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An essential tool

World Coal,

Coalbed methane (CBM) extraction uses coal seam depressurisation to extract methane adsorbed into the solid coal matrix. This can be achieved by drilling wells into the coal seam and pumping water from the well to reduce the hydrostatic pressure in the formation. The decrease in pressure allows methane to desorb from the coal and flow as a water and gas mixture to the surface. Once the groundwater level is below the coal seam level, free flow of gas is achieved. CBM production does not require conventional mining infrastructure (no opencast pits or underground workings and no discard dumps or slurry lagoons, as one would find at a traditional coal mining site), while the gas is also produced at a lower total carbon footprint and can be used for power generation.

While this unconventional source of energy offers a number of benefits, it is vital that attention is paid to groundwater – the byproduct of any CBM production. Groundwater management, based on accurate prediction of aquifer behaviour, drawdown levels and estimates of volumes and quality of groundwater pumped over the life of the project, is important for both the operational and environmental aspects.

Through limiting the planning, implementing and operating a CBM project to traditional reservoir modelling outputs only, the project will have limited understanding of groundwater changes over the life of the project. Traditional reservoir modelling only considers groundwater aspects related to the coal seam and fails to account for hydrogeological interaction at all levels. There are potential long-term financial implications for the project at operational level, while there are also environmental liabilities if the hydrogeological interaction is not considered.

The solution to this is to make use of the holistic approach associated with numerical groundwater modelling as a management tool. It is important not to limit the use of the model to environmental purposes only.

Given the exploration effort required for the development of a successful CBM projects, it is important to recognise the number of tools available for use that are not yet being fully utilised. If these tools are used effectively, it is possible to build and acquire knowledge and data that feeds into a groundwater model. CBM developments can involve hundreds or thousands of wells distributed at a regional scale, with implications for both science and regulation. A “regional-scale, multi-state and multilayer model” of both groundwater and surface water assesses impacts and develops mitigation programmes, while supporting project development planning and implementation.

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Written by Dr Jennifer Pretorius, Gerhard van der Linde and Gabriel Canahai.

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