Wednesday, December 17, 2014

892 m: Premature end to DFDP-2B

Rupert Sutherland, GNS Science and Victoria University of Wellington
John Townend, Victoria University of Wellington
Virginia Toy, University of Otago

A core of cement and borehole wallrock from 471 m depth. 15/12/14. R. Sutherland.

Yesterday afternoon, we decided that it was not sensible to continue drilling in the DFDP-2B borehole. We will not drill in January 2015. The broken PWT steel casing is severely misaligned and cannot be repaired. We will securely cement it in place, and then use the well as a geophysical observatory – though with less capability than planned, and above the fault.

It would be possible to continue drilling from 463 m in the DFDP-2B hole, but only with a 4.5” (114 mm) bit, or smaller. A range of technical and safety concerns make it very unlikely this is a feasible way to achieve our goals below 1000 m depth.

Our primary concern now is to ensure that the final cementing operation is done well. This is scheduled for later today (Wednesday 17/12/14). After Christmas, we will drill out the excess cement to leave a sealed borehole to a depth of 400 m. We will install a wellhead, a seismometer, and then make temperature measurements.

This is not the end of the DFDP-2 project. We did not achieve our goals of making observations and installing instruments within and below the fault zone in DFDP-2B; but we obtained important data and samples to a depth of 893 m. These show that the Whataroa Valley is even more interesting than first thought, and that our original plan was feasible.

This project is interesting to the international science community, because we are investigating a fault that is due to fail in a large earthquake, and because we can investigate it deep enough to account for the complications of mountainous terrain above. This has not been done before.

We have learnt that the Whataroa Valley is much hotter than anticipated. The base of our planned borehole at 1300 m has a temperature of about 190°C. Temperature is fundamental in rock mechanics, and 190°C puts us in the lower half of the brittle crust, where some very important earthquake processes occur. Sampling any fault in this state would be an important world first. It is high risk, but with potential for very high reward.

More blog posts will follow…

Virginia Toy walks past a table of concrete. It was not what we had hoped for.
14/12/14. R. Sutherland

Funded by: the International Continental Scientific Drilling Program (ICDP); the Royal Society of New Zealand Marsden Fund; GNS Science;Victoria University of Wellington; University of Otago; and governments of NZ (MBIE), UK (NERC), & USA (NSF).