In developing future supercritical energy projects, the target for deeper drilling in New Zealand is supercritical fluids. It is not magmatic fluids, and definitely not magma.
What’s the Difference?
Magma is composed of molten rocks, minerals, and volatile compounds (e.g. water, carbon dioxide, sulfur) in varying relative abundances. In the central North Island, the rhyolitic magma has a temperature around 750-850°C.
Magmatic fluids are released from magmas deep within the Earth’s crust. These fluids escape from magmas during decompression (ascent) or crystallisation (when no more water can be trapped inside the liquid magma). They contain elements, compounds, ligands and neutral and corrosive gas. Their physicochemical compositions reflect mantellic and crustal processes, and temperature ranges from magmatic temperatures, greater than 700°C, to around 100°C.
New Zealand’s hydrothermal fluids, which circulate in our geothermal systems, are predominantly meteoric in origin. This means the fluids originate from precipitation, i.e. snow and rain. This water percolates down into the Earth, and is heated convectively by a heat source underground, before rising again to the surface. During this journey the fluids react with the various rocks, mix with other crustal fluids and incorporate rock forming elements (e.g. potassium, sodium, magnesium, boron, sulfur) and gas (e.g. carbon dioxide). The temperature of hydrothermal fluids vary as they circulate, from ambient at the surface to >500°C at depth. In New Zealand's TVZ geothermal systems, the proportion of magmatic fluids mixed with hydrothermal fluids is difficult to assess and can vary from 0 to 15% (based on previous scientific estimates).
Supercritical fluids refers to any fluid above its critical points (374°C and 22 MPa for pure water), where it is neither liquid nor gas. When reaching these pressure and temperature conditions, the physical properties of the fluid, such as density, vary abruptly. Above the critical point, these fluids become more liquid-like or more gas-like depending on the pressure and temperature conditions. Read more about: What is Supercritical?
Why we want Supercritical Fluids, NOT Magma?
A well drilled in the Iceland Deep Drilling Project did unintentionally drill into a rhyolitic magma body. They were aiming for supercritical fluids in hot rock below 4,000 metres, but instead the drill struck magma at 2,100 metres. The result was fluids which could not be handled, and a well that collapsed/was sealed up. These expensive lessons provide valuable insights for scientists that can be applied to new experiments and improve future exploration drilling.
This Iceland drilling incident has meant many people now associate drilling for supercritical fluids with drilling in magma – but this is not our goal. Although in Iceland some people are looking to intentionally drill magma, here in New Zealand, as part of Geothermal: The Next Generation (GNG) programme, we don’t want to "put magma in a pipe". New Zealand’s next generation of high temperature geothermal energy needs super-heated steam.
The challenge that the geothermal community faces is that we need to access fluids close to a heat source and accessible by drilling. In a volcanic active country such as New Zealand, the logical heat source is magma, but we don’t want to be too close because we do not want to tap magmatic fluids. Due to their origin and association with magma, these fluids are hugely corrosive and abrasive. Like magma, these fluids eat pipes and solid materials.
Supercritical hydrothermal fluids are low density and high energy, offering a large potential energy source. We want to tap this hot, deep circulating (supercritical) meteoric water that penetrates the rock near a magma chamber. In summary, targeting supercritical fluids does not mean targeting magma.
We know supercritical fluids have different, likely more corrosive and less understood chemistry compared to the two-phase geothermal fluids currently used in geothermal electricity generation (~320°C max). Specialist materials and processes will need to be developed to handle them safely, but we are confident these engineering challenges can be overcome.
How will we tell the difference between Supercritical Hydrothermal Fluids and Magmatic Fluids?
Both magmatic and meteoric fluids occur under supercritical conditions and can easily mix. We’ll need to be able to recognise a greater input of magmatic fluids when we are drilling deeper, so we can stop or redirect drilling.
During drilling, geologists examine the composition of drill-core samples brought to the surface. By looking at the composition of the rocks in these samples, and measuring temperatures and conditions in the hole, geologists can tell with reasonable confidence what rocks and fluids are being intersected by the drill-hole.
Due to their different origins, each type of fluid has a different chemical signature – like a unique fingerprint, that tells us where they came from. When a fluid phase escapes magma, it carries within it elements that are characteristic of magma, such as the metals lead, zinc, copper and tellurium, that don’t occur in high abundances in meteoric fluids. High-temperature fluids also alter and change the rocks they pass through and reside in, and the different mineral composition of the rocks also acts as a unique fingerprint.
We are learning from the challenges encountered internationally during deep drilling and supercritical projects, that have highlighted a need to be able to recognise what rocks look like above a magma body when drilling. In the GNG programme, we are simulating supercritical conditions in the laboratory in order to develop tools to recognise this. We are also looking at rock that has trapped magmatic fluids, so we can analyse their composition and understand where they escape the rhyolitic magma.
We need to build a dataset of what these altered rocks look like, so we can identify if drilling is getting too close to magmatic fluids and/or magma.
How is New Zealand’s situation different to international deep wells, and what we are doing in the GNG programme?
The New Zealand target is 4-6 kilometres depth. To help determine where the magma bodies, and heat sources, reside in the central TVZ, we are improving geophysical and reservoir models within the GNG programme. This includes seismic tomography, magnetotellurics and aero-magnetics. These geophysical techniques seek to deliver clearer, better refined imaging of heat sources and underground structures to ensure we hit our target of supercritical fluids - not magma.
Using geophysics techniques and in collaboration with volcanologists, we are actively trying to resolve where magmas reside in the crust in the present day. This is not a trivial thing to do, and the resolution of the 3D geophysics interpretations is still too large to precisely target exploration drilling with confidence. However, this is one of the aims of GNG Project EXPLORE: to improve our modelling to de-risk future exploration wells.
We are testing two different hypothesis for how the high heat gradient is present in the TVZ at depth: (i) shallow intrusion (below 4.5 km) or (ii) buried discontinuity that act as fluid highway in the crust.