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.