A method to collect representative samples from water-driven percussion drilling techniques: examples from RoXplorer® coiled tubing drill rig

Abstract

Water-driven percussion drilling techniques, such as that used by the RoXplorer® coiled tubing drill rig, return a rock sample as cuttings that are carried to the surface by the circulating drilling fluid. This drilling technique does not deliver a core sample. In order for the RoXplorer® to be effective as a mineral-exploration drill rig, it is critical to develop robust, representative and repeatable sampling techniques for the rock cuttings and to establish with confidence the depth interval from which the sample was derived. The RoXplorer® produces more than 7 kg/m of cuttings every few minutes (rate of penetration dependent) that are carried to the surface by the drilling fluid with a fluid return rate of 80–150 L/min. The cuttings, which range in size from <10 µm to 5 mm, require extraction from the drilling fluid and preparation prior to analysis. Subsampling of the fluid/cuttings stream can reduce the sample size to allow easier processing. A modified static cone splitter and ancillary flow-control system were designed and tested during field deployment of the RoXplorer® drill rig in South Australia in February 2017. We drilled a vertical hole with RoXplorer® starting 10 m to the northeast of an existing vertical diamond drill hole with well-characterised, shallow-dipping sedimentary and volcanic units. This allows comparison of drilling performance and sample quality against industry-standard drilling and sampling techniques. The performance of the splitter to take a representative continuous 1/8th subsample of drilling fluid and cuttings by volume was assessed based on its ability to resolve a 1 m interval of low-grade Cu mineralisation (0.74–1.03 wt% Cu) at 398 m that was detected in the reference diamond drill hole. The coiled tubing samples produced an equivalent and unbiased (chemistry and particle-size distribution) subsample, with sub-metre depth accuracy and comparable signal magnitude of the mineralisation at 398 m as detected in the diamond core. KEY POINTS Samples from the RoXplorer® drilling system were benchmarked against a twin diamond core hole that intersected a narrow band of copper mineralisation at 398 m depth A representative subsample was achieved through taking a split of cuttings as a slurry prior to dewatering Samples collected over key stratigraphic boundaries, including the narrow band of mineralisation, were accurately depth-constrained and had minimal carryover or mixing of cuttings when compared with the twin diamond core hole