Underground » Geology
The aim of the Advanced Logging Tool project was to develop a suite of Measurement Whilst Drilling (MWD) tools for the Underground In-Seam (UIS) drilling industry that can facilitate faster and more reliable holes to be drilled by providing accurate Coal Interface Detection (CID) for geo-steering. This work is part of CRCMining's broader strategy that will involve collaborating with other research partners to leverage the data for 3D geological modelling and assist with longwall shearer horizon control.
Phase 1 of the Advanced Logging Tool project commenced in 2010 with the development of the 'Superlogger' tool, a pump down shuttle tool that provides high resolution gamma density and directional passive gamma measurements. This tool has been deployed commercially in Queensland, and the phase was completed in 2012. The subsequent Phase 2 commenced in 2013, which involved the development of an intrinsically safe LWD platform and a prototype directional resistivity tool.
The UIS industry is lacking a number of the leading edge capabilities that has helped the surface drilling industries to accurately steer through complex reservoirs and provide geological surveying data for identifying and mapping the coal seam roof or floor, dykes and sills, and faults. These capabilities include:
· Geo-steering tools for coal interface detection (CID);
· Coal seam imaging tools for more accurate longwall horizon control;
· Real-time wireless telemetry that enables standard hollow drill pipes to be used;
· High performance pump down wire-line tools.
UIS drilling is currently a blind process that utilises an orientation sensor for calculating the borehole trajectory, without knowing where the drill bit is inside 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 meters, adds time, and increases the risk of a borehole collapsing and blockages. It is estimated that 20-30 per cent extra metres are drilled as a result of branches, and blockages impact the effectiveness of gas drainage which can delay mining.
Steering with vision of the rock ('geo-steering') can be achieved in horizontal boreholes using near-bit directional gamma, or directional resistivity tools with shallow and deep depths of investigation. The gamma tools provide a maximum depth of investigation of 50cm, and are suitable for at-bit rotary steerable applications, but are not suitable for UIS drilling as they would sit too far behind the bit and motor. On the other hand, modern resistivity tools provide a depth of investigation of 2m-5m. Directional resistivity has been used for imaging and steering through complex parallel bed boundaries.
Existing resistivity technologies do not meet the requirements for UIS drilling. It would be very expensive for an SME to manufacture a slimline, hazardous area rated tool. Therefore, a prototype directional resistivity tool that is optimised for lateral in-seam drilling has been developed together with a slimline logging-whilst-drilling (LWD) tool. The prototype resistivity tool has two capabilities, (1) high resolution directional imaging of the borehole surrounds with two depths of investigation, 0.4m and 1.2m, and (2) at-bit directional coal interface detection. For applications where long drill rods can be used, a longer electrode sub can be utilised, which would increase the depth of investigation.
Experiments were conducted in simulated conditions with lab prototypes of the imaging sub. 3D modelling of a full-size imaging sub was also conducted. The results of the physical experiments and 3D modelling demonstrated that the sensitivity at low contrasting roof and floor conditions was similar to published data from existing geo-steering tools. The tool also utilises the down hole motor and bit as a directional measurement electrode that increases the measurement distance and adds at-bit sensitivity to the roof and floor, dykes, and sills ahead of the bit. This report provides details of this work.
Based on the positive outcomes of this phase, one more phase of R&D will be conducted to manufacture and demonstrate the resistivity tool in an in-seam borehole. A parallel CRCMining/UQ/ACARP project will also deliver a commercially operable wireless survey tool by the end of 2016; which will be the first product that will be delivered by CRCMining for UIS measure-while-drilling.