The trials were carried out by Queensland’s Simtars and detailed in a paper by Hugo du Preez1 and Matthew Page at the International Conference of Safety in Mines Research Institutes in Brisbane this week.

The trial aimed to identify and formulate a safe, simple, easily replicable and cost-effective solution, while achieving a worthwhile reduction in respirable dust without adversely affecting production.

The seven-month trial at the Queensland mine saw water introduced via horizontal inseam boreholes previously used for gas drainage, utilising fractures and cracks in the coal to facilitate the flow of water throughout the coal seam.

Methane drainage is viewed as one of the main contributing factors of high concentrations of respirable dust at Central. This is mainly due to the large amount of water that is drained out of the seam along with the seam gases. The lack of water in the seam results directly in a greater concentration of respirable dust when coal is mined.

Simtars said the effectiveness of the trial was determined by a measure of the potential dust reduction when mining in infused and non-infused sections. The monitoring was undertaken with sampling pumps and respirable sampling cassettes.

The main geotechnical concern for researchers was identifying holes with significant floor and roof contacts. The researchers said seeking geotechnical advice was an integral part in the overall success of water infusion.

The maingate ends of the inseam holes were grouted during periods of no production and water was introduced from the tailgate end. The boreholes had to be grouted 10m to provide enough support to the water pressure.

Simtars said the main drawback with grouting the maingate end of the holes was that water was then introduced via the tailgate. As a result, virtually all monitoring and moving of the water infusion equipment was done by the longwall coordinator and the longwall deputies, due to entry restrictions on the tailgate.

Water was introduced several weeks ahead of mining to allow for saturation. Simtars said the major problem affecting the success of the trial was getting water to migrate via all available inseam holes. Numerous holes failed to hold any significant amount of water.

Simtars has claimed the project a success but maintain further refinement is required to improve the retention capabilities of boreholes. Variables that were found to affect the success of water infusion were permeability, virgin gas content, pressure of the gas in the coal seam, porosity, depth of cover, water delivery pressure and lead-time.

Simtars found holes with significant floor or roof contact should not be used, along with holes where ribs were in poor condition.

Researchers said in order to maximise the effectiveness of water migration though the panel, future water infusion systems should be designed to incorporate a pressure differential between methane drainage holes. The pressure differential could have been created by infusing every second hole with water, or infusing every second hole with water, and leaving the uninfused hole connected to the methane drainage suction plant.

They also advised that water infusion should be integrated into the mining process so it was considered as part of the drilling/drainage process to increase overall ownership and to achieve a consistent, reasonable reduction in respirable and visible dust.

Simtars also suggested further investigation was required into the possible use of packers rather than grouting and/or the methods to introduce water via the maingate side of the longwall block in order to lower costs and increase water migration.