Large, complex risk assessment needed for agricultural area

The Namoi catchment is located in northern New South Wales (NSW), Australia. The catchment is characterized by large volumes of groundwater abstraction used for agricultural irrigation, and coal has been mined within the catchment on a relatively small scale for at least a century. The NSW Government was aware of stakeholder concerns regarding coal and gas exploration and possible development and the potential effects of these activities on aquifers and agriculture in the Liverpool Plains region.

To strategically assess the likelihood of potential impact from coal and gas development on the quantity and quality of surface and groundwater resources in the catchment, the NSW Government commissioned the Namoi Catchment Water Study. The project presented a number of challenges:

• Comprehensive scope of work - The Study needed to consider issues ranging from local and short-term impacts on ecologically sensitive springs, to the whole area of the catchment and long-term impacts of climate change on water resources.
• Catchment size - The Study needed to include the entire catchment, an area of more than 42,000 km2 (roughly the size of Denmark). Previous studies and models examined only a few small areas within the catchment.
• Data type, quality, and quantity - More than 22 million surface water records, 25,000 borehole records, and 6.5 million daily rainfall records had to be collected, organised, and analysed. Derived from a large number of public and private (industry partners) sources, the data was immense and sometimes commercially sensitive and confidential. Secure data storage was essential, with some data unable to be sent on Study reports and in public presentations.
• Coal resource model - An integrated assessment of the location of coal resources didn’t exist, so it was necessary to complete this catchment-wide geological model prior to evaluating in what form, and where, and where coal resource development might occur.
• Stakeholder engagement - Highly technical information had to be conveyed to the public and more than 30 stakeholders in a manner understandable by all including local residents, landholders, farmers’ groups, coal industry representatives, and government departments.

Database and combined modelling packages applied by expert team

The NSW government chose Schlumberger Water Services as the Independent Expert to manage and complete the Study. To take on the complex project and meet the challenge, Schlumberger first created a custom relational database to manage the datasets collected from federal, state, and local governments; industry partners; and others. The final database contained more than 30 million records.

The Petrel software platform was then used to construct a 19-layer 3D geological model representing all strata from ground level to 2,000-m depth over the catchment. Several coal seams were mapped as individual layers, enabling a higher degree of interpretation and 3D visualization for these areas. Based on the Petrel model, the team created linked dynamic numerical models for both the surface water and groundwater systems using LASCAM and MODFLOW software—collectively known as “the Model.”

The Model, calibrated to historical records, is able to incorporate and assess a wide range of potential future coal-resource-development activities. It is a robust predictive tool that can be updated with new data, recalibrated as necessary, and used to assess alternative scenarios and the effects of changing other inputs and assumptions such as climate change.

Based on the position of the coal seams identified, seven different scenarios were conceived for future coal resource development. These included possible future open-pit and underground coal mines, as well as extensive areas where coal seam gas extraction might occur. The Model was required to assess the potential impact of each of these scenarios until the year 2100.

To address the need for communication with stakeholders, Schlumberger released interim Study reports at key milestones. Each milestone was presented to the various stakeholders at two separate community presentation and discussion forums.

Potential impacts quantified and areas of concern identified

The comprehensive data management, analyses, and model construction enabled prediction of the potential short- and long-term cumulative effects of mining and coal seam gas developments on water resources for the entire Namoi catchment. The Schlumberger project team identified sub-regional areas where water resources were most at risk and quantified the potential magnitude of impact for different coal resource development options. They also evaluated the sensitivity of the results to different inputs and the level of confidence in predictions.

Areas of the catchment were identified where water resources were at high, medium, or low risk to coal and gas developments. With this information, stakeholders, industry, and the government can focus on specific and much smaller areas of concern rather than spreading resources across the whole of the catchment. Stakeholders were also made more aware of the quantitative risks associated with each type of coal resource development.

Furthermore, in the course of the Study, Schlumberger identified a number of data gaps and evaluated their relative importance. A future investigation and monitoring program was proposed to provide robust catchment-specific data inputs that can be used to reduce the uncertainties and enhance confidence in predictions.