Thermal Conductivity Measurement in Support of the Implementation of Close-Loop Ground Source Heat Pumps or Geoexchangers
(Editor’s Note: The following is based on excerpts from Matt Grobe’s comperehensive report for the Alberta Geological Survey titled ‘Importance of Geoscience Information in the Implementation of Closed-Loop Ground-Source Heat-Pump Systems (Geoexchange) in Alberta’)
Discussions with geothermal-energy contractor associations and with representatives from different levels of government (municipal, provincial, federal) indicate that geoscience information is important for selecting, properly designing and implementing ground-source geothermal-energy (geoexchange) systems. Geoscience information is also critical for resolving land-use issues and potential environmental concerns related to the widespread adoption of this technology.
The variability of geological materials at the surface and at depth manifests itself in the form of differing drilling conditions and differing values of thermal conductivity and thermal diffusivity. Geological maps show the types of soil, sediment and bedrock that are likely present at most locations. However, translation of these maps into useful information for geoexchange practitioners is limited because 1) published values correlating thermal properties (thermal conductivity and diffusivity) with geological material type vary quite widely, and 2) the geological material classification does not necessarily provide useful information for assessing drillability of soil and rock layers.
In August 2007, the Alberta Geological Survey (AGS) initiated a pilot project involving the gathering of shallow temperature and thermal conductivity measurements in shallow-earth materials (surficial sediments, shallow bedrock units) in Alberta. The aim of this activity was to test the hypothesis that thermal conductivity values correlate with geological material type and that existing geological maps of surficial material can be used to estimate thermal conductivity where no publicly accessible values exist.
The project concept entailed 1) measuring the thermal properties of rock and sediment samples collected from outcrops and surface exposures, and 2) comparing averaged values of thermal properties derived from core samples in purpose-drilled boreholes to actual in situ measurements of thermal properties measured by standard methods in those same boreholes.
In addition, AGS explored the relevance of geology for the operation of a geoexchange system with a simple modelling exercise and calculated the cost effects of overestimating the thermal conductivity of a site.
Thermal Conductivity Results:
The goal of this activity was to collect thermal-properties measurements for different types of Alberta’s surface and subsurface earth materials (sediments, rocks) to test the utility of producing thermal-property maps.
The planned approach was to 1) obtain thermal-property values of rock and sediment samples collected from outcrops and core samples from purpose-drilled boreholes and 2) compare the average thermal properties from core samples to actual values measured by standard in situ methods in the same boreholes.
In order to carry out measurements of thermal properties of earth materials, AGS used a C-Therm TCi™ Thermal Conductivity Analyzer.
The thermal-conductivity analyzer is based on the modified transient-plane-source technique (Gustafsson, 1991; Mathis, 1999, 2000). It uses a one-sided, interfacial, heat-reflectance sensor that applies a brief, constant heat pulse to the sample. The heat provided results in a rise in temperature at the interface between the sensor and the sample. This temperature rise at the interface induces a change in the voltage of the sensor element. The rate of increase in the sensor voltage is used to determine the thermophysical properties (thermal conductivity and effusivity,) of the sample material. The thermophysical properties of the sample material are inversely proportional to the rate of increase in the sensor voltage: the more thermally insulative the material, the steeper the increase in voltage. The analysis is nondestructive but requires a flat contact surface between the sample and the sensor. Slight sample-surface irregularities are compensated for by the use of a contact agent (usually water).
Precision and accuracy of measurements conducted on standard materials that were provided by the manufacturer are better than 1% and better than 5%, respectively.
Testing was carried out on a variety of earth materials (sediments and rocks). In order to produce a flat contact surface, unconsolidated but cohesive sediments were cut with a knife and rocks were cut with a rock saw. Cuts were oriented perpendicular to bedding. Dry rock samples were resaturated with distilled water overnight.
Results from testing fully saturated rocks and competent sediments fall within the range of literature values (Birch and Clark, 1940; Kappelmeyer and Haenel, 1974; Roy et al., 1981; Cermak and Rybach, 1982; Robertson, 1988; Zoth and Haenel, 1988).
Thermal Conductivity of Sediments and Rocks at the Hastings Lake Pilot Site
Thermal-conductivity measurements were obtained from nine core samples of the drift sediments and 66 core samples of the Horseshoe Canyon Formation. As mentioned earlier, the drillcore had been wrapped in plastic wrap and aluminum foil to prevent it from drying out. Core intervals were individually unwrapped and samples were taken from the centre of the core. Samples of drift sediments were quickly weighed (for determination of moisture content) and immediately placed on the TCi sensor for thermal analysis. Bedrock samples were placed directly on the sensor without weighing. Samples were visually inspected and classified into broad lithological categories (i.e., sand, silt, clay, sandstone, siltstone, shale, coal, bentonite) prior to thermal analysis. Detailed textural (grain size, sorting, porosity) and compositional (mineralogy) analyses of the samples are still outstanding, and will refine lithological classification of the samples.
Measured thermal conductivities for the recovered drift sediments (classified as sand, silt and clay) ranged between 1.8 and 3.2 W/m•K, with an average of 2.4 W/m•K (Table 5). However, given the poor core recovery (<10% of 42.4 m) in the unconsolidated drift sediments (i.e., cored intervals are mostly more competent, clay-rich silt and clay, while the less competent sand and gravel were washed out during drilling), the samples were not deemed to be representative of the sediment types encountered. As a result, a thickness-weighted average of thermal conductivity for the drift sediments could not be obtained. Moisture contents ranged from 8.2 wt. % in silty sand recovered at 10.65 m depth to 21 wt. % in silty clay from 5.9 m depth (Table 2). For the silt samples except sample 1, a general trend of decreasing thermal conductivity with increasing moisture content can be observed. The low conductivity of sample 1, taken at 0.3 m depth, may be due to the effects of weathering and the presence of distributed organic matter.
TABLE 1: Ranges and averages of measured thermal conductivity (k) for sediment and rock types cored at the Hastings Lake Community Hall site. Drift-sediment samples were recovered from borehole HL-08-01, bedrock samples from boreholes HL-08-01 and HL-08-02.
1Denotes drift settlement samples.
TABLE 2: Measured thermal conductivity and moisture content for unconsolidated drift samples cored in drillhole HL-08-01 at the Hastings Lake Community Hall site.
Measured thermal conductivities for the recovered bedrock types range between 0.6 W/m•K for some coal samples and 2.7 W/m•K for some siltstone and sandstone samples (Table 1). Since core recovery in the bedrock units was good, AGS researchers attempted to calculate a thickness-weighted average of thermal conductivity (Table 3).
TABLE 3: Thickness-weighted average thermal-conductivity contributions of saturated bedrock types at the Hastings Lake Community Hall site.
Conclusion: Good Agreement
A composite thermal conductivity of 1.97 W/m•K was calculated for the 77 m of bedrock types penetrated at the Hastings Lake Community Hall site.
Due to the lack of representative samples for the drift sediments and the preliminary nature of sample lithology designation, a direct comparison of these values with those obtained from the FTC tests (see report) would be premature. However, given these uncertainties, the calculated composite thermal conductivity of 1.97 W/m•K is reasonably close to the values obtained by the FTC tests for both the shallow borehole (dominated by drift sediments; 2.01 W/m•K) and the deep borehole (43 m of drift sediments and 77 m of bedrock; 1.74 W/m•K). It appears the thermal conductivities derived from the FTC tests are lower than those that would be calculated as a composite of weighted-average values from the thermal analysis of the core samples. This is likely because the thermal-conductivity measurements were conducted on the best preserved (least disturbed) samples that could be obtained from the core. This eliminated the effect of naturally occurring, water-filled fractures (i.e., in the upper part of the bedrock section, close to the bedrock-drift interface), which would generally lower the thermal conductivity of a given sediment or rock unit (water has a thermal conductivity of 0.6 W/m•K). More opportunities for comparison of in situ FTC tests with calculated composite thermal-conductivity values derived from core samples are necessary to establish a better calibration methodology. However, the calculated composite thermal conductivity can serve as an estimate of the maximum thermal conductivity of the bedrock material at the site.
Resources & References
Canney, B., Dixon, C. and Mathis, N. (2001): Three-way thermal conductivity instrument comparison; Polyurethanes Expo 2001, Conference Proceedings, September 30–October 3, 2001, Columbus, Ohio, p. 71–74.
Gustafsson, S.E. (1991): Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials; Review of Scientific Instruments, v. 62, no. 3, p. 797–804.
Mathis, N. (1999): Comparison of destructive and non-destructive thermal conductivity results with filled epoxy; Thermophysical Properties, v. 20, p. 464–467.
Mathis, N. (2000): Transient thermal conductivity measurements: comparison of destructive and nondestructive techniques; High Temperatures—High Pressures, v. 32, no 3, p. 321–327.
This work was generated by researchers at the Alberta Geological Survey (AGS).