Case Studies

The following summary reports have been contributed by our colleagues

Performance

Case Study 1: Use of the Zero-trip Wedge in the Boliden Tara Mines During the early to mid-1990s, ongoing exploration at the Tara zinc and lead mine near Navan, Co. Meath, then owned and operated by Outokumpu Oy, led to the discovery of the South-West Extension tohe Navan deposit. The steadily increasing depth of target, down to approximately 1000 m, led the drilling contractors Priority Drilling Ltd eventually to adopt directional drilling as the primary method of collaring new holes (see Walker, 2003). In 2012, the now Boliden Tara Mines DAC, while interpreting recently acquired seismic data, identified a target some 3 km to the south of the Navan deposit (see Ashton et al., 2018). Initial drilling intersected 32.5 m @ >14 % (Zn+Pb), and subsequent drilling had by early 2017, revealed an inferred resource of 10.2 Mt grading 8.5 % Zn and 1.8 % Pb. Much of this new discovery, now called Tara Deep, lies below 1000 m depth and currently down to close on 2000 m.
These depths necessitate the continued employment of directional drilling as the primary exploration technique, which, coupled with environmental awareness, helps maintain social licence in an area dominated by agriculture and small communities. Limited rig-moves and extended site occupancy are a direct result of the use of directional drilling technology, and reduce drilling costs to the company. Since 2015, the Zero-trip Wedge to replace the existing conventional Hall-Rowe. The Exploration Section of Boliden Tara Mines DAC sees this as a direct improvement on existing technology. To explain, the substantial advantages that this new wedge offers arise from the fact that you will no longer need to prepare the hole with a Van Ruth and wooden plug, and that you will have the capability to drill past the wedge. These two advances on previous technology allow a contractor to make vast economic improvements to the utility of the drill rig and they further bring a number of benefits to health and safety, the environment and costs for both the employer and the contractor. This can be highlighted by considering the time saved when the Kick-off Point for a daughter hole might be greater than 1300 m. Current practice in Tara Deep is to drill north-west orientated ‘strike’ sections with approximately 80 m to 100 m separation between intersections outwards to around 500 m from the mother hole. This can theoretically yield 11 intersections per section over 1.1 km and three sections can be effectively drilled from one location, giving a notional 33 intersections. Exploration of the Tara Deep deposit has reached a point where the majority of drill targets will be at depths greater than 1500 m, so the economic potential of Zero-Trip Wedge technology is significant.

Case Study 2

In the 2018 drill program, 67 % of targets were drilled using directional methods, with the Zero-trip Wedge used on 92 % of these, and the traditional multi-trip method was used for the remainder. The multi-trip wedges were used in the same mother hole, due to the exceptionally poor ground conditions and some concern was expressed about using the ‘experimental’ wedge. However, these boreholes were lost and, ultimately, a Zero-trip Wedge was attempted and proved successful. During use, the Zero-trip Wedge failed to anchor three times. This was typically found to be caused by ground conditions, such as cave or rubble from higher in the mother hole, and on each occasion a second attempt proved successful. Overall the Zero-trip Wedge was shown to have a 100 % success rate. Savings The advantages of the Zero-trip Wedge come down to time. Its installation took place in one shift, regardless of depth, while the wedge cut was also typically completed in Shift 1 for boreholes with a wedge depth of 600 m to 900 m. The average for 2018 with the Zero-trip was: one shift setting the wedge, six shifts directional drilling, one shift reaming, for a total of eight shifts. An average breakdown of the time on directional drilling with the multi-trip from historic mother holes was: 3.5 shifts setting wedge, 6.5 shifts directional drilling, one shift reaming, giving a total of 11 shifts.

Over the 2018 period, directional drilling with the Zero-trip Wedge resulted in 75 shifts being saved when compared to the standard method. Note: Where possible, only mother holes where both zero and multi-trip wedges were installed have been used for time-saving calculations.

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