Deep cover. The term is regularly used in exploration circles to describe areas which have thick, barren, and often unconsolidated sediments overlying prospective exploration targets. What makes these sediments an issue is not the fact that they’re unprospective for any hard-rock metallic deposits, but rather the drilling difficulties or depth restrictions that are encountered using existing and proven drilling methods such as diamond drilling and reverse circulation (RC), respectively.
Enter the Deep Exploration Technologies Cooperative Research Center (DET CRC) which has been working to develop research projects on the subject of successful, cheaper and safer drilling technologies for use in deep target exploration through barren cover. The CRC is an Australian federal initiative setup to drive initial research into fields that can potentially deliver significant economic, environmental, and social benefits which have the potential to develop further without government funding.
The DET CRC commenced in 2010 and is planned operate until the end of 2017. The program is funded by $30M from the Commonwealth Government, $2M plus $5M in-kind from the South Australian Government, $41M in-kind from research participants, and $25M from industry participants including AngloAmerican, Barrick Australia, BHP Billiton, Boart Longyear, Imdex Ltd, and Vale.
The project is split into three streams:
- Project 1: Drilling
- Development of an operational coil tubing rig
- Drilling management and optimisation
- Project 2: Logging and Sensing
- Development of ‘logging while drilling’ technologies such as downhole magnetic susceptibility, spectral gamma, and downhole resistivity
- Project 3: Targeting
- Development of cost-effective seismic techniques
The Technology – Coil Tubing Drilling
Coil tubing (CT) drilling is not a new concept. The technology has been used for over 20 years in the oil and gas industry and is commonly utilised in conjunction with directional drilling technologies. However, CT drilling has never been successfully applied to land based mineral exploration.
The goal of the CT drilling program is to deliver a functioning and reliable drilling technology capable of drilling to 500 metres, weighing less than 10 tonnes, and at a cost of <$50 per metre.
The fundamental advantage of a coil drilling technique over conventional drilling technologies is the elimination of drill rods. Removing drill rods means no rod handling which is both faster and safer and the result is continuous drilling, maintained hole pressure (which significantly increases drillhole stability), and fast retraction of the coil which is projected to be 30 m/min.
Circulation will be conventional, with fluid pumped down the coil, through the drill bit, and up the annulus of the drillhole. The drill bit will be powered by a downhole motor due to the inability of the coil to spin. The coil is held on a large spool which spins to release or collect the coil. However, there are a number of challenges facing the development of this technology. With CT drilling relying on fluid driven sample transport within the drillhole there are two obvious hurdles; 1, the flow of water within the annulus can lead to sample contamination by the inclusion of wall-rock (especially when drilling through poorly consolidated sediments); and 2, the method will rely on clean water being pumped into the drill string. The second hurdle will require on-rig water cleaning and separation of cutting from the returning water.
Coil fatigue is a major concern in the development process of the technology. Each time the coil is bent there is an element of fatigue that impacts its longevity. Coils currently being used in oil and gas drilling applications can withstand between 30 and 200 bend occurrences, with the critical number depending on operating conditions. The DET CRC hoped to develop a coil capable of withstanding a minimum of 300 bends, allowing a single coil to last a maximum of 50 drillholes, with a minimum of 6 coil bends per drillhole. Currently, coil testing has been completed on a variety of coils with current iterations lasting over 900 bends, bringing the maximum number of drillholes per coil up to 150.
In November of 2016 the DET CRC launched the RoXplorer®, the latest version of the programs CT rig. The details of the rig are not publicly available with the rig undergoing extensive testing over the next year.
In a trial drillhole completed at the Brukunga test facility in South Australia, a 165 m drillhole was completed in 12 hours using a fluid driven percussion setup accompanied by fluid recycling with fluid being cleaned to an average of <400mg/l solids. Improvements in fluid return have been significant with 85% fluid recovery over a 65,000 litre test increasing from 45% in earlier testing.
The Autosonde™ is essentially a downhole probe with the ability to be used while rods are being pulled. Functioning sensors currently includes magnetic susceptibility and spectral gamma, but the probe will eventually include galvanic resistivity (currently in prototype stage).
The Autosonde™ is lowered inside the drill string to the bottom of the hole on completion of the drillhole and the various sensors record as the rods are pulled. The tool has many benefits especially in saving time by reducing wireline logging and core logging activities and, with the inclusion of a survey tool, time lost during survey collection will be eliminated.
The DET CRC has signed an exclusive agreement for the Autosonde™ with Boart Longyear and it is not yet known when the tool will be commercially available.
The DET CRC has no definitive projection for when CT drilling will be commercially available however Project CEO, Prof. Richard Hillis is hopeful for a date sometime before 2020. Details on commercial availability are also uncertain, with Boart Longyear and Imdex Ltd having the first right to license the technology it is likely that one of the two companies will gain exclusive rights to CT drilling, producing CT rigs for sale on the open market.
The prospect of cheap drilling through barren cover at <$50 per metre is exciting. With RC and diamond costs (contractor only) in the order of $100/m and $170/m, respectively, the opportunities in brownfields, greenfields and blue sky terranes are undeniable. Of course, only time will tell if the technology works well and at the proposed price point.
DET CRC Website: http://detcrc.com.au/