Figure 8 - uploaded by Dirk Mallants
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Equivalent hydraulic conductivity of an aquitard with a series of leaky wells (Kwell = 0.1 m/d) as a function of well failure density.
Source publication
![Table 2 Model parameters sourced from the CDM Smith [2014] Narrabri...](profile/Dirk-Mallants/publication/323883564/figure/tbl1/AS:668601182855177@1536418352474/Model-parameters-sourced-from-the-CDM-Smith-2014-Narrabri-model-Parameters-are-defined_Q320.jpg)
This research explores some of the potential consequences from inter-aquifer connectivity caused by degraded or poorly constructed bores and wells. This report uses local groundwater modelling to identify the types of compromised bore integrity that may be measureable in CSG-bearing basin.
Context in source publication
Context 1
... is a very high conductivity, typical for sands and gravel, but not representative for major aquifers such as the Pilliga Sandstone [CDM Smith 2014] In the case of a well leaking through the degraded cement annulus (Kwell = 0.1 m/d), flow is limited solely by the flow through the well for all realistic values of aquifer conductivity (Figure 7, dot-dash line). The conditions for which the background equivalent conductivity of an aquitard becomes significantly modified are shown in Figure 8: for a broad range of aquitard conductivities (10 -2 to 10 -6 m/d), the number of leaky wells/km 2 has to be larger than five. Because this is highly unlikely, the conclusion is that a leaky well with a degraded cement annulus (Kwell = 0.1 m/d) would not normally modify the equivalent hydraulic conductivity of an aquitard. ...
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Citations
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 interaquifer
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 >100 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.
