Inter-aquifer leakage may be enhanced where coal exploration bores are inappropriately decommissioned or where the integrity of water bores and coal seam gas (CSG) wells has been compromised. Such features may provide leakage pathways for groundwater from a CSG production zone to overlying water bearing aquifers. This project was designed as a rapid assessment of the potential impacts of leaky CSG wells using conservative, single phase groundwater models and analytical solutions. Two leakage pathways were considered: a leaky well with flow through the partially degraded cement annulus between the well casing and the rock matrix and a fully open, inappropriately decommissioned exploration well or abandoned production well.
Analytical solutions were developed first to represent passive flow through a vertical conduit between two aquifers separated by an aquitard. Closed-form equations were used to explore the theoretical conditions that may lead to noticeable impacts on the groundwater balance. These conditions included conductivity of the well seepage pathway and the hydrogeological properties of the adjoining aquifers and aquitard.
A complementary numerical analysis was undertaken to confirm whether or not the theoretical hydraulic conditions resulting in inter-aquifer leakage are physically plausible. For this the MODFLOW Unstructured Grid model (MODFLOW‐USG) with the Continuous Linear Network package was used with parameters representing conditions similar to the Gunnedah Basin, New South Wales. Groundwater quality impacts were also assessed using the advection-dispersion equation.
Single well numerical analyses that assumed a relatively large effective well conductivity suggest that it is unlikely that there would be a significant impact from partially degraded wells on the groundwater balance of a hydrogeological system similar to the Gunnedah Basin. It is plausible, however, that inappropriately decommissioned exploration or production bores will enhance inter-aquifer leakage. Future analysis will consider the potential cumulative effects of multiple leakage pathways on the groundwater system.