Numerical Modelling of Methane Gas Migration from Decommissioned Shale Gas Wells into Shallow Aquifers

English Abstract:

The behavior and fate of methane gas leaking from decommissioned shale gas wells into shallow aquifers is simulated in three dimensions. The main objective is to assess the efficiency of natural bacterially-driven processes to mitigate methane concentrations in both confined and unconfined aquifers. The conceptual model includes a shallow sandy aquifer pierced by a vertical well which is leaking gas-phase and dissolved-phase methane from a faulty cement seal within the underlying rock. The modelling approach couples the DuMux multi-phase simulator with the BIONAPL/3D flow and transport code for reactive transport and degradation of dissolved methane under oxygen and/or sulfate-reducing conditions. Methane behavior in the aquifer is evaluated for different leakage rates, background geochemistry and hydraulic gradients. In confined aquifers, methane will tend to persist longer than in unconfined aquifers from which the buoyant gas phase will more readily escape. Aquifers with a naturally high background sulfate concentration can significantly attenuate methane migration. Moreover, dissolved oxygen inflow through groundwater recharge in unconfined aquifers can also enhance methane oxidation. By-products of methane degradation reactions can also affect the groundwater geochemistry. 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, fracking fluids or hydrocarbons.