A deep hole exploration project is taking place in Finnish Lapland to increase Agnico-Eagle Finland's probable reserves at depth in the Kittilä gold mine. Diamond drill rigs have since the summer of 2008 been drilling NQ-size boreholes to recover core for analysis. Arctic Drilling Co's LM75 drill rig has been drilling boreholes deeper than 1,300 m and that in a quite impressive six weeks. The results will hopefully show that it will be profitable to develop further from open pit to underground mining.
In the beginning of March 2009 Devico was hired to help make branch holes from the boreholes that had been drilled in previous months. Norwegian company Devico specialises in directional core drilling and borehole surveying. It was hired to improve the accuracy of the boreholes and reduce drilling time and costs on the prohect compared to drilling from the surface.
Agnico-Eagle Finland had already in an early stage of the project considered using directional drilling to hit targets at depth. Kati Oy and Arctic Drilling, two Finnish drilling contractors, were contacted to drill NQ-size boreholes in the mafic vulcanite rocks. Several boreholes were drilled during the autumn and winter. By spring 2009 sufficient boreholes had been drilled to involve Devico more actively in the project.
Once Devico arrived at Kittilä, the process of branching from the earlier boreholes started. At that time initial planning had taken place at the office, discussing how to hit the targets in the best way. Several factors had been considered, like which order to drill the targets, at what depth to start the different branches and which borehole to give priority.
To start a branch from the mother hole, the drilling companies installed an orientated steel wedge to force the drill bit to create a new borehole. When the new branch hole was started, Devico could run its DeviDrill down the hole. The DeviDrill is a directional core barrel that fits directly on a normal NQ-wireline operated drill string. The DeviDrill can be adjusted to give any curvature, but a dogleg of approximately 9 is normal (that means 9o deviation per 30 m drilled. Higher doglegs risk possible wear and damage to the drill rods, as well as increased torque.
The directional core drilling operation is a co-operation between the Kati and Arctic Drilling drillers and field engineers from Devico. The field engineers do the orientation and borehole surveying, while the drillers drill with supervision from the field engineers. A normal directional core drilling section varies from 15 to 80 m, depending on how far from the mother hole the targets are located.
When the directional drilling section is finished and the required azimuth and inclination has been obtained, the directional core barrel is replaced with a standard core barrel. Borehole surveying is regularly performed to make sure that the borehole is not deviating more than the expected natural deviation while drilling continues towards the desired target. If necessary, a correction has to be made closer to the target. In the mean time, the field engineers from Devico have started a new correction on the next drill rig.
When it is time to hit a second target, a different technique is used to create the branch hole. It is possible to take advantage of the directional section in the borehole and drill straight out of the curve. This approach is less time consuming, since it involves less tripping of the drill string, than putting down a wedge. Slow and careful drilling in the curved section, repeated a few times, creates an edge which is the beginning of a new branch. Besides being quicker, this approach also gives a smoother borehole trajectory, and avoids all the disadvantages of using wedges. After the hole has been created, the DeviDrill is yet again run down the borehole, to create the new curve leading to the second target.
The feedback from the project so far is very good. The co-operation between the drilling companies and Devico is working fine, and Agnico-Eagle Finland is happy to hit targets in less time and with higher accuracy. www.devico.comJohn Chadwick is editor/proprietor of International Mining magazine - www.im-mining.com