October 2017

Drilling advances in underground coal

  • By Stephen Fraser, Program Director, Mining3

Advancements in drilling technology have the potential to deliver large economic gains to the mining industry

As innovation improves most areas of the mining process, drilling technology is not immune. With the development of GPS, wireless communications, sensors, real-time data and autonomous equipment, drilling technology is evolving to meet the needs of the industry. Mining3 continues to provide a range of capabilities in drilling technologies that are further enhanced by our partnership with CSIRO Mineral Resources.

Drilling remains a vital component of the mining process and is used for multiple purposes including ore body characterisation (with electronic geo-sensing technologies and sampling), surface and underground blast hole drilling, underground roof and wall bolting and cabling, dewatering and degassing.

Research over the last decade has focused on a number of key areas, but this article looks primarily at advances in drill bit technology, alternative cutting techniques and changes to the drill rod flexibility. The intention of these advances is to enable:

  • improved steering of the drill bit
  • better orebody knowledge of the rock surrounding the drill bit
  • real-time communications
  • increased flexibility of the drill string for improved handling
  • development of novel drilling methods.

The technologies seek to increase productivity by saving time, improving efficiencies and effectiveness and providing a higher level of control.

Improved steering and borehole imaging

One of the greatest advances in drilling in recent decades has been in geological sensors and high accuracy drill rod orientation sensors fitted behind the drill bit with survey electronics. They enable operators to effectively ‘see’ inside the borehole and steer the drill rod. The acquired data are transmitted to the surface at regular intervals by implementing a telemetry system that is inserted at the end of the drill rod.

Mining3 has led the way in measurement while drilling (MWD) technologies for more than two decades. Current developments have focused on slim line MWD geo-sensing tools for the coal mining industry. The outcome is a suite of MWD tools that can facilitate faster and more reliable holes drilled by providing accurate coal interface detection and geo-steering. Although funded by the coal industry, these high precision, slim line and explosive environment-rated tools have wider application. One example is the use of these tools to re-enter an old oil and gas well that is known to contain reservoirs at shallower depths.

The first of the tools commissioned by Mining3 was a high accuracy directional gamma and density probe. This tool was deployed on a wireline in two 1300 m surface inseam (SIS) boreholes in 2011. It was shown to have a resolution that surpasses capabilities of existing shuttle tools used in Australia. It provided good results when imaging the rock through the steel casing, and is ready for commercial applications.

The next tool developed is a directional resistivity measurement sub for coal interface detection (CID) for inseam lateral drilling. This probe uses electrical fields generated in four different directions and can detect different rock types surrounding the borehole, as well as the direction of the response. The resistivity tool has been developed into a prototype, with a proven maximum penetration depth of 1.5 m. The results closely matched the performance of existing geo-steering tools on the market.

One of the greatest challenges that exists in underground coal mining is the ability to accurately automate a longwall miner to follow a known target horizon. Longwall automation has now been achieved through the LASC Longwall Automation project conducted at CSIRO. However, the location of the coal seam horizon is still uncertain. Horizon prediction at the face requires high confidence and a high level of accuracy. An accuracy of 30 cm is required 20-30 m ahead of the coal face. Mining3’s development of geo-sensing tools is part of a broader strategy involving collaboration with other research partners to leverage the data for 3D geological modelling to assist with longwall shearer/plough horizon planning.

Geological imaging can range in depth and resolution. Modern imaging tools can exceed 5 m in depth penetration, albeit with poor resolution. However, more sophisticated tools can provide resolutions of better than 30 cm at a depth penetration range of 2 m. This depth of measurement provides valuable information that can help develop an image of the coal seam that is more detailed than that achieved through seismic surveying and radio imaging. The tools that Mining3 are developing will be optimised for accurate imaging of the coal seam roof and floor from within an inseam borehole.

Underground in-seam drilling

MWD tools for underground coal seam drilling have received strong industry support over the years. The underground inseam (UIS) industry is lacking several leading-edge MWD capabilities that have helped the surface to inseam drilling industry improve the quality of metres drilled and provide geological surveying data for mine planning. These capabilities include real-time wireless telemetry with high accuracy geo-sensing tools to be connected directly behind the drill bit. The telemetry system allows measurements from sensors, such as high accuracy orientation sensors and geo-sensing units to be sent to the driller whilst drilling. They can use the orientation and geological measurements to steer the drill bit through the target rock layers. The data are downloaded after a borehole is drilled and used for more accurate mine planning.

One inherent weakness in current UIS drilling practices is their inability to maintain the bit within the confines of the coal seam. As a result, coal seam boreholes tend to undulate, frequently drilling out of the seam. This requires pullbacks and branching, which duplicates drilling metres – and establishing a branch takes time. It is estimated that 20-30 per cent extra metres are drilled as a result of branches. Branching also carries a risk of borehole collapse, so is not desirable from a gas drainage perspective.

Mining3 has developed what is believed to be the first wireless survey tool for the UIS market capable of transmitting data in a matter of seconds. It is slim line, intrinsically safe (for explosive environments), modular and scalable, and has been designed to interface with third party geo-sensing tools.

This advanced logging tool incorporates a low energy, slim line battery and fits into most drill rods on the market. The whole assembly, including the batteries, logging unit, power distribution unit, orientation unit, and Inseam Wireless Drill String (ISWDS) communication unit is approximately 3 m long and has a 69 mm outer diameter.

Flexibility for improved handling

Coiled tubing drilling differs from conventional drilling in that the drill string is a continuous, malleable steel coil, as opposed to being comprised of individual steel rods that must be connected and disconnected.

Research into coiled tube drilling began in the mid-2000s. Mining3 developed small coiled tube drilling rigs for underground drilling. Large-scale rigs for deeper ore body exploration have been developed by the Deep Exploration Technologies Cooperative Research Centre (DET CRC).

A next generation drill rig for greenfield mineral exploration that incorporates the coiled tubing technology recently underwent successful trials in South Australia by DET CRC. At a cost of $50/m with a depth of 500 m, it delivers rapid drilling with no string rotation, no one near the drill string, no rod handling and minimal fluid loss on the surface. As well as being fast and cheap, it offers a substantial improvement in safety because individual rods do not need to be manually handled (moved or connected).

Water jet drilling technology

High-pressure water jets have been a focus of research for applications over a wide range of activities related to mining and excavation technology. Investigations by Mining3 over the years have indicated increased drilling rates over conventional drilling, and potential significant savings in drilling costs – especially for deep land wells. For small hole drilling such as blastholes, anchors and roof bolting, water jets are being investigated. A variety of materials can be drilled using jet cutting including limestone, sandstone, granite, taconite, oil shale and tar sands.

V2H Australia is a spin off company of Mining3 that was established to deliver the first effective radial and water jet drilling technology to the coal and oil and gas industry. The technology sees a traditional drill head replaced with a high-pressure water jetting system that rapidly creates an extensive pattern of horizontal tunnels for oil and gas to flow directly into a vertical well.

The innovative drilling technology has been engineered with a flexible, high performance, instrumented jet head. The system is steerable, controlled, flexible, robust and reports to the operator in real time. It is able to turn from vertical to horizontal within the confines of a well bore and is strong enough to mill through steel casing.

Conclusion

Drilling technologies remain a core area of research and offer productivity and safety gains with every advancement. By providing better visibility of the surrounding material as well as being able to control, manoeuvre and monitor drilling progress while increasing the speed of advancement, technology improvements can deliver large economic gains to the mining industry.

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