THERMAL PROPERTIES OF A HERCYNIAN GRANITIC ROCK: DATA FROM A DEEP BOREHOLE (ALMEIDA, CENTRAL PORTUGAL)

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1 Energy for Sustainability 2015 Sustainable Cities: Designing for People and the Planet Coimbra, May, 2015 THERMAL PROPERTIES OF A HERCYNIAN GRANITIC ROCK: DATA FROM A DEEP BOREHOLE (ALMEIDA, CENTRAL PORTUGAL) Mafalda M. Miranda 1 *, Alcides S. Pereira 2, José J. Costa 3, Nelson V. Rodrigues 2, Catarina R. Matos 1 1: Department of Earth Sciences, Faculty of Sciences and Technology, University of Coimbra Rua Sílvio Lima, Coimbra mfldmiranda@gmail.com, web: 2: CEMUC, Department of Earth Sciences, University of Coimbra apereira@dct.uc.pt, web: 3: ADAI-LAETA, Department of Mechanical Engineering, University of Coimbra Rua Luís Reis Santos, Coimbra jose.costa@dem.uc.pt, web: Keywords: Deep borehole, Hercynian granitic rock, thermo-physical properties, TPS method, deep geothermal potential, Portugal. Abstract. The accurate knowledge of the thermal properties of rocks is of primary importance for modelling the deep geothermal field in target areas. In the present work, measurements of the thermal properties were carried out in 33 core samples collected in a 1000 meters borehole drilled in a Hercynian granitic rock for thermal water exploitation (Almeida, Central Portugal). The well log shows a relatively homogeneous rock locally transformed by metasomatic processes. Therefore, the set of samples was divided into two groups Group I (less altered) and Group II (with more pronounced, visible metasomatic alteration). The thermal properties were measured using the thermal-constants analyser Hot Disk TPS 2500 S under standard temperature and pressure conditions (20 ºC and 100 kpa) with dry samples. The main parameter of interest, the thermal conductivity, shows mean values of ± W/mK for Group I and of ± W/mK for Group II. It is concluded that metasomatic alteration does not significantly change the thermal conductivity values. The results obtained are included in the range of variation usually obtained for similar rocks, and has high relevance for a precise evaluation of the potential of geothermal energy in Central Portugal, namely through the use of numerical models.

2 1. INTRODUCTION The use of geothermal energy could be a sustainable way to produce electricity in the near future. The recovery of thermal energy from deep reservoirs, estimated to be located at a depth of 3 to 6 km in most of the situations, would be supported through the technology known as Engineering Geothermal Systems (EGS). With this technology it is expected that the drilling phase would be responsible for the main part of the project costs and, so, positively correlated with the reservoir depth. To reduce the cost and improve the efficiency, it is crucial to find the best locations for the exploitation site, which usually would be the places where the reservoir is closest to the surface. For this goal it is necessary to determine precise site-specific data for several variables that will constrain the thermal models, particularly the ones related with transport and storage of heat in the subsurface, as is the case of the thermal properties of the host rocks of the reservoir. The knowledge of the thermal properties of Earth s materials is important to characterize the thermal structure of the Earth [1]. So, and since heat conduction is the main mode of thermal energy flow in the solid portions of the Earth, thermal conductivity is an important parameter. It controls the heat flow and the temperature field [1], [2], making this property of the Earth s materials one of the main input parameters in geothermal modelling and its knowledge one of the prerequisite for the designing of geothermal installations [3], [4]. Thus, due to the importance of the thermal properties, in the present study, data is presented about the thermal conductivity and diffusivity as well as heat capacity obtained in core samples from a deep borehole drilled in a Hercynian granite outcropping in central Portugal (Almeida) Geological data The Hercynian granites of Almeida area belong to an extensive outcrop of the Beiras Granitic Batholith located in the Central Iberian Zone in central-northern Portugal (Fig. 1), a geostructural unit of the Iberian Massif. The age of those rocks, according to Cortez [5], is posterior to Middle Gzhelian (Upper Carboniferous), classifying those granites as tardihercynian to post-hercynian. According to the mineral composition and the high levels of plagioclase feldspars, these rocks may be included in the monzonitic granites [5], [6], [7]. They are porphyritic and biotite is the dominant mica. From a structural point of view, the batholith is cut by a dense fracture network dominated by the NNE-SSW and WNW-ESE systems [5]. Those systems promote locally the rise of thermal waters, as is the case of the thermal spring of Fonte Santa (Almeida). The water emerges along a fault zone lying between 268 to 700 meters deep [5], as observed in the borehole core drilled with the purpose of increasing the hot water flow rate for the spa. In most of the well log, the granite is relatively homogeneous showing locally macro and microscopic evidences of metasomatic alteration (episienitization). 2

3 Figure 1. Geological map of the studied area, with the location of Almeida and the studied borehole AQ Methods and techniques Thirty-three granitic core samples were taken from the borehole AQ1 (Fig. 1) and selected, taking into account the lithological and structural variability observed in the well log. According to the degree of metasomatic alteration, the set was divided into two groups, I and II. The first one comprises the less altered samples and Group II the samples where the metasomatic alteration is more pronounced. The thermal properties were measure with the thermal-constants analyser TPS 2500 S manufactured by Hot Disk AB. The TPS method is based on the use of a transiently heated plane source, the hot disk sensor, which acts both as a heat source and as a resistance thermometer for recording the time-dependent temperature increase. More detailed explanations about the method and all the theory behind it can be found in, for instance, Gustafsson [8], Gustavsson et al. [9], He [10], and at the Hot Disk Thermal Constants Analyser Instruction Manual. To obtain the values of the thermal properties, the hot disk sensor 8563 was used, with a radius of mm and Kapton an insulating material, with a heating power of mw and using a measuring time-step of 80 seconds. The measurements were done at ambient temperature and pressure conditions (20ºC and 100kPa) using dry samples. This transient plane source (TPS) method gives information on the thermal conductivity, the thermal diffusivity as well as the specific heat per unit volume of the material under study, and meets ISO This method has been used already for the study of other rock samples by another authors [11], [12], [13] showing reliable results. 3

4 2. RESULTS AND DISCUSSION 2.1. Thermal conductivity The thermal conductivity measurements for Group I have a mean value of 2.90 ± 0.26 W/mK, and Group II shows a mean value of 2.95 ± 0.19 W/mK. The results showed no significant differences between the two groups of samples and are included in the range of values referred elsewhere for similar lithologies [14], [15]. The low standard deviations in both cases indicates a low variability and are in accordance with the macroscopic observations of the well log that pointed to a relatively homogeneous composition of the granitic rock along the borehole Thermal diffusivity, thermal capacity and specific heat capacity Thermal diffusivity data showed a mean value of 0.99 ± 0.21 mm 2 /s, for Group I, and a slightly higher value of 1.05 ± 0.22 mm 2 /s for Group II. Thermal capacity average values for Group I is 3.06 ± 0.67 MJ/m 3 K, being the average of Group II slightly lower 2.95 ± 0.81 MJ/m 3 K. At last, specific heat capacity for Group I shows a mean value of 1.15 ± 0.25 kj/kgk, with Group II showing a mean value of 1.12 ± 0.19 kj/kgk. The obtained values are statistically similar in both groups of samples, thus demonstrating the homogeneous character of the well log, and also that the metasomatic alteration does not significantly change the values of thermal properties. And, as in the case of the thermal conductivity, similar to those ones referred elsewhere for the same type of rocks [16]. 3. CONCLUSIONS The results obtained in the study of 33 samples of granitic rocks from the Beiras batholith (Central Portugal) enable to conclude that the thermal properties obtained from the transient plane source method with the equipment TPS 2500 S and a hot disk sensor, show a good accuracy and reproducibility and reliable results since the obtained results for the Beiras granite are included in the interval of values referred in the literature for similar Hercynian granitic rocks. Moreover, the results also allow arguing that the metasomatic alteration in the studied samples does not seem to influence their thermal properties, as the values of the studied samples analysed in two different groups on the basis of their different alteration degree are statistically similar. In conclusion, the data obtained in this work represent a contribution to the estimation of the potential of geothermal energy in Central Portugal, namely as a site-specific input for the numerical models able to evaluate later the energy production from the geothermal reservoir. 4. ACKNOWLEDGMENTS This work has been framed under the Initiative Energy for Sustainability of the University of Coimbra and supported by the project Energy and Mobility for Sustainable Regions - EMSURE (CENTRO FEDER ). 4

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