Measurements and analysis of the thermal properties of a sedimentary succession in Yangtze plate in China
Introduction
Geothermal energy refers to the heat resources stored in geological materials or groundwater [1]. The use of geothermal energy has developed rapidly due to technical advances in high efficiency heat extraction and its low environmental impact. The ground source heat pump (GSHP) system is an emerging technology that uses the ground as a heat source or a cold sink by coupling ground heat exchangers (GHE) into the ground. The ground materials have a large thermal capacity but exhibit little seasonal variation in temperature at depths greater than 10 m below the ground surface; thus, GSHP systems can generally operate at higher efficiency and exhibit a more stable performance than traditional air conditioning systems [2]. Currently, GSHP systems are widely adopted both in commercial or residential buildings for heating and cooling. It has been reported that the installation of GSHP systems worldwide has grown continuously by 10%–30% annually over the past several decades [3].
The thermal properties of geological materials are an important consideration in the design and planning of geothermal systems. To properly install GSHP systems, the thermal properties of the ground must be determined either through field measurements or laboratory tests. Specifically, thermal conductivity, thermal capacity and thermal diffusivity are the main thermal property parameters for estimating the geothermal potential and heat transfer ability in the materials [4]. Geological materials are usually porous or fractured media that are composed of a solid matrix, with interconnected pores or fissures and voids with partial or full saturation [5,6]. Many previous studies have reported that the thermal properties of geological materials are determined primarily by factors such as temperature [7,8], pressure [9], rock-forming minerals [10], porosity or fractures [11] and moisture content [12].
The thermal conductivity of rock varies considerably with different rock lithological types [13]. Liu et al. measured 745 drilling core samples and showed that coal samples have an extremely low thermal conductivity of 0.25 W m−1 K−1, while salt rock has the highest thermal conductivity of 4.62 W m−1 K−1. For rocks of the same lithological type, the thermal conductivity increases with depth due to the effects of compaction [14]. Shim et al. conducted a statistical analysis of the thermal conductivity of rocks in the Republic of Korea. The influences of density and porosity on the thermal properties of rocks were studied by analyzing 1560 rock samples in the laboratory. Thermal conductivity was found to decrease with increasing porosity but to increase with increasing density of the samples. Thermal conductivity varies strongly between igneous, metamorphic and sedimentary rocks. The rock-forming mineral content was a dominant factor in the thermal conductivity of the rock mass [15]. Thermal conductivity increases obviously when the soil/rock samples are saturated [16,17]. It is clear from previous works that the thermal properties of geological materials are substantially affected by many factors. Therefore, an investigation should be done with a particular focus on specific geological units and rock formations to evaluate the geothermal potential or feasibility of GSHP application.
To provide more specific information relevant to the design of geothermal systems, the thermal properties of geological materials have been mapped at various locations around the world. Stylianou et al. investigated the thermal properties of rocks for the compilation of geothermal maps of Cyprus [18]. The impact of water content in the samples on thermal conductivity, mineralogical composition, and geological age of samples was the objective of their study. Blázquez et al. mapped the thermal conductivity based on the thermal conductivity measurements taken for different rock and soil samples for the Avila region in Spain [19]. In Europe, the thermomap project has been announced that aims to map the superficial shallow geothermal resources in nine countries [20]. Somogyi et al. summarized the scientific findings for the shallow geothermal mapping in six European countries [21]. Bertermann et al. measured the ground temperature and thermal conductivity of soil samples in Germany. Consequently, very shallow geothermal potentials were mapped up to 10 m depth in the selected study areas [20]. The WebGIS platform was used in their work for the visualization of the measurements.
The measurement and mapping of thermal properties have been conducted extensively using rock samples collected from outcrops. Only limited information is available for the deep ground from these geothermal maps [[20], [21], [22]]. In a vertical GSHP system, the borehole heat exchangers (BHE) are generally drilled at depths varying between 30 m and 200 m [23]. At this depth range, multigeological layers are often encountered in sedimentary deposit areas [24]. Thus, mapping thermal properties using only local rock outcrops may not be sufficient for the planning of vertical geothermal systems. To access more useable and practical information for vertical GSHP systems installation, a detailed investigation that includes the entire geological formation sequence is necessary.
This paper presents a study on the thermal properties of the rock formations of a stratigraphic succession that contains nearly all the geological formations of the Yangtze plate. Based on previous work, this paper is outlined as follows: the geological setting of the selected stratigraphic profile and the sample collection, preparation and testing procedures are introduced in Section 2. The impacts of the composite parameters such as rock lithological type, moisture content and geological formations on thermal properties are analyzed in Section 3. Furthermore, the samples collected from nine different locations are studied, and the results are compared with the corresponding formations in the stratigraphic profile. Finally, in Section 4, the major results of this study are presented.
Section snippets
Geological setting
A geological succession of the Yangtze plate that is located in Zigui county in Hubei province in China is chosen as the case study area in this work. The Yangtze plate area is shown in Fig. 1. This succession is the west-limb wing of the Huangling anticline that covers a set of pre-Cambrian to Cretaceous geological strata. The Yangtze Plate, also known as the South China Block or the South China subplate, comprises the majority of southern China, as shown in Fig. 1. The Yangtze plate contains
Results and discussion
To determine the thermal properties for the rock formations in a geological sequence from the Yangtze tectonic plate, rock samples were collected, and the thermal conductivity and thermal capacity values were measured. The obtained results were analyzed with an emphasis on lithological types and sedimentogenesis. The homogeneity of the thermal properties for the rock formations in the Yangtze plate was then studied. Based on the measurements, the thermal properties of the rock formations for
Conclusions
Thermal properties of a stratigraphic succession that contains nearly complete stratigraphic strata from pre-Cambrian to Cretaceous in the Yangtze plate in southern China are measured. Thermal conductivity, volumetric heat capacity and thermal diffusivity values are analyzed with an emphasis on rock lithological types, and sedimentogenesis at the dry/saturated states. Moreover, the thermal properties of the rock samples and data collected in nine different locations that are randomly located in
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (NSFC) (authorized No. 41877200 and No. 41502238) and by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) No. CUGL150818. The support provided by the China Scholarship Council (CSC) during the visit to the University of California, Berkeley, is deeply appreciated.
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