The Feasibility Of Enhanced Gas Drainage For Coal Mine Operations

This report presents the findings of ACARP project C17055 which investigated the feasibility of enhanced coal seam gas drainage for coal mining. Enhanced drainage has the potential to increase the recovery of coal seam gas over normal drainage methods. It involves the injection of a gas, such as nitrogen, into the coal seam, that acts to displace the coal seam gas, which is predominantly methane or CO2. There are a number of potential benefits from this technique but key ones are that a larger proportion of the original gas within the coal is drained over normal drainage practices and the rates of drainage are also higher. For open cut mining reducing the coal seam methane present at the time of mining reduces the mines fugitive emissions.

The study presented in this report investigates the feasibility of enhanced drainage through a combination of experimental studies, reservoir simulation and economic analyses. The experimental studies combined core flooding with other characterization studies in order to estimate the reservoir properties required to perform the reservoir modelling. In the core floods nitrogen and other gases were injected into core samples with methane present at reservoir pressures and temperatures and the rates and composition of gas outflow measured. These experiments provide information on the gas displacement mechanisms such as diffusion coefficients and sweep efficiency. From this work the sweep efficiency was estimated to vary between 90-99%. In a second program of laboratory work a series of other reservoir properties were measured, such as the permeability, geomechanical properties and coal swelling. These measurements were performed on two coal samples from active mining sites; one from the Hunter Valley and another from a site in the Bowen Basin.

In conclusion this project has found that there is good evidence for technical and business case feasibility for enhanced drainage as a means of reducing the gas content in coal seams prior to mining. Technical feasibility is demonstrated through evidence from previous field trials and the experimental program of core flooding work conducted in this project. The economic analysis performed in this project found that enhanced drainage was significantly more economic than primary or normal drainage for the shallow coals seams mined by open cut and that the net present value was positive when there was a CO2 penalty on fugitive emissions of more than approximately $20/tonne CO2e. The next step in evaluating the technical merits of this process would be to conduct a field trial; a field trial would provide a more accurate understanding of reservoir sweep efficiency and the nature of breakthrough of injected gas in production wells.