Optimisation of Long Hole Drilling Equipment

The objectives of this study were to establish the factors currently restricting long hole drilling in coal seams to 1000m, and to establish what changes in technique or equipment are necessary to reach 2000m. Data from existing boreholes were analysed using sophisticated borehole survey, torque and drag hydraulics software. Longer holes than are currently being drilled were simulated with changed drilling parameters. Physical testing was undertaken on strain gauged drill rod joints to determine their yield strength under combined bending and axial load. In addition a physical model of drill rod buckling was constructed for comparison with theory and to assist in visualising buckling behaviour.

The conclusions reached were that the prime limitation on drilling was the lack of straightness of the holes and in particular the frequency with which the tool face angle is changed. Drill rod strength is also an important limitation with combined axial, bending, torsional and internal pressure loadings producing high stresses.

To drill further it will be essential to drill with less frequent tool face angle changes and reduced build up rates. To achieve this, better surveying and roof-floor location using geophysical techniques are highly desirable. Stronger drill rods are also required.

This project was conceived as a result of the need of the Australian coal industry to drill longer in-seam holes from underground for the purpose of methane drainage and for exploration of proposed longwall blocks. To cover the latter, holes of 2000m length are required. At the time of the project proposal to Australian Coal Research Pty Ltd as part of the ACARP program hole lengths at best were 1000m with many falling short of this distance.

This project was also spurred by the clear failure to understand the drill string mechanics of the surface to in-seam drilling trial at German Creek in 1991 (Gray & Phillips (1994)). The German Creek project was undertaken under a NERDDP grant to Capricorn Coal Management Pty Ltd with SIMTARS acting as technical consultants. This failure occurred despite the hire of oilfield directional drilling service companies with what was then state of the art knowledge.

This experience forced the researcher to look beyond what is considered to be state of the art in the oil industry, to develop new theory and to verify this by comparison with measurements taken in current coalfield drilling.

The purpose of the project was thus to define what was needed to extend in-seam drilling capability to 2000m.

To achieve this the following objectives had to be reached:

1. An understanding of the components of frictional drag between drill rods and borehole wall.
2. Definition of operational constraints on drilling by examining past records
3. From a) and b) establishment of the critical areas that need improvement so that the primary objective can be achieved. These were considered likely to include:

• techniques to drill straighter holes;
• drilling lubricants;
• alternative drill strings;
• survey equipment.

Conclusions

The main conclusion of this report is that changes need to be made to current drilling practice to ensure drill rod integrity at currently drilled lengths and that these changes will be essential to permit longer drilling.  It is considered that 2000m boreholes will be able to be drilled in-seam. Before this can occur however a number of developments to equipment and changes of practice will have to be made in a stepwise approach.

These changes principally involve drilling holes that have the least number of angular changes and the lowest build up rate that is possible. To achieve this two techniques can be adopted. The first is darning needle drilling, ie long intervals between tool face changes and small build rates. The second is by rotating the string at about 80RPM as is oil industry practice, so as to drill a straight hole.

The darning needle approach has considerable benefits in that if it is conducted in the vertical plane it may permit the entire seam section to be drilled through while creating a long hole. This could be beneficial for both exploration and methane drainage. The bad side of the approach is that if branches are to be avoided then the hole will certainly run into roof or floor.

Depending on the feasibility of creating gradual branches, the roof or floor may simply have to be drilled through and back into the coal. If this is the case then drilling must be slowed right down or bit load will go up. Driving up bit load requires far more thrust in long deviated holes. With it come problems of buckling and high drill rod stresses. To use the darning needle approach in the vertical plane will require Bottom Hole Assemblies (BHAs) that build up at about the same rate as they build down.

The second approach of rotating the drill string to permit a straight hole to be drilled will require several changes to current practice. The first of these is that the bottom hole assembly will have to be significantly different from that currently in use for in-seam drilling. The second is that the BHA must not cut a significantly larger hole than current assemblies. The oilfield approach to this problem is BHAs with double bends and which also use stabilisers.

Currently there are objections to the use of stabilisers in that they are likely to cause the BHA to become trapped. Improvements in hole stability through borehole pressurisation are likely to be forthcoming, while careful design of stabilisers with cutting rear faces, should enable BHAs to be rotated so as to cut their way out of borehole collapses.

Rotation, either in forward drilling, or rod pulling, will require more torsional drill rod strength than is needed in current practice. The present flip-flop drilling method, or the proposed darning needle approach, only requires the torque to rotate the string while off bottom to set the tool face angle. As the torsional strength of drill rod joints is very doubtful at present the additional torsional loads associated with rotation probably will not be capable of being withstood by NT or NQ rods. In addition to quasi-static loading conditions, rotation will bring with it a need for drill strings that will withstand fatigue loadings.

Measure While Drilling

The use of drilling techniques where angle is built more slowly will place greater emphasis on knowing the location of surrounding strata so that the hole is not permitted to deviate too far from its intended pathway. Thus measurement of survey information while drilling, as well as geophysical information, is going to be extremely important to the drilling operation.

Whatever the intentions for drilling long boreholes it is a certainty that the borehole trajectory will not be what is planned. For this reason it is important, and will become essential in longer holes, to analyse the drilling operation while it is being undertaken. This will enable the build up characteristics of the BHA, in that particular section of seam to be understood and stuck or broken rods to be avoided. Automatic monitoring of the borehole trajectory and bit thrust and torque will make this easier. The potential exists to process this information while drilling.

Drill String Stress

It should be noted that from other work conducted by Gray and Phillips (1994) that the presence of varying stiffness of drill rod sections in curved holes will lead to significantly raised stress levels at their junctions. Great care should therefore be exercised in designing drill string elements to avoid such problems.

Use of Lubricants

The coefficients of friction derived from back analysis in this report ranged from 0.105 to 0.26 with a mean of 0.17. These values are low and would not seem to justify the addition of lubricants given the other operational problems that these would produce. Unfortunately the figures refer only to drilling in the Bulli Seam. Other seams may have higher coefficients of friction, which would reduce the capacity to drill long holes.

Tahmoor vs Northcliff

The reason why the boreholes drilled at Tahmoor Colliery required a fraction of the thrust of that drilled at Northcliff is not explained. This difference is however very significant as higher bit loads reduce the distance that the drill string can be pushed. The drilling water pressures were in all cases comparatively low indicating that low torques were required. It is considered quite likely that either the flushing water coming out of the bit assisted cutting considerably or that the coal is virtually self cutting, with the possibility of mini outbursts occurring in the face of the hole. If the former is correct then the use of nozzles on the bit may reduce thrust requirements further.