Simion, Stefania (2016) Multiscale petrophysical and thermal properties analysis of rocks. [Magistrali biennali]
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Data on thermal properties of rocks such as thermal conductivity (λ), thermal diffusivity (α), specific heat capacity (cp) are necessary for many aspects of exploration and exploitation of geothermal fields (Popov et al., 2012), in both high-low enthalpy and geo-exchange systems. However, there are still several difficulties in characterize geological materials, because their thermal properties are extremely dependent on scale of measurement. From a micro- and mesoscale point of view, porosity (in sedimentary and volcanic rocks), the dominant mineral phase (in metamorphic and plutonic rocks), and anisotropy (in sedimentary and metamorphic rocks) are important controlling factors on thermal conductivity. Texture have been scarcely investigated. Anyway, thermal conductivity as a tensor depends not only on the volume fraction and thermal conductivity of rock components, but also on their distribution, on geometry and internal structure, and on the heat transfer conditions at the contacts between them (Schön, 2011). Thus, understanding the influence on thermal properties of the texture is a present-day challenge whose results could provide a huge contribution to the scientific community now involved in geothermal energy topics. The main goal of the present project is to provide an example of a new approach that could take into account the thermo-physical properties of rocks at the microscale as well as those at meso- and macroscale. Some techniques have been tested on two different lithologies, dolomites on one hand and trachytes on the other, and results have been discussed separately. μ-XRF seems to be the “turning point” technique for analyses of petro-physical properties on effusive rocks as trachytes since the image analysis on the elemental X-ray maps provides reliable quantitative information on texture and rock-forming minerals in relatively short analysis times. In the same way, a good applicability of the technique is assumed on intrusive rocks. Conversely, the micro-XRF doesn’t provide the expected results for dolomites. Among those tested, the He pycnometer technique remains the most accurate one for measuring porosity. In addition, a search on literature was made to understand how thermal properties of rock-forming minerals were and can be measured; an overview on the models for the computation of thermal conductivity of rocks starting from rock-forming minerals values is provided. A continuation of this study is necessary in order to (1) test the validity of the proposed methods on other lithologies, (2) deepen the study of texture influences on thermal conductivity, (3) contribute to the compilation of rock thermal properties database collected by several authors, (4) apply the acquired information for numerical modelling purposes.
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