2 Background to geothermal energy
Part 1 Background, context and existing data
2.1 Sources of heat, heat measurement and factors affecting heat transfer
2.1.1 Sources of heat
The heat resource that exists at accessible depths within Earth's crust is derived from a number of sources, but three dominate:
(i) Stored heat in Earth's core and mantle, which dissipates by various means into the crust. The amount of heat supplied to the base of the crust from the mantle is related to the thickness of the lithosphere (the top part of the mantle), which can vary significantly, and the time elapsed since the last geological event to cause a significant transfer of heat across the mantle-crust boundary (Polyak and Smirnoff, 1968; Chapman and Pollack, 1975; Sclater et al., 1980).
(ii) Heat generated by the motion and interaction of tectonic plates. This contribution, which commonly manifests at Earth's surface as volcanic activity, will be small to non-existent in regions of the crust that have been stable for a long time (this includes Scotland).
(iii) Heat generated by the radioactive decay of naturally occurring elements (radiogenic heat). Virtually all rocks contain small quantities of elements that undergo radioactive decay, which generates heat in situ. The quantity of heat that can be produced by a rock in this way is referred to as its heat production ( HP) capacity.
Many of the elements that undergo radioactive decay have over geological time become concentrated in the upper parts of the crust, with the consequence that the HP capacity of upper crustal rocks is generally substantially higher than it is for rocks in the lower crust and the mantle. The chemical elements potassium (K), uranium (U) and thorium (Th) are generally the most abundant sources of radiogenic heat in continental crust. The HP capacity of a rock can be calculated from analysis of its chemical constituents, using the equation:
A = 0.1326ρ(0.718U + 0.193Th + 0.262K) ( e.g. Manning et al., 2007)
where A is heat production in μW m -3, ρ is rock density in g cm -3, Uis uranium content in mg kg 1
(also expressed as 'parts per million', or ppm), Th is thorium content in mg kg -1 ( ppm), and K is potassium content in weight % K 2O. Other forms of this equation, such as
A = 10 -5ρ(9.52U + 2.56Th + 3.48K) (Rybach, 1988)
are used widely and produce the same result (but note that in this case ρ is expressed in kg m -3).
HP capacity can also be determined indirectly from well logs (spectral gamma-ray log or integrated gamma-ray spectrum) using the empirical (linear) relationship between heat production and gamma ray readings that has been shown to exist for a wide range of lithologies (Bücker and Rybach, 1996).
Potassium, uranium and thorium concentrate in the chemically evolved parts of silica-rich magmas, so granite intrusions commonly contain high (and in some cases anomalously high) concentrations of these elements relative to other lithologies. Granite intrusions with unusually high HP values are referred to as high heat production ( HHP) granites. There is no widely accepted HP threshold for distinguishing rocks with HHP character, but the term is usually used when HP values exceed 4 μW m -3. For the purposes of this report, rocks with HP values ≥ 4 μWm -3 are considered to have HHP character. A granite intrusion containing 4% potassium (as K 2O), 20 parts per million ( ppm) uranium and 40 ppm thorium would, assuming a typical density for granite of 2.63 g cm -3, generate around 8 μW of heat per cubic metre of rock per second, and hence be classed as HHP. This is a small amount of heat, but granite intrusions typically contain many billions of cubic metres of rock, and the heat may be generated continuously for many millions of years. Extracting the thermal energy stored in one cubic kilometre of hot granite such that its temperature drops from 240ºC to 140ºC would reportedly yield the energy equivalent of 40 million barrels of oil (statistic taken from the website of Geodynamics Ltd; www.geodynamics.com.au). Rocks other than granite can have HHP character, but they generally don't occur in volumes large enough to have geothermal energy potential.
The various sources of naturally occurring heat contribute collectively to heat flow, which can be defined generally as the movement (or transfer) of heat through the Earth. Heat flow is also the standard measure of the amount of heat travelling through Earth's crust, and is expressed most simply as:
q = kβ
where q is heat flow in mW m -2, K is the thermal conductivity of the rock (in W m -1 K -1) and β is the vertical temperature gradient (in ºC per kilometre). Thermal conductivity measures the ability of a material (such as a type of rock) to conduct heat, in this case expressed as a unit amount of heat (one degree Kelvin) measured in watts per second per vertical metre. The vertical temperature gradient, which can be thought of as 'the rate of temperature increase in the Earth with depth', is also referred to as the geothermal gradient. Geothermal gradient is usually calculated by measuring the temperature at two different depths in a borehole.
On average, heat flow from the mantle in continental areas is around 20 mW m −2, while the mean heat flow across all continents is 65 mW m −2 (Turcotte and Schubert, 2002); the difference is due mainly to the contribution of radiogenic heat in crustal rocks. The average geothermal gradient in continental areas is 25-30 ºC/km.
Heat flow is assumed to be essentially the same at all points within a vertical column of the Earth's crust if the heat supply is constant and heat transfer is conductive and vertical. Thus, a constant heat flow is theoretically maintained throughout a vertical column of rock spanning any depth range and including multiple rock types, because changes in thermal conductivity (due to changing rock type, permeability, pressure and temperature) are associated with coincident, proportionate changes in geothermal gradient. Put another way, any increase in thermal conductivity is matched by a proportionate decrease in geothermal gradient, and vice versa. This situation should ensure that the heat flow below any point on Earth's surface is constant, and therefore that heat flow measurements (which are made in the near-surface zone) should provide a good indication of the size of the heat resource at depth. However, several factors (described briefly below) can affect the supply of heat and how it is transferred, particularly in shallow parts of the crust. These factors reduce the degree to which near-surface measurements of heat flow can be considered a reliable basis for estimating temperatures and the size of the heat resource in deeper parts of the crust.
Local sources of heat
Although the supply of 'background' or 'stored' heat (derived from the mantle) may be broadly constant at any point in the crust at any given time, the supply of radiogenic heat can vary significantly on a local, and sometimes regional, scale. Granite intrusions in particular can contain elevated concentrations of radiogenic elements and so can be a source of thermal anomalies in the crust. Where such intrusions crop out at the surface or are concealed in the near-surface zone, near-surface heat flow may be significantly higher than background heat flow. Such intrusions are potentially important sources of geothermal energy in Scotland, and are described in more detail in section 8.2.
Topographic relief can perturb heat flow, so heat flow values calculated in the near-surface zone of hilly areas require a correction. Measured heat flow will typically be greater in valley floors than on the tops of hills. Corrections for topography are typically based on the method of Bullard (1940). Topographic corrections to heat flow data gathered from four granite intrusions in the East Grampians region of Scotland (see section 8.2) involved downward corrections of the uncorrected values of between 3.7% (borehole sited on a hillside) and 10.5% (borehole sited on a valley floor) (Wheildon et al., 1984).
Changes in surface temperature due to geologically recent climate change
The geothermal gradient is controlled primarily by heat generated within the Earth. However, the temperature at Earth's surface 'pins' one end of the geothermal gradient curve, and changes in surface temperature due to climate change must therefore cause transient changes to the near-surface part of this curve. In the last two million years, Earth has been subjected to a number of cooling-warming cycles (the 'Ice Age'), with areas in higher latitudes (including Scotland) affected most by extreme temperature fluctuations.
Within the last decade, a strong body of evidence has emerged showing that transient changes in surface temperature are recorded in subsurface temperature gradients. For example, a study of heat flow measurements from more than 2,000 boreholes in northern Europe and North America revealed a systematic increase in heat flow with depth, and indicated that heat flow values determined from boreholes less than 2 km deep could be underestimated by up to 60% (Gosnold et al. 2011). Other authors ( e.g. Popov et al. 1998; Szewczyk and Gientka, 2009) have described the same effect.
Scotland was strongly affected by the Ice Age glaciations, so warming of the atmosphere since the last prolonged glacial period is likely to have perturbed the top part of the geothermal gradient. Heat flow measurements in shallow boreholes therefore probably underestimate the actual background 'steady-state' heat flow that is assumed to exist beneath the climate-affected zone. Unpublished work by BGS (J P Busby pers. comm.), in which an estimated model of the surface temperature dating back 160 000 years was applied to the East Grampians region of Scotland, indicates that recent changes in the climate may have suppressed near-surface heat flow in the region by up to 29%.
Inefficient heat transfer through low conductivity rocks (trapped heat)
At the Cooper Basin geothermal energy prospect in Australia ( e.g. Chopra and Wyborn, 2003), a deep borehole has revealed an unusually high temperature in a buried granite intrusion (240ºC at 3.7 km). The high temperature has been ascribed to a combination of the HHP character of the granite and the low thermal conductivity of the sedimentary rocks that overlie the granite. In other words, a reservoir of temporarily trapped heat seems to have developed in the granite because heat transfer through the overlying rocks is too inefficient to keep up with the supply of heat. Heat flow measured above such a setting may therefore underestimate the size of the (trapped) heat reservoir at depth.
Some bedrock geology configurations, including inclined strata, can cause heat refraction such that conductive heat transfer deviates from vertical.
Convective heat transfer
Heat transfer through the crust occurs primarily by conduction, but heat transfer by convection in water, liquid hydrocarbon and magma, can play an important role in some parts of the crust. Heat can theoretically be transported in any direction by this process. Shallow parts of the crust can contain substantial amounts of water, and convective heat transfer is therefore likely to be important in rocks with high permeability.
2.2 Classification and exploitation of geothermal resources
2.2.1 Classification based on temperature of heat resource
Low enthalpy (low temperature) resources provide warm water that can be used for direct applications. Low enthalpy resources can be divided into two types. (i) Indirect use resources exploit relatively cool water (~8-15ºC) in the shallow sub-surface (<c.200 m depth). The low energy heat of indirect use resources is collected with a ground source heat pump either with ground collector loops in trenches or boreholes, or from water in shallow, permeable formations. Ground source heat pumps can be installed at most locations, but the design of each system depends on local ground conditions. (ii) Direct use resources comprise water that can be pumped directly from underground and used immediately for heating; these resources are generally in the temperature range 20-80º C and in the UK are found within permeable rocks in the depth range 0.5-4 km.
High enthalpy (high temperature) resources yield hot water capable of driving turbines and generating electricity. The concept underlying the exploitation of high-enthalpy resources is simple: sink one or more boreholes into an accessible zone of hot rocks, extract the thermal energy in the form of hot water/steam, and use this to generate electricity at the surface. In parts of the world where there is active or recent volcanism, hot water circulates at shallow depths in highly permeable fracture systems where it can be exploited directly and relatively easily. Electricity is generated directly from these conventional 'hydrothermal' systems in Iceland, Japan, parts of continental Europe, western USA and a few other parts of the world, but they are globally rare. More usually, the permeability and/or fluid content of a hot rock mass needs to be enhanced artificially in order to extract the geothermal energy in commercially viable quantities. Concepts and projects requiring the use of advanced technology to recover a high-enthalpy geothermal resource are known as Enhanced (or Engineered) Geothermal Systems ( EGS).
Some heat is lost in the process of transferring it from the extraction zone to the power-generating facility, so the minimum target temperature for a deep geothermal resource capable of generating electricity economically using conventional (steam-driven) turbine technology is commonly stated to be around 150 ºC; however, an operation capable of generating a substantial power output over a period of decades using conventional power-generating technology may require an even hotter resource, perhaps in the region of 200ºC. In parts of the continental crust exhibiting the average geothermal gradient of 25-30 ºC/km, this temperature would be encountered at a depth of approximately 7 km, assuming a mean annual surface temperature of around 10ºC. Modern drilling technology is capable of reaching depths in excess of 10 km; however, the technical challenges and associated cost of drilling rise steeply with increasing depth. The economic viability of an EGS prospect therefore depends critically on minimising the depth to target temperature. Most concepts and established experimental facilities for EGS are based on a target depth range of 4 to 5 km. A consistent ('steady-state') geothermal gradient of between 38 and 48 ºC/km would be required to achieve a temperature of 200ºC within that depth range.
In recent years, the development of binary cycle power plants has greatly improved the potential for recovering geothermal energy from prospects that previously would have been considered marginal or not viable. A heat exchange system at the power plant is used to transfer thermal energy from hot water into a second liquid, usually a butane or pentane hydrocarbon liquid, which has a lower boiling point than water. Vaporisation of the second liquid drives the turbines, hence electricity can be generated using water that is cooler than 100ºC. The range of water temperatures that can be used in binary cycle power plants extends down to less than 40ºC. However, the efficiency of such systems decreases significantly with decreasing temperature, and a commercially viable EGS utilising a deep geothermal resource and binary cycle power plant technology is likely to require water of a temperature significantly above the lower limit at which it is possible to generate electricity.
2.2.2 Classification based on setting of heat resource
Geothermal resources can also be classified according to the setting in which the heat resource resides. Sinclair Knight Merz (2012a) defined two main geological settings:
- hydrothermal systems where the heat resource is contained within a water aquifer, e.g. abandoned mine workings and hot sedimentary aquifers;
- petrothermal systems where the heat is contained in the rocks, e.g. hot dry rocks.
Abandoned mine workings (mine water)
Mine water in abandoned workings in Scotland's Midland Valley might form an important low enthalpy resource for space and domestic hot water heating, and related uses. Mining creates "anthropogenically enhanced aquifers" (Banks, 1997) with additional permeability within strata that otherwise typically have significantly lower permeability. Mine waters are exploited using GSHP technology. Mine workings often spanned a significant depth range (up to several hundred metres), enabling water to be abstracted from one depth interval and returned to the ground at a different depth. This vertical separation can increase the time before the returned water, at lower temperature, starts to arrive at the point of abstraction (thermal breakthrough), which can improve the efficiency of a scheme. Mines can extend to relatively deep levels, so in some cases they can provide easy access ( e.g. via remnant shafts) to higher temperature water. For example, a borehole at the Solsgirth Colliery in Clackmannanshire recorded at temperature of 21.5°C at a depth of 387 metres.
Hot sedimentary aquifers
In the Hot Sedimentary Aquifer ( HSA) concept, the heat energy is contained in permeable, water-bearing sedimentary rocks (aquifers), and is recovered by simply sinking one or more boreholes into the resource and extracting the hot water. Although the aquifer water holds a substantial amount of heat, the main heat store resides in the host rocks, and water drawn into the aquifer to replace that drawn out via a borehole will absorb heat from the host rocks. The best HSA prospects will exist where a natural system of circulating groundwater yields a high and sustainable flow rate of heated water.
Hot dry rocks
Much of the world's accessible high-enthalpy geothermal energy exists in crystalline (non-porous) rock at depths exceeding several kilometres. Such rocks are generally assumed to lack open fractures and consequently have very low permeability. They are therefore essentially dry, hence they are known as Hot Dry Rock ( HDR) resources ( e.g. Batchelor, 1982; Lee, 1986). The EGS concept for exploiting HDR resources relies on creating open fractures to hydraulically connect two or more boreholes drilled some distance apart into a hot rock zone. Cold water pumped down one or more 'injection' wells flows through the fracture system, absorbing as it does the geothermal energy held in the enclosing rocks, and is recovered as hot water from one or more production wells ( Figure 1). The thermal energy stored in the water can be converted into electricity at the surface in various ways.
Hydraulic fracturing (injecting water at high pressure through a borehole to open existing fractures and/or create new ones in deeply buried rock) is used to develop the system of open fractures - a process usually referred to as stimulation. The fracture walls then act as heat exchange surfaces, and an engineered geothermal reservoir is created as cold water is pumped into the system. The position, shape and volume of the developing reservoir is monitored using micro-seismic survey techniques, which locate the origins of the seismicity induced as fractures open during hydraulic fracturing. In operational mode, water is pumped through the injection well under high pressure, which keeps the fractures open and forces the water to circulate through the system in a closed loop, arriving in the form of hot water (or steam) at a power plant on the surface. The same water, relieved of its heat, can then be re-cycled back into the injection well.
In recent years, major projects in Australia, France and the US have demonstrated the considerable potential for generating electricity using EGS to exploit HDR resources. All of these projects have exploited the same particularly favourable geological setting, where a thick layer of sedimentary rocks overlies an intrusion of granite with elevated concentrations of radioactive elements. The sedimentary rocks act like a thermal blanket, trapping beneath them the heat generated in the granite, which builds up over millions of years into a substantial geothermal resource at a relatively shallow, and therefore accessible, depth (~4-6 km). The potential for 'buried hot granite' resources to exist in Scotland is assessed in section 8.2.4.
Figure 1 Diagram from Lee et al. (1984; Figure 1.1) illustrating the "classic" concept of a two-well hot dry rock system. Note that in this representation the HHP granite extends from the reservoir to the ground surface, i.e. it is not buried beneath a layer of low thermal conductivity rocks.