Monday, February 7, 2011

Finishing off - and next time we will go deeper

Our project has now moved into completion and demobilisation mode. We drilled boreholes DFDP-1A to 100.6 m and DFDP-1B to 151.4 m. The cores we collected have been carefully packed and taken back to the University of Otago in Dunedin, and the processing tents and most of the people are now also gone.

Wireline logging took several days because of borehole instability issues and yet more technical obstacles. We did not get clear runs through the whole borehole on some of the tools, but we have collected a very nice dataset with a wide range of tools: natural gamma, calliper, electrical resistivity, spontaneous potential, density,
porosity, full waveform sonic, dipmeter, and induction flow meter.

Installation of observatory equipment in borehole DFDP-1B is still under way. We successfully installed a 2 Hz seismometer at the base of the borehole. We have 4 piezometers in place to monitor pore fluid pressure, and will install a fifth tomorrow. Twenty five sensors monitor the average temperature gradient in the borehole and detailed thermal structure immediately above the fault. We will continue with observatory installation and testing tomorrow.

First photo: Full waveform sonic tool and wireline logging truck

Right: Observatory installation team: Jennifer Eccles, Mike Hasting, Simon Cox, myself (kneeling), Alex, John
Townend, and Jeremy Cole-Baker.

An initial observation that I found interesting was confirmation that fault gouge is an effective hydraulic seal. A preliminary estimate of pore pressure difference across the fault in DFDP-1B is equivalent to 33 m of water head. This partly explains borehole instability when the hole was left open: there was a “waterfall” within the borehole trying to wash fractured rocks above the fault into and down the borehole. When trying to clear the borehole in DFDP-1A, to grout a cement-bentonite seal into the fault, we punctured a temporary breach with a long plastic pipe.

There was the most extraordinary slurping noise as water was sucked out of the drill string into the lower pressure gravel aquifer beneath.

Egor lowers casing with a shoe bit and casing advancer inside

Over the next few days we will wind up field operations, demobilise equipment, and secure the permanent observatory. It is gratifying to know that, despite substantial challenges, our long hours, good planning, and excellent contractors have led to success for all of our main technical objectives. We collected lovely cores across the fault, ran a full suite of wireline logs, and installed a fault zone observatory. I specially thank the students involved and our visitors from Liverpool University. Everyone worked long hours and behaved in a very professional and collegial way.

The whole operation was very slick. You really would not have known what a steep learning curve we
had actually all been on. Finally, I give massive thanks to Egor our “senior” driller. He worked hard and long hours, never had a day off, solved the many technical challenges we created for him, and mostly kept a smile on his face too (though he didn’t like torrential rain).

This is Rupert signing out for now. Over following months the massive quantity of samples and data we collected will need to be carefully analysed and written up, so it may take some time before I poke my head up above water again. The next phase, DFDP-2 (>1 km), already has advanced plans, but we will certainly want to improve them now that we have had this practical experience and learnt so much about what we might find at greater depth.

Egor and Alan

Finally, in case you haven't seen it, here is a short introduction to the Alpine Fault Drilling project:

Tuesday, February 1, 2011

Alpine Fault drilled through again!

The customised system of open-hole drilling devised by Alan Speight (our head driller) worked fantastically. We rapidly progressed from 45 m to 93 m in 18 hours, including one unsuccessful attempt to core. We  changed back to coring again at 93 m because we noticed an obvious change in cuttings and mud colour.
A sample of cemented cataclasite is carefully extracted from the core catcher

Coring progressed smoothly this time, even though one of our drillers walked out on us for no apparent reason. For the last two days I have been working as a drillers offsider, trying to keep us on track with the project, but it has been tough to balance every demand.A sample of cemented cataclasite is carefully extracted from the core catcher.

Drama continued again last night when a sudden storm passed through at 2-4 a.m., flattening our core processing and logging facility. Fortunately, nothing catastrophic was lost or damaged.We rebuilt better and were ready to drill again by 7 a.m.

Cores of the Alpine Fault footwall emerge for the first time
Skies cleared during the morning, lovely cores continued to emerge from the borehole, and life was looking good. At around 4 p.m. today we ran into a slight problem: we hit an obstacle, obtained no core, and could not advance any further. We tripped out all the drill rods and checked the bit. It confirmed the suspicion that I had after inspecting the core catcher from our previous run. Light grey clay gouge was blocking mud circulation. Was it just a gouge-rich zone within the thick (>30 m) cataclasite zone, or was it the main fault?

At about 7 p.m. we collected the first core from beneath the problem zone (thanks to the skill of our driller on the levers, “Egor”). This core and the subsequent two confirmed that we had just crossed the principal slip surface of the Alpine Fault at a depth of 128 m. Fantastic!

Both of our boreholes have obtained high-quality cores across the fault, including the precious fault gouge of the primary fault surface. There is a clear sense of excitement and achievement, and a big group has decided to work through the night to see what is going to appear.The rock cores currently emerging are really interesting, because the rock types immediately beneath the fault are not well known, in comparison to the well exposed and studied sequence above.
A beautiful core of the Alpine Fault from 128 m

It is premature for me to say what the rocks are that we cored, but brilliant white and green shades indicated that we had crossed the fault. We decided to keep going for another  shift, and over the next 24 hours a preliminary geological description will be constructed.

If all goes to plan, we will have finished our main phase of drilling tomorrow. We will move onto
wireline logging and geophysical observatory installation. I personally now need a few hours
sleep after the drama of the previous two nights.

John Townend, Richard Norris, Rupert Sutherland, and Virginia Toy consider achievement so far