Well, we’ve done it: successfully traversed the potential perils of the Coal Measures – that’s to say the 235m of strata which might have contained old mine workings. I was up at the site on Friday afternoon (March 11th) as the drill bit was approaching a depth of 234m, which was the depth at which we anticipated encountering the Brockwell Seam.
Apart from a few wee spots where the slightly deeper Marshall Green and Victoria seams were worked (e.g. around Widdrington and Sacriston respectively), the Brockwell Seam is the deepest (and oldest) coal seam to have been routinely worked in the Great Northern Coalfield.
Bang on cue, the drill bit suddenly speeded up, and the rig cut through the Brockwell Seam like a knife through butter. That was great, because if it had cut like a knife through thin air, we’d have had even greater problems maintaining flushing of the borehole with water.
Ever since we hit broken sandstones at 161m depth, some 7m above the Beaumont (Harvey) Seam, we had struggled to get cuttings back up from the base of the borehole. They were rising up the borehole, of course, but heading off into the fractures instead of coming to surface. Using a high pressure water flush, Drilcorp managed to advance the borehole anyway, but we could only estimate what we were going through by the speed of drilling.
As it turned out the Beaumont Seam was intact (we’ve since looked at it with CCTV), so the only explanation I have for the major fractures 7m above that seam is what we call the “angle of draw”. What this means is as follows: when a mine void collapses, it doesn’t just crack the ground right above it: the cracking also spreads out diagonally above the edge of the void, usually at an angle of about 35 degrees from the vertical.
So, although we drilled through intact Beaumont coal, it’s possible there was an old mine void in the same seam, but about 5m away from the line of the borehole. If this collapsed, then its “angle of draw” would cause large fractures at about 7m above the seam. (This is simple trigonometry in action by the way: 7 x tan 35 = 4.9).
Anyway, the Beaumont seam was intact, and so were the Tilley Seam (at about 184m depth), the Busty (at about 207m), the Three-Quarter (at about 222m) and, finally, the Brockwell Seam. So that’s it – the Complete Works of the Coal Measures in central Newcastle. We carried on to 245.5m depth, and then withdrew the drilling tools.
This morning we had specialist contractors on site – European Gephysical Surveys Limited – running special tools (which we call “sondes”) down the borehole to confirm the geological sequence and give us a sneak preview of the temperature gradient. We should hear how that went later this week.
Next, we will install a casing – basically a very long steel tube, fitted together section by section – and grout this into place. Then we will be ready for the BIG RIG to come, and continue drilling through virgin strata towards our total target depth of 2000m.
We are ahead of schedule and loving it! Let’s hope those virgin strata allow us to keep things that way.