Published in Australian Longwall Magazine

In December 2003 trials of newly developed geophysical tools were about to kick off at a central Queensland minesite when an unexpected cyclonic storm dumped 75mm of rain in under two hours. The researchers, some of whom had travelled from Germany for the trials, packed up their hi-tech gadgets and went home.

Six months later, with the help of some additional ACARP (Australian Coal Association Research Program) funding the project was back on track and the trials were completed.

Now, for the first time ever, these new tools have been tested in a horizontal borehole - pre-drilled into the highwall at the German Creek mine - with some very promising results.

Geophysical tools are used in vertical boreholes to source information about the hole, such as the density, natural radioactivity and electric properties of the rock and seismic wave behaviour.

While directional drilling is widely conducted in Australia for gas drainage and exploration purposes, no geophysical instrumentation is used in underground coal mine drilling, according to Peter Hatherly, a senior geophysical researcher with CSIRO and CRC Mining.

As a consequence, valuable geological information is lost from the drilling process. Furthermore, the inability to accurately steer a drill rig in-seam adds a 20-30% increase to drill costs associated with drilling going into the roof and floor.

Known as coal interface detection (CID) technologies, these new geophysical tools could change all that, and make inseam drilling much more effective and cheaper.

The main application would detect the coal/rock interface boundary, allowing improved drill steering in-seam. Controlled drilling within the seam may also bring benefits in terms of gas flow rates from in-seam holes.

“In the ideal world you’d have the intelligence at the drill bit, performing like a self-guided missile that stays in the seam. Then you wouldn’t have to worry about the control or intrinsic safety issues at the collar of the hole,” Hatherly said.

“But there are a number of difficulties. If you want to introduce the sensing tools onto the drill they must be robust enough to withstand the environment.”

The origins of this work date back ten years to the start of the ACARP program when an industry workshop established a long-term goal to further work in geophysical logging and sensing for in-seam drilling.

Geophysical tools are routinely used for oilfield logging during which logging data is supplied in ‘real time’ back to the drilling operator and used to determine the path of the borehole. The oilfield geological model is then updated and reactive decisions made on the path of the borehole.

However, underground directional drilling falls a long way short of oilfield drill management, as steering and logging, and amending the geological model based on in-seam drilling data is virtually unheard of. This is largely because of the intrinsic safety barrier to development and few research dollars. Furthermore, little has changed in terms of drilling practice in the past ten years.

Hatherly said exploration of coal mining leases by directional drilling has been carried out inconsistently and was not widely used. This was due to the poor quality of downhole data received, and the inherent risk in drilling long distances in coal with expensive, non-retrievable equipment (in particular the survey tool and downhole motor).

“The method has its adherents, and is generally utilised in a reactive manner according to specific mine planning requirements. Seldom is exploration drilling carried out on a routine, systematic basis. Often it is a by-product of gas drainage drilling - a consequence of surprising and adverse circumstances.”

Core is rarely collected and the only formal record of the borehole is data from the survey instrument and the written records of the driller. According to the skills of the operator, the notes are usually very basic and open to interpretation.

In-seam directional drilling to distances over 800m is rare in coal field drilling and the majority of degasification holes drilled are less than 300m.

In underground directional drilling the only approved survey system currently in widespread use is Advanced Mining Technologies’ DDM MECCA system but it offers no geophysical instrumentation.

AMT recently released an approved new generation survey system - the Drill Guidance System - which offers directional gamma as an option. The system is growing in acceptance with five units currently on order for delivery to New South Wales operations.

The new technologies trialled at German Creek included a directional gamma system and a borehole shuttle (with density, gamma and acoustic calliper) developed by the German research group Deutsche Montan Technologie (DMT); CSIRO DEM’s spectrometric gamma system; and CSIRO TIP’s radar and dielectric tools.

Trials involving the DMT borehole shuttle using directional gamma proved successful, Hatherly said. This tool is a post-drilling logging tool, inserted into a pre-drilled borehole, within the drill string, which is then retracted.

The gamma tools measure the varying gamma radiation in rocks and coal. As coal has a low gamma count the gamma signature for the roof and floor typically varies, with usually higher gamma responses associated with the floor.

The use of natural gamma as a means of coal interface detection is already established, over the past two years it has focused mainly on medium radius surface to in-seam drilling. However, to date, directional gamma has not been applied.

The DMT tool uses two orientated gamma detectors, which provide data for the roof and floor. The test results showed the sensor looking up had a higher gamma count as the roof was approached than the one looking down and vice versa.

“An unexpected bonus from the testing was the suggestion that individual coal plies had a unique gamma signature, thereby enabling the position of the sensor within the coal seam to be determined,” Hatherly said.

The tool has the ability to distinguish whether the drill is in the roof or the floor, a valuable and significant improvement on current methods. Also, during the trials the DMT data revealed dykes at 165m and around 350m, which are not readily identified by drilling alone.

So what exactly have the trials delivered? Hatherly said while the trials were a success there was no silver bullet. Yet.

“We still don’t have a real-time sensor you can put on the drill bit, able to deliver a straight hole without the problems of doing extra drilling or hitting friction points, that lets you know exactly where we are,” he said.

Greater confidence in geophysical tools could make exploration by directional drilling more attractive, though the development of surface to in-seam drilling, which has made significant progress in Australia in the past 24 months will also have an impact on future directions.

Hatherly said there had now been about 60 successful surface to in-seam holes drilled, mainly in Queensland.

“Is the industry moving to a situation where most of its gas drainage and exploration is done from the surface? It could be the way things go,” he said.

Either way, intelligent geophysical logging systems deployed in horizontal boreholes will be part of the future.