Thursday, May 18, 2017

High temperatures and pressures explained

Rupert Sutherland, GNS Science and Victoria University of Wellington
John Townend, Victoria University of Wellington
Virginia Toy, University of Otago
Figure 1. View from State Highway 6 of the DFDP-2 drill site and Whataroa valley, Westland. Photo: J. Thompson, GNS Science.
DFDP-2 drilling involved surprises that created challenge and opportunity (Figure 1). We discovered some of the most extreme underground conditions on the planet, and the hot fractured rocks caused technical difficulty. Temperatures and fluid pressures were much higher than anyone had predicted. Temperatures of 100 degrees Celsius (°C), hot enough to boil water at the surface, were discovered at just 630 m depth. It has taken us two years to process and understand the data, and then simulate our findings using computer models. Our observations and analysis are now published in the prestigious scientific journal Nature (doi: 10.1038/nature22355). More scientific publications will follow.
It usually gets hot if you go deep underground. Petroleum wells that were drilled near DFDP-2 (nearer to the coast, near Franz Josef Glacier and Harihari) have geothermal gradients of about 30 degrees Celsius per kilometre (°C/km). This is typical by global standards and means that the temperature at 1 km depth is about 42°C (30°C added to the 12°C average surface temperature). The hottest 1% of boreholes on Earth have a geothermal gradient more than 80°C/km, and are mostly found in volcanic regions (Figure 2). We discovered a geothermal gradient at the DFDP-2 site of about 150°C/km (Figure 3), which is similar to the hottest geothermal energy boreholes drilled into volcanoes of the central North Island; but there is no volcanism in Westland. 

Figure 2. Global database of geothermal gradients in boreholes, with circles showing post-earthquake fault zone drilling experiments.

Figure 3. Temperature and fluid pressure observations, and a geological log of borehole DFDP-2B, modified after the paper published in Nature (doi: 10.1038/nature22355)

We explain the extreme underground conditions at our drill site by considering the combination of two processes, both of which are related to the Alpine Fault, which is what we were drilling into. The Alpine Fault moves in earthquakes every 291 ±23 years, and last moved in AD 1717. The earthquakes have two important effects: (1) mountains get moved up higher, and (2) shaking fractures the rocks. Over time or/and during the earthquake, the fractured rocks landslide and are transported to the sea by rivers – limiting how high the mountains can get. This movement process has operated for millions of years, with the height of the mountains staying about the same. Eventually, hot rocks from great depth (about 30 km depth, 550°C) were transported to the surface. The movement process is fast by geological standards, and the rocks have not had time to fully cool. This is one part of the explanation for such hot conditions. The other process, rock fracturing, allows rain water and snow melt to percolate into the mountains so fast that it can move heat towards the valley, where water discharges. The flow needs to be fast enough that the heat is not lost by conduction along the way, just as a water pipe at home moves heat from a hot water cylinder to your bath before having time to cool. The water flow concentrates heat and raises fluid pressure beneath the valleys, but is invisible at the surface because it discharges into and mixes with shallow cold groundwater (at about 50 m depth at DFDP-2, Figure 3). Most Westland valleys have warm springs that hint at this deeper process. The combination of heat transport by both rock and water allows us to explain the hot conditions, and it explains the difference in conditions between our two drill sites. Some frictional heating on the fault surface may occur during earthquakes, but it is not required to explain our results. Details and references are given in the recent Nature publication.
The unexpected results of our research are important to other scientists. Nobody had predicted such extreme conditions, but they exist in an environment that many believe to be quite normal for a plate boundary fault. Other faults around the world may have extreme conditions that have never been investigated – it isn't an easy or cheap technical job! Such conditions may have existed in the past on many geological faults, and our observations could explain a wide range of geological, geochemical, and geophysical observations that are otherwise enigmatic. Perhaps most significantly, we are able to estimate ambient conditions on the geological surface that will rupture in the next earthquake. This is fundamental input to computer models of earthquake rupture, and it is the first time this has ever been done on a major fault that is due to fail in coming decades. The fact that these ambient conditions were so unexpected is particularly interesting.
It is common for scientific research to result in surprise, and some unexpected outcomes have greatly benefitted society. Penicillin, the most famous antibiotic, was discovered by Alexander Fleming when an experiment 'went wrong' due to a dirty dish; microwave ovens were invented after a Raytheon engineer noticed the chocolate bar in his pocket had melted while working in front of a vacuum tube; and Viagra failed in its intended primary purpose – to relieve chest pain – but its side-effects now underpin a billion dollar per year business. The extreme underground conditions we discovered in our drilling project were not expected, but may represent a substantial geothermal energy resource. We expect that similar types of resource may exist in other nearby valleys and in other places in the world. More work is needed to determine just how big the resource near Whataroa is, and how best to use it. We are excited to have discovered a new class of sustainable clean energy, but it is now up to others to decide how much effort and cost is warranted to start the next phase of understanding and assessment. 

Figure 4. DFDP-2B drill rig at dusk. Photo: Jack Williams, University of Otago.

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).

Monday, December 15, 2014

Calamity 3, broken casing

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

HWT (4.5”, 114 mm) steel casing. 13/12/14. R. Sutherland.
Nobody likes a sequel, and a sequel to a sequel is even worse. Yes, we dropped another heavy steel tube to the bottom of our borehole. This time it was worse, because our recipe had 40 tons of cement added to the 25 tons of steel in the hole.

It was a complicated casing operation last Wednesday, and it was done carefully. We lowered PWT casing (5.5”, 140 mm) with only a minor hitch – the float shoe apparently failed with a third of the pipes still on the rack. Before this, mud had been displaced from the borehole as each piece was added. A float shoe is a one-way valve at the bottom of the pipe that stops mud entering, giving the pipe some buoyancy (air inside it) and reducing its effective weight, and preventing backflow when the pipe is cemented.

Float shoe for PWT casing. 10/12/14. R. Sutherland

Next, a BQ steel pipe (2.25”, 56 mm) had to be threaded 890 m to the bottom of the borehole (8.5”, 210 mm diameter), running alongside the PWT casing (5.5”, 140 mm). Simple maths tells you that a 14 mm tolerance over 890 m of rubbing against jagged rocks had a small chance of success. But it went in!

Next, a stainless-steel tube with delicate optical fibres inside was threaded into the BQ pipe. That worked, and we tested our sensors. OK too!

At 4 a.m. on Thursday, it seemed that everything had gone as planned. Just in time too, because the trucks showed up at 5 a.m. to set it all in concrete.
Concrete appears at wellhead. PWT and BQ pipes visible, and fibre optic cable held by clamp. 
11/12/14. R. Sutherland

By 9 a.m., the joyous mood had changed. Cement appeared back at the surface much earlier than it should have, and pump pressures were lower than predicted. We quickly deduced that the PWT steel casing had broken.

Our immediate action was to flush cement out as best we could before it set. Fortunately, we had waste pits prepared for just such an emergency.

Cement pumped from the hole. 11/12/14. R. Sutherland
The next step was to determine where the break in casing had occurred, and what we could do about it. Over the last few days we have been trying to determine the state of the borehole 430-470 m below us.

We have analysed records of pump volumes and pressures, and digital data from the drill rig computer. This shows that a significant event occurred at 11:42 a.m. on Wednesday, and it seems this is when the casing broke apart. The depth to the broken joint is estimated to be 436 m.

The drillers made up an HWT drill-string (4.5”, 114 mm) that fits snugly inside PWT pipes, and carefully lowered this into the borehole with a core barrel and bit. We touched an object at 436.0 m, pushed it to 437.0 m, and then cored it. It was the cementing plug. This was the last thing pumped after the cement. It floats, so its position seems consistent with our depth estimate for the break.

We advanced farther and eventually started coring rock below 470 m. This is bad news. It means that the pipes are not aligned.

Cement plug and concrete core. 13/12/14. R. Sutherland.

It is still too early to tell exactly what this means for the project. Several options are available to us. Our top priority is to remediate the immediate situation and secure the upper casing. However, we need to think through all possibilities before putting more cement in the borehole.

Everyone is looking forward to a break over Christmas. It has been physically and mentally very demanding for the last 16 weeks, and it still seems like there is so much more to do.

The latest addition to our team is as big as Virginia but not as ferocious.

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).

Monday, December 8, 2014

892 m, decision time, steel casing

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

The view downstream. It is 600 m to the bridge. 4/12/14.  Photo J. Townend.
We reached a very important decision point today. At 8 a.m. this morning at 892 m depth, we decided to stop drilling and to cement casing into the hole. A bunch of things now have to happen so that we are ready to start the collection of rock core.

We first have to clear the borehole of rock chips and cool it by circulating mud. The mud emerges at 52°C. How hot would it be if we weren’t drilling? We will know in the next week or so.

Before we cement PWT steel casing (5.5 inch = 140 mm diameter pipe) into the hole, we will use wireline tools to log the hole. This is our last look at the borehole wall before steel and cement get in the way. Logging will start about midnight tonight.

New PWT casing is ready to go. Photo J. Townend.

The process of putting casing in the hole is fairly complicated. It has to get down and around the J-bend in one piece. The bottom of the hole is now 250 m horizontally towards the road bridge and away from the drill site (see our last blog). Next, we have to run another smaller steel pipe next to it – this is going to be a tight fit. Then, we have to insert a fibre optic cable down the second pipe. Finally, we have to pump 25 cubic metres of cement down the inside and back up the outside of the casing without it leaking into the rock or setting hard before we are finished.

If all that happens without a hitch, then we will be hoping to collect our first core this weekend.

View from the geologists’ cage.  J. Townend.

Sunday, December 7, 2014

DFDP-2 @ 850 m, mylonite, borehole shape

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

Are we there yet?  They were born when we started and are growing up fast. 5/12/14.  Photo R. Sutherland.

 It was a slow start yesterday as the reconditioned drill reamed its way into the hole, but by this morning we were making new hole and producing fresh rock cuttings. We are now making about 3 m of new hole every hour and passed 850 m at lunch.

The good news is that analysis of rock cuttings is indicating we are not far from the fault. We know this from fragments of mylonite, which is a rock formed at temperatures of more than 300°C by the smearing out and recrystallization of quartz grains.

In the meantime, geophysical logging of the borehole (see last blog) and analysis of data is producing interesting results. 

The borehole is J-shaped, with the bottom deviated 40° from vertical (animated figure). This places the bottom of the hole below the Whataroa River about 200 m northwest of the drill rig. This is exactly what we had hoped would happen: the deviation is taking us directly towards the fault. The onset of mylonite rocks is right in line with predictions that we should be getting close.

Borehole geometry to 825 m.  R. Sutherland

We have been able to make a very detailed analysis of the inside surface of the borehole (example shown in animated figure). We have discovered that the layers in the rock consistently dip southeast at about 60°. This is a little steeper than predicted and a slight concern, as it could mean that the fault is deeper than predicted. The mylonite cuttings are giving us a clue that we are close, but we will soon find out where the fault is.

Borehole surface over a 1.5 m interval.  J. Townend

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).

Thursday, December 4, 2014

DFDP-2 @ 828 m, wireline logging, schools and locals

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

A lovely morning with drilling progressing smoothly.  2/12/14.  Photo R. Sutherland.

We made solid progress over the last week. Our plan was to stop and set steel casing at 800 m depth, but we always knew that a detailed analysis of rock cuttings would be needed to confirm the depth of the fault. We reached 828 m depth after dinner yesterday.

The cuttings geology team is telling us that we should drill a bit deeper before we cement casing into the borehole, but we took a break from drilling last night for several reasons:
(1) drilling equipment needs to be checked and maintained;
(2) we wanted to survey the orientation and state of the borehole; and
(3) geophysical measurements made deep in the borehole today will inform our decision about what to do next.

Geophysical instruments (sondes) are lowered into the borehole on a wire rope from a special winch that precisely measures depth. Cables inside the wire transmit data to a computer at the surface. This process is called ‘wireline logging’.

Wireline logging sonde.  Photo J. Thomson

There are many different types of sonde. Today we will be measuring: electrical conductivity and temperature of the mud in the hole; seismic wave-speeds and electrical conductivity of the rock; natural rock radioactivity; borehole orientation; borehole shape; and images of the borehole wall that reveal rock fabrics and fractures.

This video gives a bit of an explanation of some of the wireline logging tools we use:

We have had several open days recently with lots of locals and two schools coming to visit.

Carolyn Boulton explains rock cutting and crushing to Hokitika High students. 4/12/14 Photo J. Townend

Jennifer Eccles explains earthquake sensors to Hokitika High students in the rain. 4/12/14 Photo J. Townend

DFDP-2 is 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; NZ Government (MBIE); UK Government (NERC).

Tuesday, December 2, 2014

DFDP-2 @ 760 m, rock cuttings team

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

Naoki Kato (Japan) cleans rock cuttings, Mike Allen (UK) makes microscope thin-section slides, Carolyn Boulton (UK) analyses dried rock cuttings under a microscope, and Tim Little (NZ) records analysis. 28/11/14.  Photo J. Townend.

The sun is out and drilling is going smoothly. We passed 760 m before lunch. The drillers are happy.

Tony Kingan, Head Driller, looking happy with progress. Photo R. Sutherland

The rock cuttings team tell us what the rock is that we are drilling through, and how far we are from the fault. We’re getting closer, but we are not quite there yet. The plan is to start coring 100 m from the fault or at a depth of 1000 m, whichever comes first.

The rock cuttings team collects, cleans, dries, sorts, describes, counts, glues, and grinds the cuttings, analyses them under a microscope and then enters the data into a computer. Everyone has had to work hard in shifts over the last few days to keep up with drilling, but it is more interesting than waiting for equipment problems to get solved. This video shows what's involved:

Many of the team have now been here for more than two months — and some of us have been here most of the time since August! It is becoming clear that we will be here well into January, but only after a brief and well-earned Christmas break.
Rewi Newnham (left) talks to Rupert Sutherland about the DFDP-2 drilling project.  Photo J. Townend.
Lisa Craw and Alan Cooper are given a site induction by Alex Pyne.  Photo J. Townend.

Primary funders of the DFDP-2 project are: the International Continental Scientific Drilling Program (ICDP), the Marsden Fund of the Royal Society of New Zealand, GNS Science, Victoria University of Wellington, and the University of Otago.