Potential for deep geothermal energy in Scotland: study volume 2

5 Key conclusions and recommendations arising from Part 1

5.1 Conclusions

1. Scotland occupies an essentially stable geological setting and doesn't obviously have a substantial accessible resource of deep geothermal energy. However, in common with all parts of the globe, the crust beneath Scotland contains a vast store of heat. The challenge is to understand how that heat is distributed, laterally and vertically, and to identify where resources occur that are sufficiently large and sufficiently accessible to form commercially viable prospects.

2. Heat flow measurements are the primary means of assessing the size of the heat resource. Thirty-five published heat flow values for Scotland are discouragingly low. The values range from 29 to 82 mW m ^-2, with a mean of 56 mW m ^-2. The mean value is somewhat lower than the mean across all continents (65 mW m ^-2), and significantly lower than values usually associated with exploitable resources of deep geothermal energy.

3. A growing body of research has shown that heat flow values can be significantly affected by local perturbations in the shallow geothermal regime. In particular, the effect of recent, post-glaciation warming on the shallowest part of the geothermal gradient means heat flow values in Scotland (and other areas affected by glaciation) are likely significantly to underestimate the heat resource at depth. 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 Preliminary, unpublished work by BGS indicates that heat flow values in the East Grampians region of Scotland may be suppressed by up to 29%.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 previously assumed.

4. Bottom-hole temperature ( BHT) measurements in boreholes provide an alternative set of measured thermal data which previously have not been assessed in the context of geothermal energy. BHT versus depth (T-z) data for sixty-one onshore boreholes in mainland Scotland and seventy-two offshore boreholes (wells) in the North West Margin area (north of the Scottish mainland and west of the Orkney and Shetland islands) define a single linear trend that extends to a depth of at least 5 km. None of the 133 BHT data lie significantly off the trend, suggesting that rock temperature in large parts of the crust beneath Scotland increases with depth in a broadly consistent, and therefore largely predictable, way. The trend is not obviously affected by the fact that the dataset includes temperature measurements made in onshore and offshore settings, basin and ridge settings, and in crystalline and non-crystalline rocks. The geographical spread of the data is patchy; however, their considerable geographical extent (over 900 km from north to south) and the continuity of the trend throughout the full 5 km depth range raise the possibility that the trend represents a general regional geothermal gradient for Scotland.

5. The trend defined by the T-z data is slightly curved: in the top 1.5 km (defined mainly by data from onshore boreholes) the gradient is approximately 30 ºC/km; between 1.5 and 3.5 km it is 36 ºC/km and between 3.5 and 5 km it is 47 ºC/km. Climate warming since the last glaciation may account (at least in part) for a lower geothermal gradient in the shallowest part of the curve, while the proximity of some offshore areas to the continental margin (where the crust may be thinner and heat flow therefore may be higher) may account (at least in part) for a higher geothermal gradient in the deepest part of the curve.

6. In the North West Margin area the temperature at 5 km is around 190ºC. If the trend defined by the T-z data represents a regional temperature gradient, similar temperature values should be encountered at the same depth in onshore settings. Even if onshore settings do not follow the higher temperature gradient of offshore settings below 1.5 km, the gradient of 30 ºC/km defined by the data from onshore boreholes suggests temperatures of around 150ºC would be encountered at 5 km in onshore settings. These results suggest the temperature gradient in the crust beneath Scotland is significantly higher and more consistent regionally than has been thought hitherto.

7. In onshore parts of Scotland, deep geothermal energy prospects can be classified in terms of three broad settings: abandoned mine workings, Hot Sedimentary Aquifers ( HSA), and Hot Dry Rocks ( HDR). The first two of these are hydrothermal systems ( i.e. the energy resides in heated water), and the last is a petrothermal system ( i.e. the energy resides in hot rock).

7.1. Mining creates additional permeability within strata that otherwise typically have relatively lower permeability; the heat within mine water can be exploited using Ground Source Heat Pump ( GSHP) technology.

7.2. In the HSA concept, the heat energy is contained in naturally permeable, water-bearing rocks (aquifers); warm or hot water extracted from the aquifer can be used for heating.

7.3. In the HDR concept, the heat energy is contained in rocks that have very low permeability and are therefore essentially dry; an Enhanced (or Engineered) Geothermal System ( EGS) must be developed to introduce cold water into the hot rocks, recover heated vapour and water from them, and generate electricity.

5.2 Recommendations

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. The following steps should be considered.

R1 Model the effect on the geothermal gradient of post-glacial warming

The extent to which the geothermal gradient at shallow depths is affected by geologically recent climate change (specifically warming since the last glaciation) needs to be determined, so that: (i) accurate corrections can be applied to past and future measurements of heat flow in shallow boreholes, and (ii) meaningful extrapolations can be made of heat flow values to greater depths. Current knowledge of the spatial and temporal distribution of glacial, periglacial and interglacial conditions in Scotland, and the ground surface temperatures associated with each, could be used to model mean surface temperatures during and at the end of the last glacial period, from which can be deduced the magnitude of post-glacial warming and its likely effect on the shape of the geothermal gradient curve in the shallow subsurface. Some academic research addressing this issue in Scotland may already be underway.

R2 Improve the heat flow dataset for Scotland

Contours on the present heat flow map of Scotland are constrained by only a handful of widely scattered (and locally closely clustered) data, leading to a generalised representation. The effect of so few data is to draw attention to a few areas of apparent above average heat flow, and simultaneously to deflect attention from large parts of the country for which there are no heat flow data. Most of the existing heat flow data for Scotland come from relatively shallow boreholes, and none come from anywhere close to the depth required (>2 km) to exceed the potential limit of a transient post-glacial warming signal. More heat flow data should be gathered to improve the dataset for Scotland in terms of both depth (the deeper the better) and geographical coverage. Deepening existing onshore boreholes might provide a relatively cheap means of obtaining heat flow data from relatively deep levels in some parts of the country. The possibility should also be explored of obtaining heat flow data from offshore deep hydrocarbon exploration wells, particularly those that penetrate basement rocks.

R3 Extend the bottom-hole temperature versus depth (T-z) dataset to include all available data for Scotland

This would improve our understanding of the degree to which the geothermal regime beneath Scotland is characterised by a single regional geothermal gradient (as is suggested by the data presented in section 4.2). Most of the T-z data presented in section 4.2 come from offshore wells, but the dataset is limited to published data from the North West Margin area. T-z data for the numerous wells that have been drilled in other parts of offshore Scotland (principally the North Sea but also around the Northern Isles and Hebrides) should be obtained to augment the dataset; these data are probably largely unpublished but will be held by the Department of Energy & Climate Change ( DECC). The assessment presented in section 4.2 includes all of the available T-z data for onshore boreholes in Scotland, but the possibility should be considered of expanding the dataset by measuring bottom-hole temperatures in existing accessible onshore boreholes for which no temperature data currently exist.

Part 2

Deep geothermal energy potential in Scotland

In Part 2 of this report, the deep geothermal energy potential in onshore parts of Scotland is considered in terms of three main settings: abandoned mine workings; hot sedimentary aquifers; and hot dry/wet rocks.

Although at this stage it must be treated with caution (at least in some parts of the country), the regional temperature gradient derived from borehole temperature data (described in section 4.2) nevertheless currently provides probably the most reliable insight into the size and distribution of the heat resource at depth beneath Scotland. It is therefore used in Part 2 to help assess the geothermal energy potential in different parts of the country. The small degree of scatter shown by data that define the gradient at all depths down to 5,000 metres means that isotherms (surfaces representing a constant temperature) in a 3D model would be represented simply as horizontal planes that ignore changes in bedrock geology. A 3D model illustrating this simple situation is not particularly helpful, therefore, so the distribution of areas that may have deep geothermal energy potential is instead illustrated in the following sections of this report using maps.