Potential for deep geothermal energy in Scotland: study volume 2

Footnotes

1. Heat flow is the standard measure of the amount of heat travelling through Earth's crust, and heat flow measurements (usually made in shallow boreholes) are the standard means of gauging the size of the heat resource at depth. Heat flow values are generally expressed in milliwatts per metre square ( mW m ^-2).

2. The friction caused by drilling, and the effect of introducing lubricating fluid, can temporarily change the temperature of the rock lining a borehole. Immediately after drilling, the rock temperature at shallow depths is typically hotter than it is naturally, while at deeper levels it is typically cooler than it is naturally. It can take many months or even years for rock temperature to return to its natural equilibrium condition after drilling. For this reason, it is generally best to measure borehole temperature some time after drilling has ceased. In many cases this is not possible, so a correction factor is applied to the measured temperatures to account for the temporary change.

3. This is significantly lower than the value suggested in this report for onshore boreholes in Scotland (30.5 °C/km; section 4.2), which was interpreted from a dataset dominated by the same boreholes in the Midland Valley. The difference is because the data have been interpreted in different ways. Browne et al. (1987) 'pinned' the top of their interpreted geothermal gradient to a surface temperature of 10 °C (representing the present day surface temperature at Grangemouth); this approach makes the gradient significantly lower than it would be if the top of the gradient is not pinned in this way. The top end of the temperature gradient (or averaged geothermal gradient) reported in section 3.4 of this report is not pinned to surface temperature.

4. Granite sensu stricto is a single rock type. The term 'granite' is also commonly used sensu lato, to encompass other similar rock types, including granodiorite and tonalite. Individual types of igneous rock, including granite, can be identified in two ways: by the relative proportions of key minerals as they appear in microscope analysis of solid rock; and by the proportions of key chemical elements as they appear in chemical analysis of finely crushed rock.