Coal seam gas (CSG), or coal bed methane, developments in sedimentary basins such as the Great Artesian Basin (GAB) in Australia, have the potential to impact on aquifers overlying and underlying the target coal formations. The extent to which this may occur depends upon the degree of hydrogeological connectivity between the coal formations and the surrounding aquifers or aquifer systems, with general implications for groundwater management. In southeast Australia, one such aquifer system, the Condamine Alluvium (CA), overlies the Walloon Coal Measures (WCM), which is a formation of the GAB and also a target for CSG production. To investigate the connectivity between the two systems, multiple lines of investigation were employed involving field investigations, data gathering and analysis (including reinterpretation of geology, multivariate hydrochemistry analysis, regional water-level mapping, drilling and coring across the contact zone, multiple piezometer installations, long-term pumping tests, groundwater-level monitoring and local-scale modelling). The study found a low level of connectivity between the GAB and the overlying CA. A layer of undifferentiated basement clay (referred to as the 'transition zone')-a mixture of alluvial clay and weathered basement-provides an effective impediment to flow across the CA and the underlying GAB formations. Results from the study potentially have wider application across the GAB and sedimentary basins where younger alluvial sediments associated with river systems frequently overlie the erosional surface.

The groundwater resources of the Condamine River Alluvial Aquifer (Condamine Alluvium) in Queensland have been developed in the past 60 years for irrigation, stock, domestic and other uses. Extraction of more than 55,000 mL/yr from this aquifer is critical to supporting the local irrigation industry. Existing and proposed coal seam gas development in the Surat Basin extends to underneath the western edge of the CA footprint and involves depressurisation of coals that form up to 10% of the Walloon Coal Measures (WCM) at some depth below the alluvium. Reduced groundwater availability from the already-stressed Condamine Alluvium is thus an oft-quoted concern of landholders when further development of the CSG industry is considered. An extensive study, led by the Queensland Office of Groundwater Impact Assessment (OGIA) and supported by Arrow Energy, was carried out to provide quantitative assessments of vertical hydraulic conductivity between the alluvium, coal measures and intervening formations. This extended abstract focuses on one element of the aforementioned study led by Arrow Energy, specifically, aquifer testing undertaken at two representative sites overlying the Condamine Alluvium. This included drilling and collection of core; geophysical, geomechanical and geochemical testing; test pumping and monitoring; and, modelling. An important element of this project, aside from sharing of data and results—was the involvement of local landholders in development of the investigations, and attendance at field days during drilling and presentation of results. Key findings included estimated ranges of vertical hydraulic conductivity derived from parameter estimation modelling that were lower than previously suggested.

Development of conventional and unconventional gas resources, including CSG, has many similarities; however, there are some key differences that may lead to potential impacts on groundwater supply. Conventional reservoirs are usually located at greater depths than commonly exploited groundwater resources and develop when gas becomes trapped by impermeable bounding layers. In the CSG industry, reservoirs are often lower in permeability than aquifers that commonly provide groundwater supply for agricultural, industrial, urban and other uses; the CSG plays may be located above and below these aquifers. The management of reservoir/aquifer interconnections necessitates a program of focused-data acquisition. For example, groundwater transfer from shallow aquifers to underlying CSG reservoirs may occur where CSG-containing units subcrop beneath shallow aquifers, possibly at significant distances from operating wells. Arrow Energy is undertaking investigations of connectivity between aquifers, including the connection between the Condamine Alluvium and the CSG-containing Walloon Subgroup. This extended abstract explores the use of analytical and numerical modelling to represent the aforementioned situation as follows: The behaviour of numerical models at the point of contact between CSG-containing units and overlying alluvial aquifers is being examined in detail—there is specific focus on how horizontal and vertical hydraulic conductivities are assigned at the interface between aquifers. This localised sub-region within a regional scale groundwater flow model has the potential to control the estimations of flows that are most interesting to stakeholders. The relationship between modelling approaches used by reservoir engineers and hydrogeologists is also explored.