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Geothermal Energy Basics

The centre of the Earth is hot and largely composed of molten and semi-molten rock.  This heat is the result of a few different processes including: left over heat from the formation of the planet; the gravitational pressure of the Earth itself (but not enough pressure to support fusion reaction of Hydrogen to Helium and make us a star), and the radiogenic decay of various elements in the mantle and crust. Convection of the molten interior (the mantle) is the main driver of tectonic plate movement. Heat flows from the mantle into the base of the crust and then via conduction to the surface where it is lost to space.

Because heat is constantly flowing outward from the Earth’s centre, temperature increases with depth into the crust everywhere. But in some places it gets hotter shallower, because molten rock (magma) is very close to, or at, the Earth’s surface such as along the edges of tectonic plates and Mid Ocean Ridges for example.  New crust is constantly being created at the ridges and consumed along the margins, resulting in magma being emplaced close to the Earth’s surface and heating the rocks above and around it.

An example is the Pacific “Ring of Fire”, a connected series of tectonic plate margins well known as the site of many active earthquake and volcanic zones.  The Pacific “Ring of Fire” is also the source for many producing geothermal energy sites like The Geysers in California, USA and the Taupo Volcanic Zone in New Zealand. These are all examples of “conventional hydrothermal” geothermal energy, the most common form of geothermal energy used for electricity generation today.This style of geothermal resource was first used for electricity production at Larderello Italy in 1904 with the first plant being built in 1914.  During WWII United States and New Zealand forces saw the Larderello site and realised their nations had similar potential.  The oldest field in New Zealand – Wairakei – celebrated its 50th anniversary in 2008.  World-wide annual production of electricity from resources of this type is on the order of >10,000MWe, with the largest single producer being the USA at about 3000MWe per year.  Geothermal energy is unique amongst renewable energy resources because not only is it sustainable and low emission, but it is truly base-load, meaning it is able to produce electricity reliably 24/7.

Geothermal Energy has also been used throughout history (e.g. Ancient Greece, China) for other applications such as bathing and health spas, central heating, food processing, etc.  The largest direct use application of geothermal energy currently is for the heating of homes & businesses.  World-wide geothermal energy supplies >21000 MWth for direct use applications.

Innovation in geothermal energy

So the use of geothermal energy isn’t new.  It is a well understood, proven, mature technology.
What is relatively new is the rising interest in two forms of geothermal resource, Engineered Geothermal Systems (EGS) also known as Hot Rock or Hot Dry Rock geothermal energy, and Hot Sedimentary Aquifers (HSA).

Fundamentally all Geothermal Resources need 3 key ingredients to be viable, forming a continuum in terms of the amount of energy available to be used, and the technology needed to bring that heat energy to surface for use.  The key ingredients are: a heat source; a permeable reservoir within hot rocks; and a working fluid which can migrate through the reservoir to transport the stored heat.  Other important considerations are the availability and sustainable management of groundwater resources to accumulate and transfer the heat, and the proximity to electricity transmission infrastructure and/or local markets for electricity or direct use applications.  Two (or more) wells are drilled to a depth where the ambient temperature is adequate for economic use – for the production of electricity this is roughly about 200oC or greater. A network of fractures throughout the rock or intrinsic high porosity and permeability in the rock, connects the wells and acts as a pathway for fluid to circulate through the hot rock mass.  The fluid is injected down one well, becomes heated as it flows through the rock, and transports the heat to the surface via a production well.  Once at the surface the heat in the fluid is used to generate electricity or in any number of other industrial applications, and the cooled fluid is re-injected and continuously recycled through the reservoir.

Conventional hydrothermal geothermal resources have all 3 ingredients naturally occurring at levels which readily enable commercial production.  The limitation of conventional hydrothermal geothermal systems is that they are geographically discrete.  Very little of the world’s population live near plate tectonic margins or other areas of active volcanism where this type of geothermal resource is available for exploitation.

Hot Sedimentary Aquifer resources are geographically more common than conventional hydrothermal resources and are generally deeply buried, fluid filled sandstone or limestone reservoirs with relatively high natural (primary) porosity.  The deeper the reservoir the higher the ambient temperature will be, but there is an economic trade-off between the cost of drilling to greater depths and the amount of additional heat energy which can be produced.  This trade-off is exacerbated by the effects of diagenesis.  The basic premise behind an HSA development is that the reservoir already exists, but as sedimentary rocks are buried they undergo mechanical and chemical metamorphosis (diagenesis) whereby they become increasingly compacted and lithified, and the original minerals in the sedimentary rock begin to recrystallise and form new, more stable minerals.  These processes all lead to a gradual reduction and finally total loss, of the original porosity and permeability of the reservoir.  The depth at which these processes begin is highly variable, but as a result HSA resources are usually cooler, may be less permeable, and deeper than conventional hydrothermal resources, and therefore have additional challenges to make them economic.  Many of the HSA resources investigated to date have been identified through historical oil and gas drilling - an example is the Great Artesian Basin in Australia.

Engineered Geothermal System resources are effectively everywhere, but currently are the most difficult resource to bring in to commercial production.  The basic concept of EGS is simple and elegant, but up until the 1970s it was the stuff of science fiction writers.  Yes there is a lot of heat below our feet but how do we get it out?  As a rule of thumb, the average geothermal gradient is about 30oC/km, and pressure increases with depth at about 1 kbar per 3 km.  So to get temperatures of around 200oC needed for power generation, we would need to drill to about 6-7 km deep.  At these depths rocks are generally hard, abrasive, and impermeable metamorphic or igneous rocks.

The 1970s Oil Crisis was the catalyst for governments to seriously consider supporting research into EGS, leading to the first field tests at Fenton Hill in the USA by scientists at Los Alamos laboratory. It wasn’t just the oil crisis though.  Significant leaps in technology had to be made to be able drill deep, large diameter wells into hard rock while controlling the well.  Furthermore in an EGS resource, the key factor which is most often missing is adequate reservoir permeability, and technology had to be developed to enable controlled fracturing of the rock to create a suitable reservoir.  This remains the main technical challenge faced by companies wanting to develop an EGS resource.

The ideal reservoir is comprised of a large tortuous network of small interconnected fractures which permeates the rock mass – similar to the thousands of tiny tubules which make up the radiator of a car.  What is required is a huge surface area throughout the rock which is able to exchange heat with the transporting fluid.  This is achieved by a technique called hydraulic stimulation.  Once the first well is drilled in to the target reservoir, large volumes of fluid are pumped at high pressure down the well and into the rock.  If the rock already contains fractures the pressured fluid enters these and causes them to open, extend and connect.  If no fractures are present, the pressured fluid breaks the rock under tensile stress, creating new fractures.  Most of this technology development was, and is still being, advanced by the petroleum industry.
Although most of the world’s population does not live near active volcanic environments, there are many areas where the geothermal gradient is much greater than average, enough to be prospective for either HSA or EGS resources.  This can occur in areas of thinned crust where the mantle is closer to the surface so background mantle heat flow is above average or in areas where high heat production from crustal rocks adds to the background mantle heat flow.  This additional heat is produced from rocks with higher than average content of the radiogenic elements Potassium (K) Uranium (U) and Thorium (Th).

These elements are present everywhere and are constantly releasing heat energy as they undergo radioactive decay over time.  Most rock types contain some K, U, Th, but granites tend to be the rocks most commonly enriched in these elements.  Even in granites, the amounts of these elements are very small, however when the volume of rock is extensive (cubic kilometres) collectively the amount of heat produced is significant. The average continental heat flow is on the order of 60 – 80 mW/m2 but there are local areas throughout Australia for example, with heat flows approaching 120mW/m2 or more, likely due to the presence of buried high heat producing rock bodies.

South Australia in particular, has large regions of interpreted high crustal heat flow associated with buried high heat producing granite bodies at depths >3 km and these form key exploration targets for Engineered Geothermal Systems (EGS). In deeper parts of the Cooper Basin, for example, geothermal gradients reach 55–60oC/km which is significantly higher than the average geothermal gradient of about 25 – 30oC/km. Other potential geothermal energy targets include Hot Sedimentary Aquifer systems (HSA) in the Great Artesian Basin in north-eastern South Australia and the Otway Basin in the State’s south-east.