The Development of a Geosteering Tool for In-Seam Drilling

Geosteering is an oilfield term that refers to directional drilling guidance systems that have on board sensors to determine the rock type either being drilled or in proximity to the drilling head. Geosteering technology allows drills to follow target horizons and return information on the nature of the rocks being drilled. The inability to provide equivalent geosteering capabilities that could allow in-seam drill holes to maintain an intended trajectory within a coal seam is one of the outstanding problems in in-seam drilling. In 1996, Sigra and CMTE began a joint research project aimed at developing a tool to provide geosteering options for in-seam drilling. The tool was to provide a survey capability, drill torque, thrust and rpm measurements, and geophysical measurements of rock resistivity and the spectrum of the natural gamma radiation. It was intended that the tool could reside behind the drill bit and communicate through the drill string via existing communication systems or a cableless system that Sigra was proposing to develop.

The development of geosteering tools has proved to be far more difficult than originally expected. There have been technical issues, issues of developing tools for the intrinsically safe environment and difficulties in being able to undertake trials on drill rigs or in holes, from either the ground surface or underground. Over $1,000,000 has been expended on this project and it has only been able to deliver prototype systems which require significant further development before the ultimate goal of a seeing these tools in routine use can be achieved.

The results from various tests, however, are extremely encouraging and indicate that geosteering is achievable given appropriate commitment. Sigra has demonstrated the acquisition of rock resistivity and drill parameters - torque, pressure and thrust while drilling is underway. The expectation is that the resistivity measurements can indicate the approach of the coal seam roof or floor from the borehole. CMTE has established through the logging of in-seam and vertical boreholes, that spectrometric analysis of natural gamma radiation allows a characteristic seam profile to be monitored and that stone bands in the seam can be distinguished from the roof and floor. The ratio of the counts due to Thorium and Potassium, is particularly diagnostic and when combined with the information on the total radiation, meaningful information about the position of the hole within a seam can be achieved.

All tools have been designed and constructed in consideration of the rigours of the down-hole environment, the needs for intrinsic safety, the needs for power, modularity, down-hole processing and communications. Detailed discussions on these considerations and their implications are provided in the report.