Geothermal energy is simply the natural heat that exists within our planet. In some parts of the world the existence of a geothermal energy resource is made obvious by the presence of hot springs, and such resources have been exploited in various ways for millennia. More usually, there is no direct evidence at Earth's surface of the vast reservoir of stored heat below, and geothermal energy has remained largely ignored and untapped in most parts of the world. Now, its potential as a renewable source of energy is being recognised increasingly, and technologies and concepts for exploiting it are developing rapidly along two lines: low enthalpy (low temperature) resources, which exploit warm water in the shallow subsurface to provide heat either directly (as warm water) or indirectly (via heat exchange systems); and high enthalpy (high temperature) resources, which yield hot water, usually from deeper levels, that can be used to generate electricity.
The potential for harnessing electricity from geothermal energy has long been recognised; the potentially substantial reserves, minimal environmental impact, and capacity to contribute continuously to base load electricity supply make it an extremely attractive prospect. The ongoing drive to develop renewable sources of energy, coupled with anticipated technological developments that will in future reduce the depth at which heat reservoirs are considered economically viable, means there is now a pressing need to know more about the deep geothermal energy potential in Scotland. This report contains the British Geological Survey ( BGS) contribution to a collaborative project between AECOM and BGS to produce a qualitative assessment of deep geothermal energy potential in onshore Scotland for the Scottish Government. BGS's role is to provide the Stage One deliverable "Identifying and assessing geothermal energy potential", comprising an assessment of areas in Scotland most likely to hold deep geothermal resources based on existing geological and geothermal data sets.
The report is divided into two parts. Part 1 sets out the background to geothermal energy, describes the geological context, and presents an analysis of the size and accessibility of the heat resource in Scotland based on existing geothermal data. The potential for exploiting deep geothermal energy in three settings in inshore areas of Scotland (abandoned mine workings, Hot Sedimentary Aquifers, and Hot Dry Rocks) is examined in Part 2.
The heat resource in Scotland
Scotland sits on a geologically stable part of Earth's crust and has none of the obvious features - such as hot springs or volcanic activity - that would indicate the presence of a substantial heat resource in accessible parts of the subsurface. The mean of 35 heat flow values  reported for Scotland of 56 mW m -2 is lower than the mean value for all continents (65 mW m -2), and significantly lower than values usually associated with exploitable resources of deep geothermal energy.
In order to calculate a heat flow value, the geothermal gradient (the rate at which temperature increases with depth in the Earth's crust) must be measured in a borehole. Within the last decade, research has shown that warming of the ground surface since the last period of widespread glaciation (the 'Ice Age') has perturbed the geothermal gradient within the top 2 km of the crust, with the result that measured geothermal gradient values (and therefore heat flow values) are reduced. Scotland was strongly affected by the 'Ice Age' glaciations, so existing heat flow measurements in Scotland probably significantly underestimate the true size of the heat resource that exists beneath the climate-affected zone. Published data suggest that recent changes in the climate may have suppressed near-surface heat flow by as much as 60% in some parts of northern Europe and North America. Preliminary, unpublished work by BGS indicates that heat flow values in the East Grampians region of Scotland may be suppressed by up to 29%. These findings suggest that heat flow below the climate-affected zone in Scotland (which may extend to a depth of around 2 km) is significantly greater than was previously assumed.
Temperature data measured in boreholes provide the best currently available alternative means of examining the size and distribution of the heat resource beneath Scotland. Borehole temperature data from 133 boreholes ranging up to 5 km deep and representing both onshore and offshore parts of Scotland display a well-defined trend when plotted as temperature versus depth. The geographical extent of the data and the consistency of the trend throughout its depth range suggest the trend may represent a regional temperature gradient for Scotland. The gradient is slightly curved and increases with depth, from 30.5°C/km in the shallowest third to 46.7°C/km in the deepest third. These values, which equate to temperatures of 100ºC and 150ºC at depths of approximately 3.0 and 4.0 km, respectively, suggest there is a significantly larger heat resource at accessible depths beneath Scotland than has been suspected previously. However, the data defining the trend come mainly from offshore boreholes in sedimentary rocks, and caution should be exercised in extrapolating the same gradient to deep levels onshore, particularly in crystalline rocks. More research is needed to test this result; nevertheless, the borehole temperature trend suggests the temperature gradient in the crust beneath Scotland may be significantly higher and more consistent regionally than has been recognised hitherto.
The regional geothermal regime beneath Scotland is still relatively poorly understood. A better understanding of the regional distribution of heat, both laterally and vertically, in shallow parts of the crust is needed before decisions are made regarding the location and design of more detailed, site-specific studies.
Recommendations for further work:
R1 model the effect on the geothermal gradient of post-glacial warming
R2 improve the heat flow dataset for Scotland
R3 extend the borehole temperature dataset (bottom-hole temperature versus depth) to include all available data for Scotland.
Geological settings for exploiting geothermal energy
The geothermal heat resource beneath Scotland can be considered in terms of three main settings: abandoned mine workings (low enthalpy), hot sedimentary aquifers (low and possibly high enthalpy), and hot dry rocks (high enthalpy).
Abandoned mine workings
Scotland's Midland Valley is underlain in many parts by a network of abandoned mines. These once employed thousands of miners to extract coal, ironstone and other minerals and are the basis and location of many towns and villages. They provided the energy and raw materials that powered industry in the 19 th and 20 th centuries, and the fuel to heat domestic properties. With industrial change and economic decline the mines closed so that there are no underground mines still in operation. The mines could play an important role in future in energy supply, providing access to thermal reservoirs which could help to heat homes and other buildings, and contribute to the energy mix of a low carbon Scottish economy based substantially on renewable energy.
Two installations of GSHPs currently tap mine water in Scotland: Shettleston in east Glasgow and Lumphinnans in Fife. Both are small schemes, each serving less than 20 dwellings, and have been operating since 1999-2000.
Most mine workings were collapsed in a controlled way soon after the resource had been extracted, but these rubbly collapsed layers can store and transmit significant volumes of groundwater. Larger voids remain underground in the form of old mine shafts and roadways, as many of these were constructed to a high standard and are still propped open.
During their operation, very large volumes of water had to be pumped from the mines. This water, held naturally in the rocks as groundwater, entered the mine workings, but with pumping, the level at which groundwater occurred around the mines was greatly lowered so they could function in relatively dry conditions. When mining ceased, pumping ceased also in most instances allowing the natural levels of groundwater to re-establish. As a result, most of the abandoned mine workings become flooded, and remain so today. The abandoned mine workings now contain significant volumes of water. In addition and arguably more importantly, they provide potential access to the very much larger volumes of water held in the rocks within which the mines occur. The volumes of water in the surrounding rocks are of the order of 100 times greater than the volumes of water in the mine workings.
Therefore, the abandoned and flooded mine workings and their surrounding rocks have very large subsurface volumes, which in turn contain very large volumes of water. It is the water which and provides significant potential for heat exchange. This, and the associated potential high abstraction rates as a result of the mining-enhanced permeability, makes them potentially suited to large, open-loop ground source heat pump ( GSHP) systems. These open-loop systems would not be open to the atmosphere as the chemistry of the mine waters, and particularly their potentially high iron content, could result in iron precipitation if exposed to atmospheric oxygen.
Mine workings typically worked several relatively flat lying seams of coal (etc) in a vertical succession. These often spanned a significant depth range (up to several hundred metres). This could enable water to be abstracted from one depth interval for example towards the bottom of the mine, and returned to the ground at a different depth for example at a significantly shallower depth and after heat has been extracted from the water. This vertical separation can be advantageous in increasing the time before the returned water, at lower temperature and shallower depth, starts to arrive at the point of abstraction where water warms at greater depth. This can in turn 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 a temperature of 21.5 °C at a depth of 387 metres. Deep boreholes (800 metres) at Heerlen in the Netherlands intercepted water of about 35 °C.
In 2004, the Scottish National Mine Water Potential Study assessed the largest 62 coal mines in Scotland; the number of men that worked down the mine was used as a proxy for the volume of mine water that might be available. The study considered the potential for mine water heating at Shawfair, near Edinburgh, as part of a consideration of the wider potential across the Midland Valley of Scotland. It was calculated that mine waters could contribute up to 1,708 gigawatt hours ( GWh) per annum of heat, if grants were provided. This equates to about 3% of their estimate of Scotland's total annual heat demand in 2004.
This assessment of the geothermal potential of mine water in Scotland draws on recent BGS work, including 3D modelling, and utilises other available data to provide an estimate of potential borehole yields and the extent of the likely resource across the Scottish coalfields. The appraisal is based on the best estimates of geological and hydrogeological properties of the mined areas, providing a different viewpoint from earlier studies. Ironstone and oil-shale mines within the Midland Valley also have the potential for use in mine water heat recovery. From BGS 3D modelling, borehole data, and Coal Authority extent-of-mining data, we estimate the volume of the mine-worked area ( i.e. from the base of the mine workings to land surface) to be 600 km 3. It is this approximate volume of rock, and the mine workings themselves, that groundwater can be abstracted (pumped), and from which heat can be extracted.
The two key parameters which influence the potential for getting heat from mine waters are the flow rate at which water can be abstracted from the subsurface without significantly depleting the resource, and the temperature of that water.
It is very difficult to predict the likely flow rate (borehole yield) that a particular borehole might obtain due to the great variability in mining-enhanced aquifers, but analysis of available data has enabled us to establish a typical range of yield. In general, it is thought reasonable to expect a yield of about 10 litres per second (l/s) in mining-enhanced aquifers, from an individual borehole penetrating a reasonable water-saturated thickness of strata (minimum of 50 metres). For the purposes of the calculations of potential heat abstraction, a range of 5 to 25 l/s per borehole seems reasonable. Assuming that this yield could be achieved in boreholes spaced at 4 boreholes per km 2, we suggest that the groundwater could be exploited at a rate of 20 to 100 l/s/km 2. This rate of pumping would be far in excess of the natural recharge rate and would be unsustainable without re-injection of the water to the aquifer (ie. the shallow mine workings) after heat has been extracted.
The temperature of mine waters generally increases with depth according to the geothermal gradient. It is not simple, however, to predict the temperature of water pumped from a borehole, as water of different temperature may be entering the borehole at different depths. A compilation of mine water temperatures for boreholes in the Midland Valley shows a fairly narrow spread of temperatures from 12 to 21°C, with a mean (and median) of 17°C. However, this may not accurately reflect the higher temperatures that may occur in some of the deepest mine workings in Scotland.
We estimate that some 2.5 megawatts per kilometre square (MW/km 2) could be obtained on average using open-loop ground source heat systems in the mined areas of Scotland. Multiplying this value by the number of square kilometres in the mined area (4.8 x 10 3 km 2) gives a very approximate estimate of the accessible heat of 12 000 MW (12 GW). We consider this to be an approximate estimate of the maximum potentially accessible resource, i.e. how much heat energy could theoretically be extracted from all the mined areas of the Midland Valley, bearing in mind the geological constraints on how much water can in practice be abstracted.
On this basis, mine waters could theoretically provide the equivalent of approximately one third of Scotland's heat demand. However, the actual contribution is likely to be significantly less for three main reasons:
- heat cannot be transported efficiently over large distances, so would only be used above or close to suitable mine workings (although many towns and villages in Scotland's Midland Valley lie directly above mine workings, reflecting their historic roots);
- a proportion of mine workings will not be suited to heat extraction;
- heat delivered by GSHP is most effectively used in new-build properties; existing building stock would likely require extensive upgrading to benefit from mine water heat.
We have not selected areas within the Midland Valley as having more favourable prospects for mine water heat recovery as economic and technical factors are likely to play a greater role in site selection.
Mine water in abandoned workings in Scotland's Midland Valley presents a potentially important geothermal resource which might be used (by heat exchange) for space and domestic hot water heating, and related uses. However, there are many assumptions and generalisations in this assessment and further work is recommended.
R4 One or more site-specific studies should be conducted utilising existing information (including Coal Authority and BGS data), developing a detailed 3D model of the mine workings, gathering new information on borehole yields and permeability, and assessing the technical feasibility of installing an open loop GSHP system perhaps in combination with other forms of energy. The Glasgow area would be an obvious target given the availability of BGS 3D geological models and other previous studies as well as the expected scale of the ongoing developments and urban regeneration.
R5 The GSHP industry should be encouraged to deposit key data on installed schemes in a national archive, similar to that for borehole data held by BGS to facilitate further modelling and potential exploitation of available thermal resources.
Hot sedimentary aquifers
Aquifers are bodies of permeable rock that can conduct significant quantities of groundwater. The largest and most conductive aquifers generally occur in sedimentary strata, and any of these that are hot enough and have sufficient productivity to constitute a potential geothermal resource can be termed a Hot Sedimentary Aquifer ( HSA). HSA resources are likely to exist, in general, down to depths of around 4 km, and most will yield water in the temperature range 20-80ºC. The hot water can be used for heating, either directly or indirectly (by heat exchange). The first, and so far only, successful HSA system developed in the UK is in Southampton. Opened in 1986, the system exploits warm water (<80°C) at a depth of nearly 2 kilometres in sedimentary strata. A combined heat and power ( CHP) system delivers sustainable supplies of heat (district heating), chilled water and electricity.
Most of Scotland (including much of the Highlands and Southern Uplands areas) is underlain by relatively impermeable crystalline (non-sedimentary) rocks, which have no HSA potential. The Midland Valley is the largest onshore part of Scotland to be underlain by sedimentary strata, and the best HSA prospects are likely to be here. However, the Midland Valley is geologically complex, and it can be difficult to make geological correlations between boreholes and to extrapolate surface observations below the ground surface. Interpretation in some areas is complicated further by the influence on groundwater flow of abandoned and active coal mines. The aquifers are typically of variable lithology, intruded by relatively impermeable igneous rocks, fractured, faulted and generally complex.
The best HSA prospects in Scotland are probably in Devonian sedimentary strata (roughly 420 to 360 million years old) underlying the northern part of the Midland Valley and the southern onshore margin of the Moray Firth Basin, and in Permo-Triassic strata (roughly 300 to 200 million years old) filling small geological basins in parts of south-west Scotland. It is emphasised, however, that current understanding of the distribution and properties of aquifers in Scotland comes very largely from surface and near-surface observations, and we know relatively little about aquifer distributions and properties at depth. The ability of water to move through rocks can change significantly with depth, and testing in deep boreholes will be required to gauge the suitability at depth of any setting with HSA potential.
Relatively high temperatures at the bottom of some coastal boreholes suggest that hot water from offshore sedimentary aquifers may have migrated locally to shallower levels in onshore margins of the aquifers. It may prove possible in some places to access HSA prospects in offshore (near-shore) sedimentary basins by drilling inclined boreholes from onshore coastal locations.
R6 More detailed investigation of parts of the Midland Valley to identify specific targets with HSA potential.
R7 Further investigation of possible thermal anomalies in some onshore margins of large offshore basins such as the Moray and Solway firths and the Firth of Clyde.
R8 Testing in deep boreholes to gauge the actual permeability and overall productivity at depth in settings with HSA potential.
Hot dry rocks
In Hot Dry Rock ( HDR) resources, heat is extracted from 'dry' crystalline rocks by fracturing them, injecting cool water into the hot fractured rock, and extracting the resulting hot water. This requires the development of an Enhanced (or Engineered) Geothermal System ( EGS). The EGS concept typically involves developing a 'loop' in the hot rock consisting of boreholes at either end of a network of connected, open fractures, through which cold water is introduced and hot water is removed. HDR resources typically yield hotter water (100-200°C) than HSA resources, and the thermal energy stored therein is converted into electricity at the surface.
HDR projects are currently being developed in several parts of the world, including Australia, France, and the USA. Some of these have the potential to yield substantial amounts of energy but as yet none is being operated on a sustainable, commercial basis. Two projects to exploit HDR prospects are currently being developed in the UK, both in the granite intrusions of Cornwall: the United Downs Project and the Eden Deep Geothermal Energy Project.
HDR resources typically will be deeper and hotter than Hot Sedimentary Aquifer ( HSA) resources. Consequently, they have the potential to produce electrical power, but they will probably present a greater technical challenge than HSA resources. In recent years the development of binary cycle power plants, in which electricity can be generated using water that is cooler than 100ºC, has greatly improved the potential for recovering geothermal energy from HDR prospects that previously would have been considered marginal or not viable.
The 'regional geothermal gradient' for Scotland (see The heat resource in Scotland, above) suggests a temperature of 150°C would be reached at a depth of approximately 4,000 metres, which is within the widely quoted practical lower limit for exploiting HDR resources (5,000 metres). If the more conservative geothermal gradient suggested by borehole temperature data from onshore boreholes is used (30.5°C/km) then 150°C should be encountered at approximately 4,900 metres, still within the 'accessible zone'. However, the lack of temperature data for crystalline rocks in deep (> 2 km) onshore boreholes means that caution should be exercised in applying the regional temperature gradient to potential HDR settings.
The best HDR prospects in Scotland are likely to exist in geological settings where heat produced by radioactive decay of elements like uranium (radiogenic heat) in the crust augments the background heat flow, producing localised thermal anomalies. There are numerous exposed granite intrusions in Scotland, and a small proportion of these produce significant quantities of radiogenic heat. These 'High Heat Production' ( HHP) granites occur mainly in the East Grampians region, and two crop out to the north of Inverness. A previous investigation of the HDR potential of the East Grampians HHP granites reported disappointingly low heat flow values, but this work predated the research described above showing that heat flow values in Scotland probably underestimate the size of the heat resource beneath the climate-affected zone. The possibility remains, therefore, that some of the exposed HHP granite intrusions in Scotland have HDR potential.
Granite intrusions can be buried beneath a thick cover of younger sedimentary rocks. Where HHP granite intrusions have been buried in this way, some of the heat passing through and generated within the granite may become trapped beneath the sedimentary rocks, particularly if they are poor heat conductors. Over geological time, large reservoirs of trapped heat can potentially develop in this way. Based mainly on geophysical evidence, buried granite intrusions are inferred to exist in Caithness, beneath the East Grampians region, and in south-east Scotland, but to date no buried intrusions of HHP granite have been proved in Scotland. Based on the distribution of HHP granite at outcrop, intrusions of HHP granite sitting beneath thick piles of sedimentary rock may exist beneath the Moray Firth and its onshore fringes.
The evidence base for assessing the potential for Hot Dry Rock prospects in Scotland is far from adequate. Most significantly, we need to improve our understanding of the distribution in Scotland of exposed and buried intrusions containing HHP granite.
R9 Compile heat production data for all the granite intrusions of Scotland to identify all intrusions that have HHP character at outcrop.
R10 Conduct research to identify whether some of the exposed intrusions that do not have HHP character would have had HHP character in now-eroded portions, or may have HHP character in still-buried portions.
R11 In offshore areas, use seismic survey data and information from wells to identify buried intrusions and intrusions exposed on the sea floor.
R12 In onshore areas, re-interpret existing regional geophysical data and 3D geological models using modern methodologies and up-to-date knowledge of the surface and subsurface geology to identify possible buried granite intrusions (and other potential HDR settings).
R13 Characterise the fracture network in exposed HHP intrusions.
R14 Conduct a programme of deep drilling to provide measured and observed, factual data from within the deep geothermal regime; to penetrate beyond the influence of surface- and near-surface effects on the geothermal gradient, a deep geothermal borehole would probably need to extend to a depth of at least 3 kilometres, and ideally to 5 kilometres.
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