Setting new directions in straight line drilling

The science for controlling and changing the direction of an operational drill, whether for oil or minerals exploration, has advanced significantly but it is still imperfect, leaving operators with problems to solve.

Recently CSIRO lodged a patent application for the invention of a new drilling method and assembly that takes drill control to a new level of scientific excellence.

The invention relates to methods and assemblies that employ ?feed force? control (feed force being the thrust in the direction of the tool) in conjunction with an asymmetric drill bit to maintain the direction of the drill bit as it advances along the drilling line, thereby avoiding deviation from that line. The invention is particularly concerned with drilling of rock bodies, although other applications may also be contemplated.

The new method and assembly builds on the existing drill control method, eg:
? The placing of wedges or ?whipstock? at the base of the drill hole to deflect the bit in the desired direction, which is time-consuming and can result in reduced drilling rates.
? The use of articulated drill pipes where bent drill strings are used to force the bit in the desired direction which can necessitate the halting of drilling to allow a survey to take place and appropriate alterations to take place.
? The locating of a hydraulically actuated ?shoe? behind the drill bit and the use of drill fluids to push the shoe out and apply force to the sides of the hole, pushing the drill in the right direction.
? The use of a directional head which results in the drill bit rotating and drilling in a spiral path and which produces satisfactory results only in soft materials.

According to the leader of the drill project, senior research scientist Dr Gary Cavanough of CSIRO?s new Earth Science and Resource Engineering division, ?the invention aims to provide a method and assembly that will identify deviation from a drilling line and make corrections accordingly?.

?For example, gravity and ground conditions may cause deviations when drilling long holes and up-holes in hard rock mines,? he said. ?This can result in large, oversize rock fragments being produced by drill and blast operations. The method of this invention may be used to compensate for the effect of gravity and ground conditions by dynamically adjusting drill trajectory.

?Reverse circulation (or RC) drilling may also find the invention useful, particularly to drill deep holes where large diameter drill bits are required to maintain a drilling line,? said Gary Cavanough. ?The large size bits/rods require large volumes of compressed air while deep holes require multiple truck-mounted compressors to provide sufficient air volume.

?It is envisaged that the method of this invention may decrease the required size of the drill bits which may increase drilling rates and save significant energy due to reducing the need for compressed air.?

The invention?s method for maintaining a drilling line can be summarised simply:
? Imparting a feed force from a drill to an asymmetric drill bit.
? Identifying deviation of the asymmetric drill bit from the drilling line.
? The feed force imparted to the asymmetric drill bit is controlled such that it is not synchronised with rotation of the asymmetric drill bit if the bit is following the drilling line but is synchronised with rotation of the drill bit if the bit is deviating from the drilling line, so deviation from the drilling line is corrected.

The directional drilling assembly of the invention includes a device to control feed force that synchronises the feed force imparted to the drill bit and is applied either by a cylinder or even an electric motor.

In order that the assembly may effectively maintain drilling line and avoid deviations, the assembly includes a guidance system that identifies any deviation of the drill bit from the drilling line. The guidance system generally communicates with the feed force control device. Communication with the device results in an immediate response in the control of feed force when the drill bit deviates from the drilling line.

Advanced sensors are increasingly being used in drilling and one could be utilised with this invention to identify any deviation of the asymmetric drill bit from the drilling line, for example, a transmitter or other locating device attached to or near to the drill bit which identifies its location.

According to Gary Cavanough, the preferred method would be to ?pass a laser beam between an origin point and the asymmetric drill bit whereby deviation of the laser beam is indicative of deviation from the drilling line?.

?More particularly,? he added, ?the laser beam is preferably passed from the origin point, for example located on the drill, down at least one drill string associated with the asymmetric drill bit and is reflected back to the origin point by a reflective target located on the asymmetric drill bit.

?If the drill strings maintain the drilling line the laser beam will maintain unchanged engagement with the reflective target but if there is deviation from the drilling line, the laser beam will deviate too and this deviation can be monitored and identified,? he said.

The invention has already stimulated interest from industry and CSIRO has been in discussions with a drill manufacturer.

This article is reproduced with the kind permission of the CSIRO?s new Earth Science and Resource Engineering division. It was originally printed in Earthmatters, No. 22, March/April 2010.

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