A multi-stage screening approach to evaluate risks from inter-aquifer leakage associated with gas well and water bore integrity failure

Abstract

Integrity failure of wells in gas resource developments poses a potential risk to groundwater resource quality and quantity by enhancing the connectivity and fluid migration between a gas reservoir and overlying aquifers, which may support groundwater dependent ecosystems or be used as a water source. This study assessed the potential for impact on groundwater resources from flow pathways created by well integrity failure in decommissioned coal seam gas (CSG) wells, deep water bores or gas wells repurposed as water bores and legacy coal exploration drillholes using an example based on a proposed CSG development near Narrabri, Australia. The study only considered the flow of water. A multi-stage screening method for likelihood and consequence assessment was used, which included (i) a semi-quantitative risk prioritisation of potential pathways for inter-aquifer leakage and development of metrics for impact assessment, (ii) assessment of the consequences of each of these pathways through an analytical model, and (iii) numerical groundwater modelling of single and multiple leaky wells. All three approaches indicated that increases in inter-aquifer leakage, drawdown in upper aquifers are not likely to be significant for high flow leaky gas wells (effective well conductivity, Kw <10−1 m/d) based on conditions reported in previous studies and for the case study. It is theoretically possible that extremely high flow leaky wells (Kw >102 m/d) could have an impact where aquitard conductivity is 10−4 m/d or less, or well failure density is higher than 1 well per km2, however Kw values for compromised gas wells of >1 m/d have not been identified in previous studies. The most extreme case tested, open legacy coal exploration drillholes or petroleum bores repurposed into water bores across an aquitard, should it occur, has the potential to deplete or contaminate groundwater resources in connected aquifers, the magnitude of which will depend on aquifer and production zone transmissivity. However, current Australian regulations for petroleum exploration make this an extremely unlikely case. More effort is required to determine Kw and failure rates of gas wells, water bores and exploration drillholes in Australian conditions to better quantify the potential risks associated with leaking infrastructure.