Development of a high pressure waterjet drilling system for coalseams (1990)

nnnnn Previous work has shown that high pressure waterjets can be used to drillnshort holes through coal seams. Because of the advantages of using high pressurenwaterjets it was proposed that this technique could be extended to drill longholesnthrough coal seams. To determine the potential of this technique a series of testsnwere carried out with three commercially available high pressure waterjet nozzles.n nnnnn The first nozzle tested contained two jets. One jet pointed directly ahead of thennozzle whilst the other jet reamed the hole to the required size. The nozzle and drillnstring were rotated using a hand held, air operated borer.n n nnnnn The two other nozzles tested were both self rotating. The first self rotating nozzle tested contained two jets which diverged from the face of the nozzle. Thenother self rotating nozzle contained four jets. Two jets converged across the face ofnthe nozzle, these were the main cutting jets. The two other jets were located towardsnthe base of the rotating head and were used to enlarge the hole drilled by the twonforward jets. Because both of these nozzles were self rotating they were used withnnon rotating drill strings.nnnnn Preliminary tests revealed that the cuttings produced by the cutting action ofnthe high pressure waterjets in coal ranged in size from fines to very large qblockyqnunits. Because of this it was necessary to ensure that the hole was large enough tonallow the larger cuttings to flow out of the hole to prevent the drill string fromnbecoming jammed in the hole.n nnnnn Both self rotating nozzles were able to drill larger holes. However, the firstnself rotating nozzle tested could not drill through the stone bands within the coalnseam. Tests with the second self nrotating nozzle were more successful because the jets were directed across the centre of the hole and the nozzle was less affected by thenharsh environment at the cutting face.n nnnnn As the length of the holes increased it was necessary to ensure that sufficientnflowrate was available to prevent the larger cuttings from settling out of the cuttingsnslurry along the hole and causing blockages. Reducing the drillrate and the numbernof interuptions to the drill cycle assisted in the cuttings removal process.n nnnnn The tests culminated in a series of field trials at German Creek Mine in thenGerman Creek Seam. The longest hole drilled during these trials was 38m. Thenmaximum drillrate achieved was 3 m/min, although 1 m/min was more typical. Thenholes were roughly square in shape and 70mm x 70mm in size. The holes were drillednusing pump pressures between 400 to 700 bar.n n nnnnn At these pressures the waterjets were able to drill through the stone bands innthe seam but could not drill through the harder seam roof or floor. Because eachnhole dipped towards the seam floor, drilling had to be abandoned whenever the seamnroof or floor was intersected.n nnnnn To prevent the holes from dipping it is proposed that a bent sub member benlocated behind the nozzle. For drilling longer holes, changes to the design of the selfnrotating nozzle are also recommended. These changes are aimed at producing smallerncuttings in a larger hole. The modifications proposed should also result in annincrease in the drillrate capacity of the nozzle. To drill through hard seamninclusions and the seam roof and floor it may be necessary to enhance the waterjetsnwith abrasives.n