Review of Inseam Drilling Practice

It has long been recognised that current inseam drilling practice in Australia has serious limitations and likely results in higher than necessary costs to mining operations.  Failed boreholes due to "boggy ground" and poorly performing individual borehole drainage are a common feature of the current inseam drilling scene. Major weaknesses associated with current practice include:

· Lack of balance control during drilling;

· Limited geosteering and logging capability;

· High 'lost-in-hole' risk;

· Limited 'well completion' approach to optimise drainage; and

· Safety issues associated with manual rod handling.

A number of these issues can be addressed by removing the predrainage function 'out of the pit', to surface using medium radius directional drilling technology (MRD SIS).  Although this methodology has proved popular in recent years, a commitment to MRD SIS will not preclude many operations from doing supplementary inseam drilling, and for others, surface access issues will mean that the vast majority of inseam drilling and drainage will still need to be carried out underground.  It is clear that inseam drilling will remain an important part of mine gas drainage for safety and production purposes for the foreseeable future.

In addition, the problems associated with current inseam drilling practice are only likely to be exacerbated by higher reservoir pressures, and "softer" structurally disturbed coal, all of which can be expected as mining goes deeper, and works in more challenging terrains.

A step change approach is required for inseam drilling practice that addresses the major issues listed above.  The best and most practical solution with a chance of fast tracking appears to be Coiled Tubing Systems (CTS), which may be adapted from mature conventional oil and gas practice. The CRC Mining is already advanced in developing a CTS prototype for underground inseam drilling and drainage, and it is recommended that this project be further supported by industry.

Development of a newly developed CFD model for simulation of gas release from coal through inseam horizontal boreholes has been achieved using the common CFD code FLUENT.  The model operates as a two phase flow regime and the gas phase is considered compressible.  Instead of treating gas flow as concentration driven diffusion (the conventional approach) gas release is dealt with as a mass source. A relatively simple but innovative equation is derived to describe the generation rate for the gas source according to the gas content - pressure isotherms.  The simulation illustrates the procedure of gas release and the development of flow variables within the coal seam and the borehole, which assist in providing an improved understanding of the mechanism of gas release and borehole flow.