Improved Efficiency of Gas Capture from Boreholes Under Active Longwall Panels

Mines can be affected by seams with a high gas content underlying the target mining horizon. Additional gas can be liberated from these seams during longwall extraction making it difficult to maintain an efficient drainage program. If the permeability of the lower seams is low, then pre-drainage is not a practical option and a system of gas drainage boreholes drilled below the goaf is necessary.

The effectiveness of gas drainage boreholes during longwall operations is essential to the productivity and viability of the mine.

The purpose of the cross-measure boreholes is to intersect gas migration from the coal seams below the seam prior to its reporting to the goaf and face area. This requires an understanding of the fracture permeability, stress redistributions, gas sources, the fracture connectivity and borehole stability.

The aim of this study has been to investigate geotechnical and hydrological factors which impact on gas drainage effectiveness below longwall panels. The field site chosen for the monitoring and study was Appin West, located on the NSW South Coast Coalfield. The depth of mining is approximately 500m-550m. The primary gas source in the floor is the Wongawilli Seam, which is located approximately 35m below the extracted Bulli Seam.

The work program was:

· Undertake computer modelling of the:

o fracture formation, ground deformation and stress conditions which occur in the floor during longwall extraction,

o determine the permeability of the fracture systems formed in the floor strata during longwall extraction and subsequent goaf compaction,

o assess the gas flow from simulated boreholes which intersect the lower seams and those which only intersect the rock strata,

o assess the deformation of boreholes;

· Review field monitoring of gas flows from borehole capture in past longwall panels at Appin West mine;

· Measure gas pressure and flow from a number of production boreholes which were drilled to drain gas from the lower seams during longwall extraction. This aspect of the work required designing and trialling a system which could be used in a hazardous environment;

· Review the information from the computer modelling and field measurement and assess the nature of the gas migration within the floor strata; and

· Assess optimisation strategies for gas drainage.

In general, the modelled results mirrored the field measurement in terms of flow rate and time. Also, the pressure distribution along the borehole was similar to that monitored. The results provided confidence that the model was estimating the key features of the deformation and conductivity of the Wongawilli Seam in particular.

The modelling indicated that the key factor for drainage was transecting the highly conductive zones in the Wongawilli Seam.

Models of horizontal boreholes were also undertaken. The results indicated that these may not be as effective as cross-measure boreholes. This was based on the fact that the borehole needs to intersect the lateral high flow areas in order to be effective.

It was concluded that if lateral boreholes are to be used, a validated concept of the conductivity geometry within the coal seams and interburden should be defined. Once this is done, then a range of borehole geometries can be evaluated.

In general, the computer modelling was considered to replicate the field data and gives confidence for a more widespread application. The field measurement system can be optimised and improved, however, it served a very important purpose in providing valuable information to undertake back analysis.

Additional field studies are recommended to obtain more downhole data to assess the effects at other sites and modified geology.