Underground » Ventilation, Gas Drainage and Monitoring
In-seam drilling for gas drainage is now an essential part of operations at many Australian underground coalmines. Gas drainage is required to reduce inseam gas content to below threshold levels for safe roadway development and panel extraction. Failure to drain coal increases the risk of outbursts and may cause unacceptably high gas emissions into the mine atmosphere. As mines are forced into accessing deeper reserves they generally encounter higher seam gas contents and higher in-situ stresses that are linked to lower permeability. Both of these characteristics tend to increase the drainage time required to lower the seam gas content to below critical threshold levels.
The focus of the longhole water-jet drilling project is to design and trial a coal drilling technology that has productivity characteristics far exceeding that of current drilling systems. This report presents the results obtained from water-jet drilling field trials, experimental conclusions and subsequent recommendations. This project was funded by ACARP and the CMTE.
The objective of this project is to develop and trial a new drilling method for the accurate and efficient installation of long inseam boreholes (>1000 metres). This involves the integration of pure water-jet drilling technology (i.e. not water-jet assisted rotary drilling) developed within the CMTE's water-jet drilling activities over the last seven years, with conventional directional drilling technology. The system was similar to conventional directional drilling methods, but instead of relying on a down-hole-motor (DHM) rotating a mechanical drill bit for cutting, high pressure water-jets were used.
In the project field trials, the new CMTE water-jet drilling system demonstrated improved functionality when compared to previous industry research. Kennerley and Just conducted previous drilling using only water-jets and a rigid drill string in the early 1990's. The major improvements over the older technology included;
- Significant improvements of the design of the cutting head and swivel body has resulted in reduced pressure drop through the system.
- The pressure drop improvements have also resulted in an increased engineering safety factor for the system.
- Drilling productivity that was comparable to down hole motor drilling.
- A new drill string design was implemented that was lighter and stronger. In addition it had an increased factor of safety for use in longhole water-jet applications.
- The survey tool and wireless communication system improved in system functionality and productivity.
- New down hole drilling tools can be easily added to the system and electronic data can be transmitted in real time during all drilling operations.
- Controlled directional drilling with a 0.5 degree bent sub was demonstrated.
In summary, the testing of the system did not achieve the full objectives set down in the project plan. A borehole greater than 1000 metres was not achieved. The first trial site had coal that was weathered, oxidized and dry. These conditions significantly affected the ability of the drilling tool to stay 'in-seam'. Due to the poor conditions at the first trial, many experimental objectives were forwarded to the second field trial. In the second trial drilling difficulties were experienced, this was due to the interaction between the confinement of the borehole and the dimensions of the down hole drilling assembly. This ultimately reduced the productivity of the system and the distance that could be drilled within the specified trial periods. Testing in the first field trial did not show any indication that the system would have this difficulty.