WorldWide Drilling Resource

Directional Drill Reaches Depth in One-Shot Pass Adapted from Information by Boart Longyear Drilling Services To maximize underground mining, it is important to stabilize mined stopes with backfill. This allows mining to continue safely by preventing stope failure from a lack of structural support. Boart Longyear Drilling Services was awarded the directional drilling job to develop a paste fill hole at Paulsens gold mine in Australia. The paste fill hole acts as a delivery system, allowing a paste-like mixture of cement and mine tailings to backfill the underground zones where mining and excavations occurred. Further mining of adjacent stopes is optimized due to additional ground support and pillar recovery provided by the paste fill hole. The traditional method of drilling this type of hole would be to drill a pilot hole with diamond drills, then enlarge the hole using rotary drilling methods on a second and potentially even a third pass. This method could take as many as 45 to 50 shifts to complete the drill. In this case, the paste fill hole needed to be drilled to a 12¼- inch diameter to allow it to be internally lined down its entire length with an 8-inch-diameter casing. An additional challenge for this project was for the drill operator to make sure the hole would intersect within the 16-foot-diameter tar- get in the underground development zone. This meant concise and accurate surveying was needed during directional drilling. Because of the unusual requirement for a larger hole, a regular exploration drill rig could not be used. Boart Longyear decided to use a water well-style drill rig. It had the ability to accommodate large-diameter casing and a deeper drilling capability via a powerful 130,000-pound pullback. An additional booster compressor was provided as the entire hole was drilled using conventional down-the-hole (DTH) hammers. The booster compressor was essential to keep the DTH hammer efficient, as well as lift and clear the cuttings from the hole. The crew set up the rig and was operational within one week of arriving on the jobsite. As drilling commenced, the hole was collared and cased with an 18½-inch-diameter drill bit for the top 20 feet before reducing to 15-inch-diameter. This was progressed to a depth of around 180 feet before the hole was once again cased. The bottom hole assembly was then reduced to the final diameter of 12¼ inches, and drilling recommenced. As the hole progressed, Surtron Technologies conducted mul- t ishot surveys and Boart Longyear adjusted drilling parameters as needed to ensure the hole trajectory remained on target. Actual drilling operations required a total of 39, 12-hour shifts to reach the target depth of 2150 feet. Most impor- tantly, the hole successfully intersected its required target at only an estimated 7 feet from center. The average drilling per shift was around 75 feet, and the single best shift produced 295 feet. Regular rod pulls were conducted for drill bit inspection and replacement, as well as surveying of the hole. As ground formations became harder, they pulled the rods to switch out drill bits and DTH hammers, providing an optimized com- bination for the drilling conditions. With the use of a water well-style rig, Boart Longyear was able to achieve a one-pass shot, helping the owner gar- ner approximately $5 million in future gold value per mining level for this par- ticular pillar extraction. DIR 8 FEBRUARY 2019 WorldWide Drilling Resource ®