December 28, 2020

Team Profile:
Water-Rock Interaction Geochemistry
contributor(s)

Peter Rendel

photo credit:
Peter Rendell

As part of the Geothermal: The Next Generation research program, water-rock interaction is being studied in the laboratory to investigate the interaction between supercritical water and Aotearoa reservoir rocks.

What is water-rock interaction geochemistry?

Chemical reactions can occur between hot water and rock materials at high temperatures and pressures in the earth. These “water-rock interactions” are studied by geochemists in order to understand the composition of these high temperature fluids in the earth’s crust.

At low temperatures, water and rocks do not appear to interact chemically, at least on the human time scale, yet, at high temperature, chemical reactions are much faster and significant interaction between these two materials can occur.

Water-rock interactions are one major process shaping the earth’s various environments and landscapes. They may also have played a crucial role in the development of early life forms and are currently essential to maintenance of life on our planet.

Water-rock interactions take place on and below the earth’s surface, in the oceans, in lakes and rivers, above our heads in the open skies, and they even take place in our own body. Although these reactions are common, most of us may be unaware of their existence.

Scanning electron microscope (SEM) image of quartz crystallised in a high temperature and pressure vessel in GNS Science's Wairakei laboratory.

Rocks are the building blocks of our planet. Their composition consists of a variety of different crystalline materials called minerals which determine the rock’s properties, such as colour, strength, texture, and in some cases even their taste.

Minerals are made of different combinations of the 94 natural elements (from the 118 known), however, a mix of only eight elements make up 98% of our planet’s mass. These include oxygen (O), silicon (Si), aluminium (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na) and potassium (K).

Water covers two thirds of our planet’s surface and is essential to sustain life. Water is an excellent solvent and has the ability to dissolve many materials. It can detach atoms or molecules from minerals surfaces in a process called dissolution. This is a type of water-rock interaction. The amount of material dissolved depends on the temperature and chemical composition of the water, for example, its acidity and its salt content.

At some point, the water reaches an equilibrium or balance, which prevents it from dissolving further material.  In a way, the water dissolves minerals until it “runs out of space” and cannot further dissolve more material. This amount is referred to as the equilibrium concentration, and this equilibrium is highly sensitive to environmental changes. For example, an increase in temperature may affect the speed of dissolution of minerals or trigger a new reaction that does not occur at lower temperature, leading to a new equilibrium state. The rate at which reactions take place is of high importance and plays a major role in many water-rock-Interactions. For example, the mineral quartz (SiO2),which is a very common mineral, has a higher equilibrium concentration (i.e. more of it can be dissolved) and faster dissolution rate, the hotter the water is.

What happens when we lower the temperature of water containing an equilibrium concentration of quartz? In this case, the reverse process takes place, where dissolved SiO2 in the water begins to recombine to form new crystals of quartz, this process is referred to as mineral precipitation.

Both mineral dissolution and precipitation are common in nature. For example, rainwater contains relatively little dissolved material. When this water collects and flows downstream in creeks and rivers, or through the ground, small amounts of mineral material are dissolved.  The water eventually becomes enriched in various dissolved compounds (solutes). These solutes ultimately become highly concentrated forming the salt found in seawater.

It is common to have mineral precipitation and dissolution occurring at the same time. Although dissolution and precipitation rates can be different, in coupled systems both reactions will eventually reach a certain equilibrium.

Water-rock interactions also take place below the surface of the Earth. Groundwater penetrates through soils layers and rock formations feeding subsurface aquifers. The slow movement of subsurface water through porous media enables the water to reach a chemical equilibrium by dissolution of mineral material in the surrounding rocks.  

Scanning electron microscope (SEM) image of Wairakite crystallised in a high temperature and pressure vessel in GNS Science's Wairakei laboratory.

Why is this important for geothermal energy development?

During the generation of geothermal power, high temperature subsurface water is extracted in order to generate steam which is used to generate electricity. As the water flows upwards through the geothermal wells, the temperature of water decreases due to boiling, shifting the chemical equilibrium in the fluid. This shift in the temperature may induce mineral precipitation, which is the cause of mineral scaling in geothermal wells, heat exchangers and pipelines. The formation of mineral scales can have serious negative effects on the efficiency of power generation.

Steam generated by boiling of geothermal fluid is used to generate electricity. After use, it is condensed back into water, combined with the remaining cooler liquid portion of the fluid (brine), and re-injected back into the geothermal reservoir. This combined fluid no longer has the composition or temperature of the original water extracted and thus is not in equilibrium with the rocks. This can induce added water-rock interactions leading to further mineral scaling.

As part of the Geothermal: The Next Generation research program, water-rock interaction is being studied in the laboratory to investigate the interaction between supercritical water and Aotearoa reservoir rocks. The aim is to illuminate possible future challenges in the production of supercritical waters for geothermal energy production. Our lab is equipped with state-of-the-art experimental equipment able to operate at high temperature, pressures, and with corrosive fluids, in order to simulate the environmental conditions existing at depths up to 6 km. During the experiments, fluids can be passed through rocks under supercritical conditions, allowing us to learn how water-rock interaction changes fluid chemistry in deep geothermal reservoirs and how re-injection of geothermal fluids into these rocks affects rock properties.

Outcomes from this research will promote our understanding on New Zealand’s deep supercritical geothermal resources, and how they can be used in the future to enhance geothermal energy production in a sustainable and responsible manner.

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Geochemistry

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