Evaluation of In-Seam Borehole Stability in Coal Mines

This report presents an analysis of techniques used in Australian and overseas mines to stabilise gas drainage holes drilled horizontally in coal seams, with particular reference to the Bulli Seam.   Technologies used for short hole and longhole in-seam drilling in Australia and overseas are described and requirements for drilling efficient drainage holes are considered.  Theoretical analyses of the factors influencing borehole stability and the interpretation of theoretical and experimental observations are included.  The report also analyses practical borehole stability methods used in:

• the petroleum and gas industries;
• in soft strata drilling for river crossings, installation of pipelines and communication cables ;
• in civil engineering.

Finally, recommendations are made for practising gas drainage engineers on possible methods that can be used under conditions existing in the Australian coal industry. 

Conclusions

Collapse of holes during drilling and/or after withdrawal of the drill stem may result in one of the following:

1. Inability to reach the desired target and as a result inability to drain the area beyond the point of collapse. 
2. Filling of the hole with the failed material resulting in decrease in effectiveness of the hole. Holes which collapse with time after having been completed give a false sense of security in regard to effective drainage. 
3. Build up of gas pressure behind the closed portion of the hole may lead to sudden ejection creating hazardous conditions. 
4. Loss of drilling equipment such as the drill string or drill motor in the hole, not only results in economic loss but may also create problems in subsequent mining if the equipment is within the block to be machine mined. 
5. Collapsed deviated holes may present serious problems if high gas pressure is maintained beyond the blocked hole and the hole is subsequently excavated. Intersection of the hole may result in explosive ejection of the material, through the rib when the distance of the hole from the rib line is small

Change in the stress regime during the mining of longwall blocks result in large scale ground movement around the block. Ineffective pre-drainage of the coal seam under extraction using in-seam holes may result in the collapse of the holes. Floor and roof holes are influenced by block movement. Holes which may have sheared may re-open again. The gas composition from these holes may also change considerably. As such these holes require close monitoring, to ensure maximum gas recovery.

Many mines on the South Coast of New South Wales, particularly when drilling downwards, routinely case their long boreholes using thin walled, slotted steel, PVC or approved FRAS casing to maintain borehole stability. Boreholes that are not cased tend to experience a reduction in gas production. West Cliff, Tahmoor and Tower Collieries have installed copper tubing to bridge unstable borehole zones before resuming drilling. The difficulty of this procedure increases with borehole depth.

Improved longwall technology is putting increasing demands on drainage requirements. Investigators are endeavouring to see whether 1,000m longwalls can be drilled to pre-drain a number of blocks (3 - 4) and at the same time pre-drain the lower/upper lying seams using drainage holes drilled from the ends of the panel. This system requires not only close control on direction and horizon but also maintenance of the holes particularly when these come under the influence of the longwall. It is doubtful if the holes could ever be maintained under the effect of a retreating longwall particularly if the vertical distance from the seam mined is small. The deviation in these holes should be less than ½ the spacing, but their maintenance within the desired horizon is extremely important. A number of trials have been made and so far these have been fairly successful.

Drainage of water in long holes is extremely important, otherwise their drainage efficiency is greatly reduced. However, no water drainage pumps are available which can be effective in long holes. Some simpler systems using gas pressure have been tried, but these restrict gas flows. Further work is needed in this area. Drilling holes to rise may be a partial solution in some cases.

Three different technologies are used in the drilling of gas drainage holes.

• Rotary drilling with the drilling machine placed in the heading.
• Rotary drilling with the drilling motor placed in the hole (in hole motors).
• High pressure water-jet drilling.

Rotary drilling machines are quite efficient and have a capacity to drill holes up to 300m. The rate of drilling varies with the length to be drilled, but in general an average 200m hole can be drilled in a shift including setting up of the machine.

Small portable rigs (like the Proram) can drill up to 150m in one shift. Larger rigs with separate electro-hydraulic power packs with track mounted machines can drill up to 450m in coal. These machines can also drill holes in rock up to 200m.

Drilling at depths beyond these lengths presents several problems needing solutions:

• Requirement of higher power packs and motor, for increased thrust and pull. 

• Loss of torque due to friction along the drill seam. 
• Larger diameter rods to avoid deviation, however these add weight and are difficult to handle. 
• Better and independent thrust and rpm control methods. 
• Larger water volumes for flushing, recirculation and disposal of cuttings.

Increased power behind the system would take away the 'operator's feel' of the material being drilled. This in turn would necessitate a sophisticated logging and machine monitoring system.

High pressure water jet drilling has been tested for hole lengths up to 400m. This technology claims to maintain holes along its planned trajectory, but whether a longer hole will stay within the coal seam if the seam dips away from the start needs to be proven. Considerable work is needed in this area. This may be an effective technology for shorter holes (50-100m) where a change in dip is not so important if hole trajectory can be maintained.

On the other hand this system is inert and gives no 'feel' to the driller about the rock type or structure through which the machine is drilling. A number of questions related to the safety of the system for underground use must also be answered. The cost of the system is extremely high and it remains to be seen if it will be a viable system for underground coal mines.

The mechanism that seems to control borehole failure is the tensile stress developed at the skin in the crown of the borehole, or high shear stress developed slightly behind the borehole wall. The presence of high fluid pressures (water or gas) as well as stress anisotropy further decreases stability. Thus holes drilled at right angles to the major principle stress are less stable.

Hole stability improves when the gas pressure drops. As such, redrilling of holes after a certain lapse of time will ensure borehole stability. Strength of the medium (rock) in which holes are drilled is the most important factor that controls borehole stability. Lower strength materials and particularly those affected by water are prone to collapse. Materials sensitive to water should be drilled dry.