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Global Geothermal Energy

Published by , Deputy Editor
Energy Global,

It is generally accepted that a wide range of sustainable and integrated energy sources are going to be needed to reach the greenhouse gas (GHG) targets set by the world community at COP meetings, in order to prevent global temperatures from rising to 2°C above pre-industrial levels. To this end, the use of solar and wind power has increased dramatically over the last few years, but what happens at night time, when it is cloudy or the wind drops?

Step forward energy from within the Earth itself. Originating in the heat generated when the planet was first formed and constantly regenerating through radioactive decay, geothermal energy is always there and fully sustainable. It has been used as a local heat source for millennia, as well as more recently to generate electricity in places where there is a high heat flow close to the surface of the Earth.

But accessing geothermal energy need not be confined to these regions – in fact, there is potential for utilising it throughout the globe.

Heat from within the Earth

The temperature of the Earth’s subsurface rises with distance from the surface. This gradual change, known as the temperature gradient, is usually approximately 25°C for each kilometre of depth, increasing up to temperatures in excess of 900°C where rock may be in a molten state – magma. Near plate boundaries and volcanic centres, such as the Pacific ‘Ring of Fire’, magma rises towards the surface where it heats underground aquifers to temperatures of 350°C or more and pressurised water escapes in the form of geysers, hot springs, and steam vents. These surface emanations have been utilised for decades in places such as Iceland, Italy, New Zealand, and California, the US. Naturally occurring hydrothermal fluids such as these can be used directly to heat buildings, greenhouses, and swimming pools, or, where hot enough, they can be used to produce steam for electrical power generation.

However, rather than rising to the surface, most of the heat remains locked in the Earth, and this is where its potential as a global sustainable energy source lies. Geothermal energy is now being developed in a variety of different geological settings throughout the world; all it requires is a system by which fluids heated within the subsurface can be accessed by drilling. These hot fluids may be naturally occurring water or brine, in pores and fractures in permeable rocks. If the hot rock does not contain enough natural cracks for the fluid to flow easily, it can be artificially fractured and a fluid circulation system developed. Sometimes the rock does not contain sufficient water to give commercially useful flow rates, in which case additional water can be pumped from the surface into the hot dry, fractured rocks, where it is heated by conduction. Once the hot fluid comes to the surface, whether by pumping or under natural convection, it can be used to produce steam for power generation, and in many cases the water cooled after use is pumped back into the aquifer to create a circular system.

Historically, geothermal developments were located close to surface expressions of natural hydrothermal geofluid circulation systems, often in igneous or metamorphic rock areas. However, recent advances in technology give increasing capability for finding hidden geothermal systems and for accessing geothermal energy from sedimentary basins.

Geoscience has the answers

Since this energy is based within the Earth, geoscience is the route to finding the optimum ways to access it. In order to do this, it is important to understand not just areas of high heat flow in the subsurface, but also how permeable and porous the rocks are and whether fluids will flow through them easily or if fracturing will be needed. Being able to recognise how rocks respond chemically and physically to heat and pressure, and how they change when fluids pass through them, makes it possible to assess how easy it will be to drill through them to access geothermal fluids.

Companies that have worked in the oil and gas industry for many decades have built up a valuable and detailed understanding of the Earth and its subsurface. One such company is CGG, a global geoscience technology and HPC leader that has been collecting and interpreting geoscientific data for over 90 years. Its geoscientists can draw on their knowledge, skills, and technologies to bring valuable intelligence and capabilities to help better understand and de-risk the development of geothermal energy throughout the world. Over the last 20 years, for example, CGG has undertaken more than 150 geothermal projects, mostly applying geophysical technologies such as analysis of magnetotellurics, gravity, and microseismicity in traditional areas such as the ‘Ring of Fire’, but latterly also helping companies explore and develop ‘hidden’ geothermal resources using these techniques.

Over the years, CGG geoscientists have collected extensive databases of the important parameters that need to be understood in oil and gas exploration, and many of these, such as temperature gradients, porosity, permeability, fluid chemistry, and flow rate, are also essential for geothermal projects. All this data has been merged with the company’s extensive seismic library to develop a geoscientific understanding of the Earth which can help identify new potential areas for the development of geothermal energy projects. Using this information, CGG recently completed a global Geothermal Resource Assessment study that can be used not only to identify and assess potential promising geothermal energy sites, but also to analyse the nature of the geothermal reservoir rock and provide information on production and monitoring solutions.

Sedimentary basins have been a major focus for the oil and gas industry, but they are of interest to the geothermal industry, not only because over the years a great deal of geological knowledge has been gathered about them, but because, unlike volcanic areas, they are often close to population centres and therefore any geothermal resource developed in them will have a ready market. The aquifers of the Paris Basin, for example, have been providing district heating for over 700 000 people for more than 30 years. However, key properties of the producing layers, such as distribution of porosity and permeability, which will identify the rocks with high rates of fluid flow that will make future development of this resource more effective, remain poorly understood. To reduce this uncertainty, a recent study by CGG, using established oil and gas techniques such as seismic inversion and recently developed rock physics-guided deep neural networks, was able to characterise the reservoirs and guide the location and design of future geothermal wells.

To continue reading this article from Energy Global’s Winter 2022 issue, click here.

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