Report: APP CMHS Project 1

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3.1.4. Directional Drilling

Australian high production coal mines deal with gas problems on the whole firstly by increasing ventilation quantities as this is the most easily instigated and most cost effective means for gas control in most situations. If a gas problem cannot be solved through increased ventilation then more costly intervention strategies are used in the form of pre drainage and post drainage of the gas through the use of various drilling and drainage techniques.

Pre drainage of gas is the most common form of gas drainage undertaken at coal mines for both development mining and longwall extraction activities with directionally controlled drainage holes drilled into areas of high gas to allow for drainage to take place prior to mining. These holes can be drilled from the surface or from within the underground workings. Pre drainage drilling offers the advantage of providing access to measure the initial and remaining gas contents (after drilling and draining) to assist in mine design and ventilation systems (all be it in the short term for inseam holes) to be planned and installed ahead of time to deal with the remaining gas.

Gas drainage techniques used to manage gas on development are typically cross panel drainage holes or conventional inseam hole (CIS) as shown in Figure 23. This method consists of drilling a series of holes typically in the order of 300 m (dependent on longwall panel width) long in a fan configuration to drain an area ahead of the adjacent gate road development face.

Figure 23 In-seam drainage patterns

The second technique of CIS holes uses a long flanking hole pattern to drain ahead of the current development face or a longwall block. The gas drainage technique used, i.e. cross panel or flanking, depends on the seam and gas conditions and the lead time available to drain the required gas. The more detailed information on directional drilling can be found in the website

The main challenge with directional drilling, particularly for long holes, that can be up to 1500 m, is that of steering (Figure 24). This is typically accomplished with ‘down-the-hole’ motor drilling equipment coupled with survey tools such as the DDM (Directional Drill Monitor, Figure 25) Mecca or DGS (Drill Guidance System). Relevant potential sites to observe and gain an understanding of directional drilling technology include Appin; Tahmoor; and NRE No.1 Coal Mines.

Figure 24 Borehole steering (UOW website)

Figure 25 DDM Mecca borehole survey tool (AMT website)

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