This program had multiple Drill technical problems associated with it. The first part of the hole was dry sand, further down the hole the sand became more stable but there was the possibility of silcrete bands in the sand that had required other drilling companies to hammer under an unconsolidated sand horizon. I understand that the other light rab/aircore rigs were unable to penetrate the silcrete with blades and found DTH Hammer use problematic. I had suggested to the client that as Bostech could put a-lot of weight over the hole (heavy masts) and our Rigs had class leading head torque this should together with our own manufactured Aircore bits enable us to drill straight through the silcrete; this proved to be the case.
The next contradictory stage of the hole was a band of clay immediately above the lignite, this band of clay required clay bits and a higher discharge pressure to ensure positive sample return. The successful drilling of the sand above requires very little hole pressure, too much bit face pressure in the sand tends to blow the sand back onto the rods and causes tight holes or stuck strings. I am certain that the clay section was drilled very efficiently with our bits as they were originally designed to work in Paleo Channels. Other methods used to drill these sections were;- R/C, Diamond, other Aircore, and sonic.
The next challenge was how to get the low specific gravity lignite out of the hole using Air drilling without either ending up with a empty sample bucket, or a bucket full of water? It was noted that the R/C rigs had trouble getting sample because of the high pressure air discharge at the skirt would have tended to displace the sample away from the return ports in the bit. This would have resulted in reduced sample. I was told that some of the other air core rigs tended to draw fluid from the lignite and end up supplying washed sample (too much water) what Bostech did was;-make sure we did not have an excess of bit face pressure compared to formation hydrostatic pressure (R/C rigs) or an bit face pressure markedly less than hydrostatic formation pressure, (face turn- around Aircore) which encouraged or allowed the sucking of fluid (water) from the formation but to balance the two pressures (bit face /hydrostatic) such that the sample characteristics would be representative of the natural in situ fluid content. We did manage this and obtained a porridge consistency.
Bostech also proved to be able to leave drill strings in the ground whilst the logging crews surveyed the holes; this meant that no time was lost waiting for the logging to be completed. This was simply done by supply extra two strings per rig and drilling until we had used them all then going back to recover them and start the cycle again.