Well that unusual red sandstone that we met about one mile down persisted – all the way to 1800m in fact. After that the rock type changed again, and we seemed to be in an unusual mixture of strata which we suppose are some of the oldest beds from the Carboniferous period. These unusual strata proved to be very hard, however, so drilling rates reduced considerably. Once it was clear that we weren’t going to go back into the enticing red sandstones anytime soon, and when we got to the point where our coffers contained only small change, we decided to declare an end to deepening the borehole. This was at a depth of 1821m – which gives us the deepest borehole ever drilled on Tyneside, and in fact the deepest direct heat use borehole in the UK (it’s a wee bit deeper than the Southampton Borehole).
Before we pulled all the tools out of the borehole we gave it a good flushing out, removing the dense drilling mud and replacing it with clean water. Then we started the lengthy procedure of demobilising the drilling rig. One pleasant surprise awaited us when we finally lifted off the Blow-Out Preventer: those red sandstones in the bottom 300m of our borehole contain water under such pressure that it rose right to the top of the well.
After cooling down a little bit, the level settled down just a few centimetres from ground level. That is potentially very useful for us, as the shallower this artesian water level is, the more heavily the deep sandstones could in principle be pumped in future. We capped the borehole off for the time being with a specially constructed cap that includes a water pressure gauge and a valve we can open to drain water out, if we ever want to (it may be, for instance, that the water pressure increases seasonally – we’ll have to wait and see). Finally, we covered the borehole chamber with a strong concrete cover, to keep everything safe and sound until we return to test the borehole in the autumn.
So now the borehole is enjoying its summer holidays: we are basically leaving it to “rest”, which means that the water in the borehole will be gradually mixing with the native groundwater, so that when we go back in for testing, it will give us representative measurements of temperature and other parameters. We remain optimistic that we have a very good chance of developing deep geothermal energy on the site – but we’ll have to wait till our autumn test work is complete before we can say quite how much heat the borehole will yield in the long term.