Underground » Ventilation, Gas Drainage and Monitoring
The objectives of this project were to develop sensors for determining the position of the drill bit within the coal seam (roof-floor proximity) and develop geophysical logging tools for indicating geological conditions within the borehole
Radiometric Tool
For the detection of the position of a hole within the seam, one possibility is to detect the radiometric emissions from the roof and floor into the seam. This is the basis of radiometric roof detection systems which have been used on longwall shearers. We have found however that typically there is only about 50cm penetration of this radiation into the coal.
In the context of in-seam drilling such close warning of the approach of the roof or floor is of limited use. However another approach for seam steering is to establish the radiometric profile for the seam and to determine the hole's position on the basis of this profile.
This latter approach has been successfully demonstrated at West Cliff colliery where a seam profile has been established. The tool that was built is a spectrometric instrument with two detectors placed at 180° to each other. The radiometric profile of the seam follows the brightness profile and the position of the test holes could be established from the potassium peak in an observed spectrum.
Special ratios such as potassium to thorium plus uranium suggest that further discrimination is possible. We recommend that a spectral capability be maintained in future tools.
Our design is for an intrinsically safe instrument. This poses special problems because of the high voltages required by the photomultiplier tubes. An approach whereby the tube itself is encapsulated and all other components operate in an intrinsically safe manner is proposed.
Various modes of operation were also considered. One option was to develop the tool as a pump down unit which could be operated in conjunction with pump down cameras. However, with the rapid acceptance by in-seam drillers of down-hole survey systems such as the AMT and Surtron systems, it is now necessary for the unit to be housed down the hole.
The mechanical design of a production tool has still to be decided. The tool must fit within an NQ rod and allow the passage of water to drive down-hole motors. The tool and detection system must also be sufficiently robust to withstand physical shocks and vibrations.
At the same time the crystal size needs to be as large as possible to ensure that the tool has the sensitivity to measure the low levels of radiation emitted from within the coal which are even further reduced by passage through the drill string. These issues will be addressed in the next stage of the development.
Radar
Two radar systems have been developed. The first was designed to operate on an HQ size rod (probe diameter ~ 82mm). The second is of NQ size (probe diameter ~ 62mm). Both generate directional signals and operate in a frequency range around 500mhz. These high frequencies are necessary to deliver resolution on features such as mylonite zones which might only be a few centimetres in width.
We have attempted to produce geological images without the need for subsequent computer processing but find that band pass filtering is need to produce a good result.
A different approach to making electromagnetic measurements down-hole was also attempted. A prototype dielectric measurement tool was developed and proved capable of detecting a mylonite zone within a test borehole at West Cliff Colliery.
No processing was required and a repeatable result was obtained. The dielectric tool appears to be responding to variations in the amount of water contained within the normal (drained) seam and the pulverised coal in the mylonite zone, which has a much higher propensity to retain moisture.
The dielectric tool is much simpler in concept and operation than the radar. Quite possibly it could assume the major role of locating mylonite zones.
Potential for Short Term Industrial Applications
While the probes that have been built in this project have demonstrated that they can perform their design functions, they are all prototypes. As such they can only be run as stand-alone instruments and require specialist operators. While they have been designed with the requirements of intrinsic safety in mind they have not been submitted for examination.
To be commercialised, design, construction and testing of production probes needs to be undertaken. This needs to be done in consideration of the mode of operation. Strategies and activities for achieving this are underway.
Recommendations
Spectrometric measurements of the natural gamma radiation emitted by coal seams provide a means of determining the position of a drill hole within a seam. The measurements can be compared with the characteristic radiometric profile of the seam. This radiation can only be detected within approximately 50cm of the boundaries. It is recommended that the radiometric probe be designed to reside permanently behind the drill bit and that it interface with the other survey instruments. A single detector is sufficient.
For the detection of geological structures in close proximity of the boreholes, continued development of radar and dielectric tools is recommended. While it is desirable to obtain the geological information as drilling proceeds, it is not essential. Development of the first generation of production probes should recognise this situation.
Conclusion
To determine the position of an in-seam borehole within a coal seam, a proof of concept spectral radiometric tool was designed, built and tested. This tool has been used to collect data from West Cliff Colliery.
The data shows that maintaining the drill bit close to the middle of the seam by sensing the variations in the natural gamma radiation emitted from dull and bright bands of coal is probably a better option than detecting the radiation from the roof and floor penetrating the seam.
The engineering problems of the tool, with special regard to IS compliance, have been addressed and the electronics of the tool should be IS compliant without sacrificing the performance of the system.
The next steps are to finalise the electronic design and integrate the system into a combined downhole tool. This will provide the industry with a general purpose drill position and performance monitoring tool, allowing more accurate downhole motor drilling.
The prototype borehole radar instruments have demonstrated that GPR can be used to map the location of the roof and floor and any structures which might be present. In addition to the radar a novel borehole capacitance probe shows promise for detecting mylonite zone.