies tend to focus on transport of hydraulic-fracturing chemicals.…”

“…Reagan et al (2015) highlight the need to investigate variations in capillarity in future work. Kissinger et al (2013), Nowamooz et al (2015), and Reagan et al (2015) identified multiphase parameters appropriate for a given location; however, parametric variation studies are needed for evaluation of the importance of multiphase parameters in solving/simulating problems of methane migration in the subsurface. Here we investigate methane leakage into low-permeability, unfractured media below a freshwater aquifer with a sensitivity analysis investigating the impact of multiphase parameters in simulating subsurface methane migration.…”

Mechanisms leading to potential impacts of shale gas development on groundwater quality

“…The potential for fluid migration along O&G wells is plausible, and incidents have been reported on impacts of fracking on adjacent wells, which has led Alberta (Canada) to develop regulations aimed at minimizing such incidents, notably through minimal distances of fracking from existing O&G wells and the verification of the integrity and completion records of nearby O&G wells. The process of fluid migration through O&G wells has been numerically simulated by several authors . Reagan et al showed that if the shale gas reservoir is at normal (hydrostatic) pressure and is being produced, then fluid flow is from the aquifer to the shale gas reservoir, and thus, it would have no impact on shallow groundwater quality.…”

Groundwater‐quality hazards of methane leakage from hydrocarbon wells: A review of observational and numerical studies and four testable hypotheses

“…Unlike Reagan et al (2015), these authors simulated overpressure in the shale-gas formation driving upward methane and brine migration along a faulty wellbore. Similar to Reagan et al (2015), Nowamooz et al (2015) found that cementation quality and hydrodynamic properties of the well annulus control travel times of methane from hydrocarbon production zones to groundwater. Travel times for methane in their simulations ranged from a few months to 30 years.…”

“…Methane leaked in the subsurface is expected to migrate upward via buoyant advection and may, therefore, reach groundwater ( Figure 1). Multiple studies have shown that, due to buoyancy, methane is a more likely aquifer contaminant than hydraulic fracturing fluids or brine (e.g., Kissinger et al, 2013;Nowamooz, Lemieux, Molson, & Therrien, 2015).…”