OR/19/015 Thermal conductivity
|Busby, J P. 2019. Thermal conductivity and subsurface temperature data pertaining to the Glasgow Geothermal Energy Research Field Site (GGERFS). British Geological Survey Open Report, OR/19/015.|
Thermal conductivity of the geological strata is required for thermal modelling and for estimations of temperatures at depth. Most measured thermal conductivity values have been undertaken in the laboratory from drill chippings. Such measurements are only representative of the strata at a specific location and only give a matrix value of the sample as opposed to a bulk value that would take into account the layering of different lithologies.
This report generates a thermal conductivity log for the vertical section of the bedrock geology that is expected to underlie the GGERFS. The geological section has been taken from Monaghan et al. (2017) and is shown in Figure 1. Measured thermal conductivities are used as the base for constructing the log, but variations in lithology are taken into account in order to derive the best thermal conductivity estimates for the stratigraphic units.
Measured thermal conductivity data
All of the available laboratory measured thermal conductivity data were taken as part of heat flow studies. Heat flow is obtained from a combination of a temperature gradient and the thermal conductivity along a section of a vertical borehole. There are no heat flow boreholes at the GGERFS, but there are five boreholes within the local vicinity and two, 75 km away in Fife (see Figure 2 and Table 1).
|Borehole||BNG co-ordinates (m)||Distance from the GGERFS site (km)||Sampled Formations|
|Maryhill||257180E, 668560N||7.6||Limestone Coal|
|Hurlet House||250100E, 661200N||11.3||Lawmuir|
Clyde Plateau volcanics
|Clachie Bridge||264470E, 683680N||20.8||Clyde Plateau volcanics|
|Barnhill||242690E, 675710N||23.4||Clyde Plateau volcanics|
|Boreholes in Fife|
|Boreland||330394E, 694222N||74.9||Upper Limestone|
|Glenrothes||325620E, 703140N||75.0||Lower Limestone|
As there is a large distance to the boreholes in Fife from the GGERFS, there are likely to be variations in lithology. Hence, where possible, these thermal conductivities are not used in the construction of a representative GGERFS thermal conductivity log.
The measured data comprise a depth to the sample, a measured thermal conductivity and a rock description. In many of the boreholes the sampling is at a regular spacing, which is often 5 m. Irregular spacing does not imply a thickness of the particular rock type. The measured thermal conductivity data from each of these boreholes are presented in Appendix 1 - Measured thermal conductivity data.
Presentation of representative thermal conductivity
A geological description has been obtained for each borehole from the literature where the interpreting geologist has subdivided the formations on stratigraphical or lithological criteria. For each of these subdivisions, a thermal conductivity has been derived as the mean of all the measured values that fall within the depth range of the subdivision. Since the thickness of each subdivision is known from the geological interpretation, a thickness weighted thermal conductivity has been derived for each formation from the subdivision thermal conductivities as a harmonic mean, i.e.
where λb is the formation thermal conductivity, λi is the thermal conductivity of the ith subdivision and φi is the fractional thickness of the ith subdivision. These derived formation thermal conductivities are presented in Table 2.
|Maryhill||Hurlet House||Clachie Bridge||Barnhill||Kipperoch||Boreland||Glenrothes|
|Clyde Plateau volcanics||2.12||2.38||2.1|
Thermal conductivity of the Coal Measures, Passage Formation and Kirkwood Formation
There are no recorded thermal conductivity measurements on the Scottish Coal Measures, the Passage Formation and the Kirkwood Formation. Thermal conductivities have therefore been estimated from measurements on similar lithologies from outside of the region.
Scottish Coal Measures Group
The Coal Measures are cyclical in their deposition and consist of interbedded sequences of sandstone, siltstone, mudstone and coal and in some cases the coal has been mined to leave a layer of fill (goaf).
There are a large number of site investigation boreholes in the Clyde Gateway area. Borehole Cuningar Loop M7 (BNG 262400E, 662600N; Registration No. NS66SW17585/M7) extends from 34.29 m to 72.54 m depth through the Scottish Middle Coal Measures. The lithological log is shown in Table 3.
|Depth (top) m||Depth (base) m||Thickness (m)||Rock type|
The thermal conductivity for this representative section of Scottish Middle Coal Measures has been estimated by combining the thermal conductivities of the individual lithological units as a harmonic mean (as described above). The lithological thermal conductivities have been obtained from measurements on samples from five boreholes in northern England that penetrated the Pennine Upper, Middle and Lower Coal Measures. These five boreholes were Rowlands Gill (BNG 416640E, 558150N), Shipton (BNG 454460E, 458579N), North Duffield (BNG 469177E, 435235N), Grove No.3 (BNG 476155E, 381373N) and Kelham Hills No.1 (BNG 475945E, 357615N). The average thermal conductivity for each lithology is shown in Table 4.
Matrix thermal conductivities are also required for coal and goaf. Herrin and Deming (1996) report on the matrix thermal conductivity of coals from the United States and report an arithmetic mean of 0.33 W m-1 K-1 from 55 samples collected across the United States. Fracturing within the coal (the cleat), which will be most likely partially water filled, will raise the bulk thermal conductivity and so a value of 0.4 W m-1 K-1 has been estimated for coal. The goaf comprises unconsolidated material and so an estimated thermal conductivity of 1.7 W m-1 K-1 has been made.
These thermal conductivities and thicknesses from Table 3 have been combined with the harmonic mean to derive an estimated thermal conductivity for the Scottish Middle Coal Measures at the GGERFS of 2.02 W m-1 K-1.
The Passage Formation is described by Monaghan et al. (2017) as dominated by sandstone, some coarse-grained, with red-purple-green-yellow mudstone, seatclay and fireclay. The adopted thermal conductivity is 2.9 as estimated by the BGS GeoReport service.
The Kirkwood Formation comprises volcanic detritus and is described as a volcaniclastic mudstone and volcaniclastic sedimentary rock. The adopted thermal conductivity is 2.1 as estimated by the BGS GeoReport service.
Thermal conductivity log
The thermal conductivity log for the vertical section of the bedrock geology that is expected beneath the GGERFS is shown in Figure 3. As mentioned, thermal conductivities from the Boreland and Glenrothes boreholes have only been used in construction of the log when no other data were available. Hence, from Table 2, the only value used is that for the Upper Limestone Formation from the Boreland borehole. Where there is more than one formation value from the Maryhill, Hurlet House, Clachie Bridge, Barnhill and Kipperoch boreholes the mean of the thermal conductivities has been taken.
- MONAGHAN, A A, O'DOCHARTAIGH, B, FORDYCE, F, LOVELESS, S, ENTWISLE, D, QUINN, M, SMITH, K, ELLEN, R, ARKLEY, S, KEARSEY, T, CAMPBELL, S D G, FELLGETT, M, and MOSCA, I. 2017. UKGEOS — Glasgow Geothermal Energy Research Field Site (GGERFS): Initial summary of the geological platform. British Geological Survey Open Report, OR/17/006. 207pp.