Abstracts

contaminant release mechanism. There are no indications of active natural preferential
fluid migration pathways, but old O&G exploration wells cut through the reservoir and
their integrity should be verified.
182 - Three-dimensional Reactive Transport Simulations of
Methane Gas and Brine Migration from Decommissioned Shale
Gas Wells into Shallow Aquifers
N. Roy 1 , J. Molson 1 , D. van Stempvoort 2 , A. Nowamooz 1 , & J.-M. Lemieux 1
1
Département de géologie et de génie géologique, Université Laval, Québec, Canada
2
National Water Research Institute, Environment Canada, Burlington, Ontario, Canada
Three-dimensional numerical simulations are presented for methane gas and gas-saturated
brine leaking from a decommissioned shale gas well into a shallow aquifer. The
main objective is to assess the efficiency of natural bacterially-driven processes to mitigate
dissolved gas contamination in both confined and unconfined aquifers. The basecase
conceptual model, based on an Alberta field site, includes a shallow sandy confined
aquifer intersected by a vertical well which is leaking gas-phase and dissolved-phase
methane from a faulty cement seal. We couple the DuMux multi-phase simulator with
the BIONAPL/3D flow and reactive transport code to simulate biodegradation of dissolved
methane under oxygen and sulfate-reducing conditions. The methane degradation
rate is calibrated for the field case while a range of literature values are applied in
a sensitivity analysis. Methane behavior in the aquifer is simulated for different leakage
rates, background geochemistry and hydraulic gradients. The confined aquifer simulation
shows that, unlike the buoyant gas phase that migrates upward towards the confining
layer, the gas-saturated brine flows at the base of the aquifer following the hydraulic
gradient. The upper gas-phase pool acts as a longer-term source which dissolves into the
flowing groundwater. Methane concentrations in both confined and unconfined aquifers
are mainly influenced by the gas/brine leakage duration, background oxygen and/or
sulfate concentration and the methane oxidation rate. At the Alberta site, considering
a continuous methane gas phase leakage rate of 0.40 kg/day, a threshold of 7 mg/L (ex.
as stipulated by the Québec Ministry of Environment for identifying wells at possible
risk) is reached 55 m downgradient after 2 years. Moreover, if the leak stops after 2 years,
under natural background SO 4
concentrations of 400 mg/L, it would take between 7 and
8 years for dissolved methane concentrations to decrease to acceptable levels. In confined
aquifers, methane tends to persist longer than in unconfined aquifers from which
the buoyant gas phase will more readily escape and in which oxygen concentrations are
likely higher. Aquifers with a naturally high background sulfate concentration can significantly
attenuate methane migration but sulfide by-products may become a concern.
The modelling approach can be extended to other contaminants emerging from deep
decommissioned or abandoned wells or from natural faults and fractures such as brines,
or fracking fluids.
IAH-CNC 2015 WATERLOO CONFERENCE
97