Waterjet Assisted Drilling of In-Seam Cross Panel Methane Drainage Holes

For many mines poor development rates for longwall coal blocks are linked closely to problems of underground gas drainage. In an effort to address current safety issues associated with methane drainage and improve longwall productivity, the CMTE has been investigating the applicability of high pressure water (20-20 Mpa) for assisting conventional rotary drilling at both Appin (BHP Coal) and Dartbrook (Shell) mines.  The aim is to drill straighter and more accurate in-seam cross panel holes for methane drainage at a productivity greater than that achievable by Down Hole Motor (DHM) drilling technology. A successful outcome will be of considerable benefit to underground coal mines as the use of DHM drilling to gain drainage hole accuracy has significant cost penalties. A High Pressure (HP) water pump (250 l/min @ 800 bar), suitable for use in an underground coal mine environment, was designed and manufactured for drilling trials. Phase 1 underground trials were conducted initially at Appin Colliery (Dunn et al, 1996), Southern coal field. Phase 2 underground field trials were conducted at the Dartbrook mine in the Hunter coal field (Liu et al, 1997).

Results

The following are the main findings of this project:

• HP Waterjet assisted drilling has demonstrated that deviation is significantly reduced to under a quarter of the benchmarked conventional rotary drilling. This equates to typically 7.6m off the intended target compared with 31.2m (conventional water pressures) in a 180m hole. As typical hole spacings for in-seam gas drainage holes are between 15 - 20cm a 7.6m deviation at the end of the hole would appear to be acceptable. 

• Waterjet assisted drilling significantly increases the rate of penetration. This is currently limited by the capacity of the drill rigs used. 
• A maximum penetration rate of 3.9m/min (Appin) - 3.6m/min (Dartbrook) was achieved in the trials to date. The results indicate a potential increase of almost 80% in instantaneous penetration rates for HP waterjet assisted rotary drilling in coal. 
• Waterjet assisted drilling consumes about same amount of water as conventional rotary drilling (120 - 170 l/min). DHM drilling at Dartbrook uses around 220 l/min. 
• Feed pressures are reduced to around a quarter of that required for conventional rotary drilling, whilst torque pressures are reduced by around 20%. Hence, bit loads are significantly reduced during HP waterjet assisted drilling. 
• The nominal coal cuttings size (50% passing) is reduced with HP waterjet assisted drilling compared to conventional rotary drilling. 
• Pure waterjet drilling (using HP water only) has been demonstrated underground and can be steered against the main cleat/jointing direction using a bent sub. Nominal particle size of cuttings is increased compared to conventional rotary drilling. 
• Most HP water assisted holes deviate downwards in the seam. There have been, however, holes from both Phase 1 and 2 trials which have not worked their way down to the floor. This indicates that further optimisation of the drilling process is required to make all holes follow a consistent horizon ithin the seam. In the Phase 2 tirals at Dartbrook an overall success rate (reaching the target distance or capable of reaching the target distance) of 81% was achieved. This compares favourably with a benchmark study conducted by Appin Colliery which indicated that 47% of rotary holes did not reach the required drilling distance.

Conclusion

From the trials conducted to date it can be concluded that the CMTE HP waterjet assisted drilling project has demonstrated improved hole accuracy and penetration rates for in-seam cross panel drilling. With additional benefits such as reduced cuttings size and reduced feed and torque forces on the rig, this leads towards the potential of HP waterjet assisted drilling being applied to longhole drilling (>1km).

This would enable improved gas drainage techniques to be adopted eg. drilling longitudinally along the longwall block which would improve gas drainage lead times and could potentially increase roadway development rates. Because DHM drilling tends to produce holes which are not straight, then there is the potential for holes to be drilled further using high pressure waterjet assistance.

Further optimisation in coal damage ahead of the HP drill bit is required before commercialisation of the waterjet assisted drilling technology can begin. As part of the C6028 Waterjet Assisted Longhole Drilling Project, optimisation of coal damage ahead of the drill bit will be conducted and a commercialisation plan put in place for the cross panel drilling technology. A video has been produced on the work completed under this project.

The benchmarking of conventional rotary drilling showed that stiffer HP BQ drill string could negotiate rolls in the coal seam. However, the stiffer HP BQ rods did not stop the holes from deviating towards the main cleat direction using conventional water pressures.

HP Waterjet assisted drilling trajectories do not appear to be influenced by the cleat/jointing in the coal seam. Comparison of conventional rotary drilling and waterjet assisted drilling showed that waterjet assisted drilling produces straighter holes.

Some HP waterjet assisted drilled holes tend to droop towards the floor of the coal seam. Most holes tend to go the full width of the longwall block and some holes move up and down in-seam which indicates the potential to keep waterjet assisted holes in-seam by optimisation of the downhole configuration.

It was demonstrated in hole 10 at Appin that a pure waterjet system is capable of drilling in-seam gas drainage holes underground. The waterjet drilling system can be steered away from the main cleat direction using a bent-sub. Pure waterjet drilling or waterjet assisted drilling with steering capability may have the potential to compete with DHM drilling. Waterjet assisted drilling in particular, has the potential to double the effective productivity of DHM drilling.

The findings from the HP waterjet assisted drilling trials are very encouraging and it appears that straighter holes can be drilled in coal with hydraulic power of around 100 kW. This would allow the use of a smaller water pump and electric motor which could be fitted onto one trailer. If a variable speed drive is utilised then there would be no header tank as well.

Two areas require further development before the technology can be applied to cross panel gas drainage:

• the survey system (pump down or real time)

• optimisation of the damage to coal ahead of the drill bit

The above points will be addressed in ACARP Project C6028 and a commercialisation plan put in place for the cross panel drilling technology.