Drilling Technologies for Soft & Low Permeability Coals

This project was designed to address two specific and separate issues affecting underground inseam drilling; soft coal and tight (low permeability) coal. These two separate problems have a common tangible consequence; difficulty in draining gas to threshold levels to enable mining to proceed safely.

Soft coal occurs when the coal structure is sufficiently weakened by tectonic processes and the borehole wall cannot withstand the pressure differential that occurs between the formation and the annulus of the borehole during drilling. Subsequently the borehole wall collapses whilst drilling. Tight coal presents no problem for drilling but a major problem for drainage. It is not a well understood phenomena but is likely a function of tectonic history, stress or petrology, or a combination of these factors.

The focus of the Drilling Technologies for Soft and Low Permeability Coals project was to investigate potential new technologies and conduct a trial using an identified technology. The review of international practice in soft coal drilling established that auger drilling and casing advance methods were the most widely used to tackle this problem, with mixed success. Borehole pressurisation also offers a good prospect for success but to date has not been tested in an underground mine. The review established that procurement and testing of soft coal drilling technologies were beyond the resources available to this project and subsequently the focus of the program was shifted to tight coal. CRC Mining's water jet slotting system was readily available, and considered prospective for enhancing gas flow in low permeability conditions. BHP Billiton's Appin / Douglas mine site was selected for field testing of the water jet slotting system. A high pressure water pump approved for use in an underground coal mine was used in conjunction with a conventional underground drill rig. High pressure water is delivered to a water jet nozzle sub assembly using a specialised drill string. The field work was carried out at Appin / Douglas Colliery in December 2003.

In the project field trials, the CRC Mining water jet slotting tool was able to slot a number of pre drilled gas drainage boreholes. Gas flows of these slotted boreholes and their surrounding boreholes were monitored. Gas flow data was collected on a daily basis two months before water jet slotting and two months after. This data was the key element to determine if borehole slotting improved gas drainage from a borehole. The field work established that enhancing drainage through a form of stimulation, such as water jet slotting, remains a tangible and realistic option to address the tight coal problem. Unfortunately, in this case the slotting failed to conclusively establish these benefits. This may be due to measurement error in gas flow sampling in horizontal boreholes, or a reflection of a genuine lack of improvement in gas flows through slotting.

• The CRC Mining water jet slotting system functioned effectively in an underground mine and demonstrated that it is able to slot to depths of 492m. Slots were cut vertically, approximately 250mm long and 20mm wide. It is suggested that in future the slotting pattern includes horizontal cuts in an effort to connect with pervasive vertical cleat systems and enhance the potential gas flow benefits of the slot.
• Gas flow monitoring standards and practices proved unreliable and irregular, and this affected the confidence in the findings of the field program.
• Gas flows appeared to fluctuate considerably over time. It is likely this is a measurement and process problem rather than a true indicator of borehole performance.
• Immediately after slotting gas flow spikes were observed. It is suggested this represents desorption of gas from the slotted surface or a response to local water jet seam invasion and is a short term phenomena.

Although the results of this project were not conclusive the deployment success of water jet slotting indicates the method is technically viable. It is recommended that further test work is carried out on an individual research contract basis with mine sites experiencing tight coal problems. Further, applications for water jet slotting are envisaged in stimulating surface to inseam horizontal boreholes or tight-radius surface to seam boreholes.