Over the last few days we have been slowly progressing through sediments containing cobbles and boulders. The team looking at small fragments of rock cuttings have done a great job in being able to tell us that we are definitely not in bedrock yet.
A couple of days ago, we became frustrated by our slow progress and removed (tripped out) our drill string from the borehole, to put a sharper bit and heavier drill string back in. During this process, the mud in the hole became diluted with water from the bottom (the hole has 12” steel casing around it), and the well started to flow (artesian).
By the time the new drill string was in the hole we were producing hot water at 5 litres/second and 43°C. This was a great opportunity for the fluid and gas chemists, and for those of us who are interested in the thermal and fluid state of the fault.
Warm water at dawn 10/10/14. (photo D. Prior)
The flow was controlled by injection of new drilling mud and drilling recommenced, but it continued to be difficult to advance the casing. We are now at 236.6 m depth. A decision was made yesterday to start installation of a new 10” steel casing string. The basic idea is that each slightly-smaller casing is nested inside and protected by the previous one. We have to make progress through this difficult-to-drill zone, and this is our only option.
We are making slow but steady progress. The chemistry and gases of the hydrothermal fluids seem similar to those of nearby hot springs in bedrock, and the high fluid flow rates suggest a quite different hydrology to the silty sediments that we were drilling through up until now. We think it likely that we are close to a contact between sediments and bedrock.
Meanwhile, the ICDP training course in Franz Josef Glacier has ended and participants have returned to Whataroa to help on-site. Our scientific facilities are almost all fully completed and providing a comfortable and efficient workspace.
Geophysical tool assembly 8/10/14 (photo J. Townend)
Events of the last few days again confirm that scientific investigations can produce interesting and unexpected results — the power output of our geothermal production was about 0.7 megawatts.