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Thermal properties of sandstone after treatment at high temperature
International Journal of Rock Mechanics & Mining Sciences 85 (2016) 60–66
Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical note
Thermal properties of sandstone after treatment at high temperature Qiang Sun a, Chao Lü a,n, Liwen Cao a, Weichao Li b, Jishi Geng a, Weiqiang Zhang a a
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100048, PR China
b
art ic l e i nf o Article history: Received 10 October 2015 Received in revised form 24 February 2016 Accepted 9 March 2016 Keywords: Thermal damage Thermal conductivity Thermal diffusivity Heat capacity Phase inversion
a b s t r a c t This paper reports the variations of thermal conductivity, thermal diffusivity and heat capacity of sandstone after high-temperature heating. Experiments were carried out to test the thermal properties of sandstone which had been heated at temperatures ranging from room temperature to 900 °C in a furnace. Temperature has a significant impact on the thermal conductivity, thermal diffusivity and heat capacity of sandstone, which is closely related to the loss of water and damage of structure caused by thermal reactions. The results indicate four phases in the variation of thermal parameters with temperature: from room temperature to 200 °C, 200–400 °C, 400–600 °C, and above 600 °C. The first and second phases correspond to the vaporization-escaping interval of adhered water, combined water and structural water. Between 400–600 °C, especially from 500–600 °C, the minerals in sandstone has thermal reactions, which are demonstrated as porosity increase, reduction of conductivity and diffusivity, and change of heat capacity. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction The thermal properties of rocks at high temperature or after heating at high temperature produce valuable information for terrestrial sciences, development and solution of applied problems in geothermy, geothermal power generation, and energetics of geodynamic processes of sedimentation basins,1 geophysics,2 geological disasters and geological structure formation,3 such as deep geological repositories for heat-generating radioactive wastes,4 exploration of geothermal energy,5–7 deep petroleum boring,8 exploitation of natural gas6,9 utilization of hydrothermal energy and underground thermal energy storage, drilling and well logging,6 and the protection of buildings against fire or building restoration after exposure to fire.10,11 In the past few decades, considerable experimental effort has been taken to quantify the relation between thermal properties factor (thermal conductivity, and thermal diffusivity, heat capacity) and temperature of rock. Intense study of the thermal-conductivity factor of rocks began in the middle 20th century in relation to the development of geothermal research aimed mainly at the evaluation of the heat flow rate. The latter is calculated from the measured geothermal gradient in wells and from the thermal conductivity of rocks recovered by them.12 Somerton and Boozer13–15 measured thermal diffusivity and conductivity of some typical sedimentary rocks using an unsteady-state conductivity method for 90–800 °C, reporting that
thermal diffusivity and conductivity decreased drastically at elevated temperatures. The studies of Vosteen and Schellschmidt,16 Hanley et al.,17 Emirov and Ramazanova1 also show that the thermal conductivity and diffusivity of rocks in general decreases with temperature. The heat transfer mechanism in rocks is very complex. It is difficult and sometimes impossible to theoretically and correctly predict the thermal conductivity of porous materials, even providing having many simplifications.2 Rocks are composed by mineral particles with various chemical compositions and different degrees of crystallization. Therefore, the thermal properties of rocks depend not only on the pressure and temperature but also on their mineralogical composition, the structure and geometry of pores, grain sizes, shapes of cracks, and their concentration. Calculation of heat transfer in subsurface of formations requires data on the thermal-expansion characteristics of rock and the heats of reaction of mineral constituents. Therefore, the research on the thermal properties of sandstone is extremely meaningful on a wide range. In this paper, the variations of porosity and thermal parameters after high-temperature heating (thermal conductivity, thermal diffusivity and heat capacity) are analyzed. Thermal expansions and heats of reaction are discussed in the temperature range of 25–900 °C.
2. Experimental tests n
Corresponding author. E-mail address: [email protected] (C. Lü).
http://dx.doi.org/10.1016/j.ijrmms.2016.03.006 1365-1609/& 2016 Elsevier Ltd. All rights reserved.
Sandstone samples were obtained from Linyi, Shandong
Q. Sun et al. / International Journal of Rock Mechanics & Mining Sciences 85 (2016) 60–66
61
province. X-ray diffraction (XRD) analysis showed that the main components of samples are quartz, feldspar, dolomite/ankerite and hematite/magnetite (as shown in Fig. 1), accompanied by a small amount of biotite and kaolinite structure. The highest content of SiO2 is 35.49%, and the next is CO2 accounting for 22.64%. The content of CaO, Al2O3, MgO, Fe2O3 and K2O/Na2O are 13.43%, 10.55%, 8.10%, 5.53% and 2.96% respectively. Therefore, the cement of the sandstone is of ferruginous, dolomitic and/or clayey type. These samples with average bulk density of 2.41 g/cm3 at room temperature were cut into Φ50 ( 70.05) 30 (70.05) mm2 cylinders. The initial content of water was bout 0.07%. In addition to room temperature samples, all the test samples were treated on 100 °C in an oven lasting for an hour before the thermal test. Therefore, there was no adhered water in the tested samples. The heating apparatus consists of a high temperature furnace (type KSL-1100X-L). The specimens used in all the following tests were placed in the furnace and then heated to the designated temperature (25 °C, 50 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C and 900 °C, respectively) at the rate of 30 °C/ min. Each specimen was kept at its designated temperature for about 30 min before the power was cut off, and the specimen was allowed to cool down naturally with the decline of temperature in the furnace. The porosity and thermal parameters of these specimens were tested before and after heating. The porosity was measured by a microporosity structure analyzer apparatus (type 9310) produced by Micromeritics Instrument Corp. The thermal parameters were simultaneously collected with a TPS test machine produced by Hot Disk equipment Co., Ltd.
3. Analysis 3.1. Variation of thermal parameters The results of thermal conductivity, thermal diffusivity and specific heat capacity are given in Figs. 2–4, respectively. After high-temperature heating, the thermal conductivity, thermal diffusivity and specific heat capacity of sandstone specimens change significantly at the temperature range of 25–900 °C. From 25 °C to 200 °C, conductivity decreases rapidly with temperature. At the range of 200–400 °C, the thermal conductivity decreases a little bit. However, when the temperature is higher than 450 °C, there is another significant decline range. The greatest 1800
D
D i ffra c t i o n i n t e n s i t y (p u l s e c o u n t s / s )
1600
D: Dolomite/Ankerite Q: Quarter K: Kaolinite F: Feldspar B: Biotite H:Hematite/Magnetite
1400 1200
Q
1000 800
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200
D
Q KH
K
400
H H H
D
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H H
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rate of decrease in conductivity is at lower temperatures. From 25 °C to 200 °C, thermal diffusivity decreases rapidly with temperature. At the range of 200–400 °C, the thermal diffusivity decreases a little bit. However, when the temperature is higher than 400 °C, there is another significant decline range. The greatest rate of decrease in conductivity is at the lower temperatures. Above approximately 600 °C, thermal diffusivity becomes essentially constant. From 25 °C to 200 °C, heat capacity increases rapidly with temperature. At the range of 200–600 °C, there is a volatility for heat capacity, but the whole is relatively stable. However, when the temperature is higher than 600 °C, there is a significant decline range. 3.2. Relationship between porosity and thermal parameters
F
B
600
Fig. 2. Variations of thermal conductivity after different temperature treated (a) test results; (b) variation level of thermal conductivity.
30
35
40
45
50
55
Diffraction angle (°) Fig. 1. XRD spectrum of sandstone sample (under 25 °C).
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The variation of total porosity versus temperature is shown in Figs. 5 and 6. Below 400 °C, the porosity changes very little. After that, it increases with temperature. At 600 °C, the porosity is nearly 1.23 times the initial value (under 25 °C). It is reported in literature that most of the mineral grains are micro-cracked at about 400 °C.18,19 Fig. 5 also shows a plot of porosity under the atmospheric conditions as a function (shown in Eq. (1)) of temperature. The increment of porosity with temperature below 400 °C is small, but becomes significant from that temperature
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40
2.0
Specific heat capacity (kJ/(kg·°c ))
30
2
-1
1.8
Average value Experimental value
-3
Thermal diffusivity (cm sec 10 )
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25 First stage 20
Second stage Third stage
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1.6 1.4 1.2 1.0 Average value Experimental value
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Temperature/ °c
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Variation level (based on average value ) Uncertainty level (absolute value)
Uncertainty level / %
Decreasing level (based on average value ) Uncertainty level (absolute value)
H e a t c a p a c i t y v a ri a t i o n l e v e l / %
-20
Uncertainty level / %
Thermal diffusivity decreasing level / %
-10
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-20
0 0
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Temperature/°c
(b)
(b)
Fig. 3. Variations of thermal diffusivity after different temperature treated (a) test results; (b) variation level of thermal diffusivity.
Fig. 4. Variations of specific heat capacity after different temperature treated (a) test results; (b) variation level of heat capacity.
onwards. The variation of porosity with temperature may be either caused by thermal expansion and altered micro-crack network, or driven by the structural damage of rocks.20 The cementing materials and initial porosity of sandstone play very important roles in this process.
n = 7.96677 − 0.00252T + 8.32664 × 10−6T2, R2 = 0.81
(1)
where n is the porosity, and T is the temperature (°C). The characteristics of porosity for sandstone have important influence for its thermal conductivity and diffusivity. As shown in Figs. 5 and 6, it is easy to see that, when the temperature is higher than 400 °C, the second significant decline range of thermal conductivity and thermal diffusivity occur, with the steeply increasing of porosity.
4. Discussion This variation in the thermal parameters of heated sandstone is caused by the variation of internal structure induced by heat. Sandstone is a sedimentary rock formed from sand-sized grains. The spaces between grains may be filled with cement of silica, carbonate, or clays. The principal mineral constituents of the grain framework are quartz, feldspar, and rock fragments.21 Since sandstone is composed of mineral particles with different thermal expansion coefficients and thermo-elastic characteristics, high temperature may lead to inhomogeneous thermal expansion of
Fig. 5. Variation of total porosity and thermal conductivity after different treated temperature.
mineral particles, thermal reaction or phase transition of some mineralogical components, generating internal stress and microcracks in sandstone. With the increase of temperature, internal defects would grow and change the thermal properties of sandstone. Accordingly,
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Fig. 6. Variation of total porosity and thermal diffusivity after different treated temperature.
variation of the measured porosity, thermal conductivity and thermal diffusivity with temperature may be divided into four phases (as shown in Figs. 2 and 3): (1) Room temperature – 200 °C. In this phase, the free and combined water would be lost and the mineral grains of sandstone be expanded (corresponding the first peaking value of internal friction temperature of Fig. 7), so porosity decreases slightly. While with the decrease of moisture, the thermal conductivity (as shown by Fig. 8) and thermal diffusivity are presenting rapid declining trend. (2) 200–400 °C. In this phase, crystal water and structural water of mineral escapes (corresponding to the second peaking value of internal friction temperature of Fig. 7), so porosity increases slightly. The content of crystal and structural water is relatively small in magnitude and, has only minor effects on heat transfer characteristics. (3) 400–600 °C. In this phase, the physical and chemical features of sandstone minerals would change. Many mineral constituents of sedimentary rocks undergo phase or related changes when heated to sufficiently high temperatures. The most remarkable reaction is the inversion of quartz from α-toβ phase at 573 °C. Although some of these reactions (such as the α–β quartz inversion) are reversible and the heat absorbed is returned to the system upon cooling, many of the major reactions are irreversible.13,14 Between 400 °C and 600 °C, especially 500 °C and 600 °C, the minerals (such as dolomite, organic matter, ankerite, siderite, magnetite, pyrrhotite, pyrite, illite and kaolinite) of rocks have chemical changes (decomposition, volatilization, oxidation).3,23 The presence of minerals such as kaolinite, illite, chlorite, and glauconite could be confirmed by small weight loss at temperature intervals belonging to the dehydroxylation of these minerals, at the roughly temperature interval of 300–600 °C.21,24 At approximately 573 °C and under atmospheric conditions, quartz has a phase transformation from α phase to β phase (corresponding the third peaking value of Fig. 7), which can be used to explain the large variation of thermal properties at the third phase (Fig. 9). From Fig. 10, it is easy to see that the thermal expansion of sandstone is closely related to quartz content. Somerton and Selim15 measured and reported thermal volume expansions and heats of reactions of three typical sandstones. They reported
Fig. 7. The variations of internal friction and thermal parameters of sandstone with temperature (data of internal friction from Xi22). (a) Internal friction and thermal conductivity (b) internal friction and thermal diffusivity and (c) internal friction and heat capacity.
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Fig. 8. The variations of thermal conductivity of sandstone with temperature.
permanent deformation or structural damage to the tested samples after heating, leading to a change in thermal conductivity of the heated sandstone samples. (4) Above 600 °C. In this phase, part of the minerals would melt,24,25 leading to enlarged defects. In the temperature range of 700–1000 °C, the second dissociation of dolomite takes place. The dissociation of carbonate and releasing of CO2 takes place at range of 750–950 °C.21 The removal of OH groups to illite, kaolinite and glauconite, breaking the crystal lattice and subsequent recrystallization occurs at temperature interval 900–950 °C.26,27 Under the influence of solid mineral inflation and fracture of metallic bonding (such as Al–O, K–O, Na–O, and Ca–O), the thermal conductivity and thermal conductivity diffusivity change slightly (as shown by Fig. 11), but heat capacity has a quick decrease. Kukkonen et al.28 and Clauser29 had studied the variations of thermal conductivity and thermal diffusivity with different treated temperature (shown as Fig. 12, rock types shown by Table 1). Results of these studies have shown the similar changing trend with our test conclusion. The main mechanism of changes in thermal properties of sandstone after thermal treatment is the loss of water and damage of structure caused by thermal reactions. Generally speaking, in the heating progress, there are a series of physical and chemical reactions. Adsorbed and interlayer water can evaporate (i.e. desorption) at the temperature in the period of 100–200 °C, and structural water loses when the treatment temperature is higher than 300 °C. Moreover, in the progress of losing structural water, some minerals are decomposed and evaporated (for example, the dehydroxylation of clay minerals), which may cause the increase of pore and fracture. Oxidation/decomposition reaction of some minerals is obviously evidenced at the range of 400–600 °C. With the increase of defects, the thermal conductivity and thermal diffusivity are reducing.
5. Conclusions The thermal parameters (thermal conductivity, thermal diffusivity and heat capacity) of sandstone after being heated at different temperatures are studied in this paper. Based on the result,
Fig. 9. Volume expansion of quartz and variations of thermal parameters with temperature (data of volume thermal expansion of quartz from Somerton and Selim (1961)). (a) Volume expansion of quartz and thermal conductivity. (b) Volume expansion of quartz and thermal diffusivity. (c) Volume expansion of quartz and heat capacity.
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Fig. 10. Volume thermal expansion of quartz and variations of porosity (data of volume thermal expansion of quartz from Somerton and Selim (1961)).
Fig. 11. The variations of thermal diffusivity of sandstone with temperature.
the following conclusions can be drawn. Thermal parameters of rock mainly depends its mineral composition, microstructure and porosity, which is closely related to the loss of water and damage of structural caused by thermal reactions (i.e the evaporation, material phase change and chemical reaction). After high temperature treated, the thermal conductivity and thermal diffusivity decrease quickly. The results indicate four phases in the variation of thermal parameters with temperature: from room temperature to 200 °C, 200–400 °C, 400–600 °C, and above 600 °C. The first and second phases correspond to the vaporization-escaping interval of adhered water, combined water and structural water. The release of water leads to the increases of connectivity of micro pore or cracks, and causes the decrease of thermal diffusivity and thermal conductivity. Between 400 and 600 °C, especially from 500 to 600 °C, the minerals in sandstone has physical and chemical reactions, which are demonstrated as volume increase, reduction of conductivity and diffusivity, and change of heat capacity. Finally, it is worth mentioning that these characteristics of variation of thermal properties were observed in laboratory tests. Resistivity and ultrasonic measurement will also be included in future work.
Fig. 12. The variations of thermal conductivity and diffusivity of rocks with temperature.28 (a) The variations of thermal conductivity of rocks with temperature. (b) The variations of thermal diffusivity of rocks with temperature.
Table 1 Sampling sites and rock types.28 Sample
Site
Rock type
JJ01 JJ02 JJ03 JJ04 JJ05 JJ06
Pielavesi Pielavesi Pielavesi Varpaisjärvi Varpaisjärvi Varpaisjärvi
Pyroxene diorite Mafic granulite Pyroxe norite Mafic granulite Enderbite Mafic granulite
Acknowledgments This research was supported by the Fundamental Research Funds for the Central Universities (No. 2015XKMS033) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
References 1. Emirov SN, Ramazanova EN. Thermal conductivity of sandstone at high
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pressures and temperatures. High Temp. 2007;45(3):317–320. 2. Emirov SN, Ramazanova AE, Zarichnyak YP. Contribution of thermal radiation in measurements of thermal conductivity of sandstone. Phys Solid State. 2013;55(12) 2463-2441. 3. Jana J, Agnes K. Thermally induced alterations of minerals during measurements of the the temperature dependence of magnetic susceptibility: a case study from the hydrothermally altered soultz-sous-Forêts granite, France. Int J Earth Sci. 2012;101:819–839. 4. Zhao HB, Yin GZ, Chen LJ. Experimental study on effect of temperature on sandstone damage. Chin J Rock Mech Eng. 2009;28(s1):2784–2788. 5. Zhang ZX, Yu J, Kou SQ, Lindqvist PA. Effects of high temperatures on dynamic rock fracture. Int J Rock Mech Min. 2001;38(2):211–225. 6. Alishaev MG, Abdulagatov IM, Abdulagatova ZZ. Effective thermal conductivity of fluid-saturated rocks: experiment and modeling. Eng Geol. 2012;135:24–39. 7. Ranjith PG, Daniel RV, Chen BJ, Pereea MSA. Transformation plasticity and the effect of temperature on the mechanical behavior of Hawkesbury sandstone at atmospheric pressure. Eng Geol. 2012;151:120–127. 8. Shafiei A, Dusseault MB. Geomechanics of thermal viscous oil production in sandstones. J Pet Sci Eng. 2013;103:121–139. 9. Wu Z, Qin BD, Chen LJ, Luo YJ. Temperamental study on mechanical character of sandstone of the upper plank of coal bed under high temperature. Chin J Rock Mech Eng. 2005;24(11):1863–1867. 10. Hajpál M. Changes in sandstones of historical monuments exposed to fire or high temperature. Fire Tech. 2002;38:373–382. 11. Wu G, Wang Y, Swift G, Chen J. Laboratory investigation of effects of temperature on the mechanical properties of sandstone. Geotech Geol Eng. 2013;31:809–816. 12. Duchkov AD, Sokolova LS, Rodyakin SV, Chernysh PS. Thermal conductivity of the sedimentary-cover rocks of the West Siberian Plate in relation to their humidity and porosity. Russ Geol Geophys. 2014;55:784–792. 13. Somerton WH, Boozer GD. Thermal characteristics of porous rocks at elevated temperatures. J Pet Tech. 1960;12(6):77–81. 14. Somerton WH, Boozer GD. Thermal characteristics of porous rocks at elevated temperatures. AIME Trans. 1960;219:418–422. 15. Somerton WH, Boozer GD. A method of measuring thermal diffusivities of rocks at elevated temperatures. AIChE J. 1961;7(1):87–90.
16. Vosteen HD, Schellschmidt R. Influence of temperature on thermal conductivity, thermal capacity and thermal diffusivity for different types of rock. Phys Chem Earth. 2003;55(12):499–509. 17. Hanley EJ, Dewitt DP, Roy RF. The thermal diffusivity of eight well-chracteried rocks for the temperature range 300–1000 K. Eng Geol. 1978;12:31–47. 18. Bauer SJ, Johnson B. Effects of slow uniform heating on the physical properties of the westerly and charcoal granites. In: Proceedings of the 20th U.S. Symposium on Rock Mechanic. Texas; 4–6 June, 1979: 7–18. 19. Tian H, Kempka T, Xu NX, Ziegler M. Physical properties of sandstones after temperature treatment. Rock Mech Rock Eng. 2012;45:1113–1117. 20. Hajpál M, Török Á. Mineralogical and colour changes of quartz sandstones by heat. Environ Geol. 2004;46:311–322. 21. Plevová E, Vaculíková L, Kožušníková A, Daněk T, Pleva M, Ritz M, Martynová GS. Thermal study of sandstones from different Czech localities. J Therm Anal Calor. 2011;103:835–843. 22. Xi DY. Physical characteristics of mineral phase transition in the granite. Acta Miner Sin. 1999;14(3):223–227. 23. Sun Q, Zhang WQ, Xue L, et al. Thermal damage pattern and thresholds of granite. Environ Earth Sci. 2015;74(3):2341–2349. 24. Xu XL. Research on the Mechanical Characteristic and Micromechanism of Granite Under Temperature Loads [Ph.D. thesis]. Xuzhou, China: Chin. Uni. Min. Tech; 2008. 25. Qin Y, Tian NX, Zhang Y, Han J. Experimental study on physical properties of post-high-temperature diorite. Chin J Geotech Eng. 2015;37(7):1226–1231. 26. Muller F, Drits V, Plancon A, Robert JL. Structural transformation of 2:1 dioctahedral layer silicates during dehydroxylation–rehydroxylation reactions. Clay Clay Miner. 2000;48:572–585. 27. Thompson GR, Hower J. The mineralogy of glauconite. Clay Clay Miner. 1975;23:289–300. 28. Kukkonen IT, Jokinen J, Seipold U. Temperature and pressure dependencies of thermal transport properties of rocks: implications for uncertainties in thermal lithosphere models and new laboratory measurements of high-grade rocks in the central fennoscandian shield. Surv Geophys. 1999;20:33–59. 29. C. Clauser, Thermal Storage and Transport Properties of Rocks, II: Thermal Conductivity and Diffusivity. Encyclopedia of Solid Earth Geophysics, Springer, Netherlands; 2011: 1431–1448.
Contents lists available at ScienceDirect
International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical note
Thermal properties of sandstone after treatment at high temperature Qiang Sun a, Chao Lü a,n, Liwen Cao a, Weichao Li b, Jishi Geng a, Weiqiang Zhang a a
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221116, PR China State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100048, PR China
b
art ic l e i nf o Article history: Received 10 October 2015 Received in revised form 24 February 2016 Accepted 9 March 2016 Keywords: Thermal damage Thermal conductivity Thermal diffusivity Heat capacity Phase inversion
a b s t r a c t This paper reports the variations of thermal conductivity, thermal diffusivity and heat capacity of sandstone after high-temperature heating. Experiments were carried out to test the thermal properties of sandstone which had been heated at temperatures ranging from room temperature to 900 °C in a furnace. Temperature has a significant impact on the thermal conductivity, thermal diffusivity and heat capacity of sandstone, which is closely related to the loss of water and damage of structure caused by thermal reactions. The results indicate four phases in the variation of thermal parameters with temperature: from room temperature to 200 °C, 200–400 °C, 400–600 °C, and above 600 °C. The first and second phases correspond to the vaporization-escaping interval of adhered water, combined water and structural water. Between 400–600 °C, especially from 500–600 °C, the minerals in sandstone has thermal reactions, which are demonstrated as porosity increase, reduction of conductivity and diffusivity, and change of heat capacity. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction The thermal properties of rocks at high temperature or after heating at high temperature produce valuable information for terrestrial sciences, development and solution of applied problems in geothermy, geothermal power generation, and energetics of geodynamic processes of sedimentation basins,1 geophysics,2 geological disasters and geological structure formation,3 such as deep geological repositories for heat-generating radioactive wastes,4 exploration of geothermal energy,5–7 deep petroleum boring,8 exploitation of natural gas6,9 utilization of hydrothermal energy and underground thermal energy storage, drilling and well logging,6 and the protection of buildings against fire or building restoration after exposure to fire.10,11 In the past few decades, considerable experimental effort has been taken to quantify the relation between thermal properties factor (thermal conductivity, and thermal diffusivity, heat capacity) and temperature of rock. Intense study of the thermal-conductivity factor of rocks began in the middle 20th century in relation to the development of geothermal research aimed mainly at the evaluation of the heat flow rate. The latter is calculated from the measured geothermal gradient in wells and from the thermal conductivity of rocks recovered by them.12 Somerton and Boozer13–15 measured thermal diffusivity and conductivity of some typical sedimentary rocks using an unsteady-state conductivity method for 90–800 °C, reporting that
thermal diffusivity and conductivity decreased drastically at elevated temperatures. The studies of Vosteen and Schellschmidt,16 Hanley et al.,17 Emirov and Ramazanova1 also show that the thermal conductivity and diffusivity of rocks in general decreases with temperature. The heat transfer mechanism in rocks is very complex. It is difficult and sometimes impossible to theoretically and correctly predict the thermal conductivity of porous materials, even providing having many simplifications.2 Rocks are composed by mineral particles with various chemical compositions and different degrees of crystallization. Therefore, the thermal properties of rocks depend not only on the pressure and temperature but also on their mineralogical composition, the structure and geometry of pores, grain sizes, shapes of cracks, and their concentration. Calculation of heat transfer in subsurface of formations requires data on the thermal-expansion characteristics of rock and the heats of reaction of mineral constituents. Therefore, the research on the thermal properties of sandstone is extremely meaningful on a wide range. In this paper, the variations of porosity and thermal parameters after high-temperature heating (thermal conductivity, thermal diffusivity and heat capacity) are analyzed. Thermal expansions and heats of reaction are discussed in the temperature range of 25–900 °C.
2. Experimental tests n
Corresponding author. E-mail address: [email protected] (C. Lü).
http://dx.doi.org/10.1016/j.ijrmms.2016.03.006 1365-1609/& 2016 Elsevier Ltd. All rights reserved.
Sandstone samples were obtained from Linyi, Shandong
Q. Sun et al. / International Journal of Rock Mechanics & Mining Sciences 85 (2016) 60–66
61
province. X-ray diffraction (XRD) analysis showed that the main components of samples are quartz, feldspar, dolomite/ankerite and hematite/magnetite (as shown in Fig. 1), accompanied by a small amount of biotite and kaolinite structure. The highest content of SiO2 is 35.49%, and the next is CO2 accounting for 22.64%. The content of CaO, Al2O3, MgO, Fe2O3 and K2O/Na2O are 13.43%, 10.55%, 8.10%, 5.53% and 2.96% respectively. Therefore, the cement of the sandstone is of ferruginous, dolomitic and/or clayey type. These samples with average bulk density of 2.41 g/cm3 at room temperature were cut into Φ50 ( 70.05) 30 (70.05) mm2 cylinders. The initial content of water was bout 0.07%. In addition to room temperature samples, all the test samples were treated on 100 °C in an oven lasting for an hour before the thermal test. Therefore, there was no adhered water in the tested samples. The heating apparatus consists of a high temperature furnace (type KSL-1100X-L). The specimens used in all the following tests were placed in the furnace and then heated to the designated temperature (25 °C, 50 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C and 900 °C, respectively) at the rate of 30 °C/ min. Each specimen was kept at its designated temperature for about 30 min before the power was cut off, and the specimen was allowed to cool down naturally with the decline of temperature in the furnace. The porosity and thermal parameters of these specimens were tested before and after heating. The porosity was measured by a microporosity structure analyzer apparatus (type 9310) produced by Micromeritics Instrument Corp. The thermal parameters were simultaneously collected with a TPS test machine produced by Hot Disk equipment Co., Ltd.
3. Analysis 3.1. Variation of thermal parameters The results of thermal conductivity, thermal diffusivity and specific heat capacity are given in Figs. 2–4, respectively. After high-temperature heating, the thermal conductivity, thermal diffusivity and specific heat capacity of sandstone specimens change significantly at the temperature range of 25–900 °C. From 25 °C to 200 °C, conductivity decreases rapidly with temperature. At the range of 200–400 °C, the thermal conductivity decreases a little bit. However, when the temperature is higher than 450 °C, there is another significant decline range. The greatest 1800
D
D i ffra c t i o n i n t e n s i t y (p u l s e c o u n t s / s )
1600
D: Dolomite/Ankerite Q: Quarter K: Kaolinite F: Feldspar B: Biotite H:Hematite/Magnetite
1400 1200
Q
1000 800
K K
200
D
Q KH
K
400
H H H
D
Q
H H
K
D H
Q
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20
25
rate of decrease in conductivity is at lower temperatures. From 25 °C to 200 °C, thermal diffusivity decreases rapidly with temperature. At the range of 200–400 °C, the thermal diffusivity decreases a little bit. However, when the temperature is higher than 400 °C, there is another significant decline range. The greatest rate of decrease in conductivity is at the lower temperatures. Above approximately 600 °C, thermal diffusivity becomes essentially constant. From 25 °C to 200 °C, heat capacity increases rapidly with temperature. At the range of 200–600 °C, there is a volatility for heat capacity, but the whole is relatively stable. However, when the temperature is higher than 600 °C, there is a significant decline range. 3.2. Relationship between porosity and thermal parameters
F
B
600
Fig. 2. Variations of thermal conductivity after different temperature treated (a) test results; (b) variation level of thermal conductivity.
30
35
40
45
50
55
Diffraction angle (°) Fig. 1. XRD spectrum of sandstone sample (under 25 °C).
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The variation of total porosity versus temperature is shown in Figs. 5 and 6. Below 400 °C, the porosity changes very little. After that, it increases with temperature. At 600 °C, the porosity is nearly 1.23 times the initial value (under 25 °C). It is reported in literature that most of the mineral grains are micro-cracked at about 400 °C.18,19 Fig. 5 also shows a plot of porosity under the atmospheric conditions as a function (shown in Eq. (1)) of temperature. The increment of porosity with temperature below 400 °C is small, but becomes significant from that temperature
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40
2.0
Specific heat capacity (kJ/(kg·°c ))
30
2
-1
1.8
Average value Experimental value
-3
Thermal diffusivity (cm sec 10 )
35
25 First stage 20
Second stage Third stage
15
Fourth stage
10
1.6 1.4 1.2 1.0 Average value Experimental value
0.8 5 0
100
200
300
400
500
600
700
800
900
0.6
Temperature/ °c
0
200
400
(a)
600
800
Temperature/°c
(a)
100
0
100
100
-30
80
60 -40 -50 40 -60 -70
80
20
-80 -90 0
100
200
300
400
500
600 Temperature/°c
700
800
0 900
80
60 60 40 40 20
Variation level (based on average value ) Uncertainty level (absolute value)
Uncertainty level / %
Decreasing level (based on average value ) Uncertainty level (absolute value)
H e a t c a p a c i t y v a ri a t i o n l e v e l / %
-20
Uncertainty level / %
Thermal diffusivity decreasing level / %
-10
20
0
-20
0 0
100
200
300
400
500
600
700
800
900
Temperature/°c
(b)
(b)
Fig. 3. Variations of thermal diffusivity after different temperature treated (a) test results; (b) variation level of thermal diffusivity.
Fig. 4. Variations of specific heat capacity after different temperature treated (a) test results; (b) variation level of heat capacity.
onwards. The variation of porosity with temperature may be either caused by thermal expansion and altered micro-crack network, or driven by the structural damage of rocks.20 The cementing materials and initial porosity of sandstone play very important roles in this process.
n = 7.96677 − 0.00252T + 8.32664 × 10−6T2, R2 = 0.81
(1)
where n is the porosity, and T is the temperature (°C). The characteristics of porosity for sandstone have important influence for its thermal conductivity and diffusivity. As shown in Figs. 5 and 6, it is easy to see that, when the temperature is higher than 400 °C, the second significant decline range of thermal conductivity and thermal diffusivity occur, with the steeply increasing of porosity.
4. Discussion This variation in the thermal parameters of heated sandstone is caused by the variation of internal structure induced by heat. Sandstone is a sedimentary rock formed from sand-sized grains. The spaces between grains may be filled with cement of silica, carbonate, or clays. The principal mineral constituents of the grain framework are quartz, feldspar, and rock fragments.21 Since sandstone is composed of mineral particles with different thermal expansion coefficients and thermo-elastic characteristics, high temperature may lead to inhomogeneous thermal expansion of
Fig. 5. Variation of total porosity and thermal conductivity after different treated temperature.
mineral particles, thermal reaction or phase transition of some mineralogical components, generating internal stress and microcracks in sandstone. With the increase of temperature, internal defects would grow and change the thermal properties of sandstone. Accordingly,
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63
Fig. 6. Variation of total porosity and thermal diffusivity after different treated temperature.
variation of the measured porosity, thermal conductivity and thermal diffusivity with temperature may be divided into four phases (as shown in Figs. 2 and 3): (1) Room temperature – 200 °C. In this phase, the free and combined water would be lost and the mineral grains of sandstone be expanded (corresponding the first peaking value of internal friction temperature of Fig. 7), so porosity decreases slightly. While with the decrease of moisture, the thermal conductivity (as shown by Fig. 8) and thermal diffusivity are presenting rapid declining trend. (2) 200–400 °C. In this phase, crystal water and structural water of mineral escapes (corresponding to the second peaking value of internal friction temperature of Fig. 7), so porosity increases slightly. The content of crystal and structural water is relatively small in magnitude and, has only minor effects on heat transfer characteristics. (3) 400–600 °C. In this phase, the physical and chemical features of sandstone minerals would change. Many mineral constituents of sedimentary rocks undergo phase or related changes when heated to sufficiently high temperatures. The most remarkable reaction is the inversion of quartz from α-toβ phase at 573 °C. Although some of these reactions (such as the α–β quartz inversion) are reversible and the heat absorbed is returned to the system upon cooling, many of the major reactions are irreversible.13,14 Between 400 °C and 600 °C, especially 500 °C and 600 °C, the minerals (such as dolomite, organic matter, ankerite, siderite, magnetite, pyrrhotite, pyrite, illite and kaolinite) of rocks have chemical changes (decomposition, volatilization, oxidation).3,23 The presence of minerals such as kaolinite, illite, chlorite, and glauconite could be confirmed by small weight loss at temperature intervals belonging to the dehydroxylation of these minerals, at the roughly temperature interval of 300–600 °C.21,24 At approximately 573 °C and under atmospheric conditions, quartz has a phase transformation from α phase to β phase (corresponding the third peaking value of Fig. 7), which can be used to explain the large variation of thermal properties at the third phase (Fig. 9). From Fig. 10, it is easy to see that the thermal expansion of sandstone is closely related to quartz content. Somerton and Selim15 measured and reported thermal volume expansions and heats of reactions of three typical sandstones. They reported
Fig. 7. The variations of internal friction and thermal parameters of sandstone with temperature (data of internal friction from Xi22). (a) Internal friction and thermal conductivity (b) internal friction and thermal diffusivity and (c) internal friction and heat capacity.
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Fig. 8. The variations of thermal conductivity of sandstone with temperature.
permanent deformation or structural damage to the tested samples after heating, leading to a change in thermal conductivity of the heated sandstone samples. (4) Above 600 °C. In this phase, part of the minerals would melt,24,25 leading to enlarged defects. In the temperature range of 700–1000 °C, the second dissociation of dolomite takes place. The dissociation of carbonate and releasing of CO2 takes place at range of 750–950 °C.21 The removal of OH groups to illite, kaolinite and glauconite, breaking the crystal lattice and subsequent recrystallization occurs at temperature interval 900–950 °C.26,27 Under the influence of solid mineral inflation and fracture of metallic bonding (such as Al–O, K–O, Na–O, and Ca–O), the thermal conductivity and thermal conductivity diffusivity change slightly (as shown by Fig. 11), but heat capacity has a quick decrease. Kukkonen et al.28 and Clauser29 had studied the variations of thermal conductivity and thermal diffusivity with different treated temperature (shown as Fig. 12, rock types shown by Table 1). Results of these studies have shown the similar changing trend with our test conclusion. The main mechanism of changes in thermal properties of sandstone after thermal treatment is the loss of water and damage of structure caused by thermal reactions. Generally speaking, in the heating progress, there are a series of physical and chemical reactions. Adsorbed and interlayer water can evaporate (i.e. desorption) at the temperature in the period of 100–200 °C, and structural water loses when the treatment temperature is higher than 300 °C. Moreover, in the progress of losing structural water, some minerals are decomposed and evaporated (for example, the dehydroxylation of clay minerals), which may cause the increase of pore and fracture. Oxidation/decomposition reaction of some minerals is obviously evidenced at the range of 400–600 °C. With the increase of defects, the thermal conductivity and thermal diffusivity are reducing.
5. Conclusions The thermal parameters (thermal conductivity, thermal diffusivity and heat capacity) of sandstone after being heated at different temperatures are studied in this paper. Based on the result,
Fig. 9. Volume expansion of quartz and variations of thermal parameters with temperature (data of volume thermal expansion of quartz from Somerton and Selim (1961)). (a) Volume expansion of quartz and thermal conductivity. (b) Volume expansion of quartz and thermal diffusivity. (c) Volume expansion of quartz and heat capacity.
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Fig. 10. Volume thermal expansion of quartz and variations of porosity (data of volume thermal expansion of quartz from Somerton and Selim (1961)).
Fig. 11. The variations of thermal diffusivity of sandstone with temperature.
the following conclusions can be drawn. Thermal parameters of rock mainly depends its mineral composition, microstructure and porosity, which is closely related to the loss of water and damage of structural caused by thermal reactions (i.e the evaporation, material phase change and chemical reaction). After high temperature treated, the thermal conductivity and thermal diffusivity decrease quickly. The results indicate four phases in the variation of thermal parameters with temperature: from room temperature to 200 °C, 200–400 °C, 400–600 °C, and above 600 °C. The first and second phases correspond to the vaporization-escaping interval of adhered water, combined water and structural water. The release of water leads to the increases of connectivity of micro pore or cracks, and causes the decrease of thermal diffusivity and thermal conductivity. Between 400 and 600 °C, especially from 500 to 600 °C, the minerals in sandstone has physical and chemical reactions, which are demonstrated as volume increase, reduction of conductivity and diffusivity, and change of heat capacity. Finally, it is worth mentioning that these characteristics of variation of thermal properties were observed in laboratory tests. Resistivity and ultrasonic measurement will also be included in future work.
Fig. 12. The variations of thermal conductivity and diffusivity of rocks with temperature.28 (a) The variations of thermal conductivity of rocks with temperature. (b) The variations of thermal diffusivity of rocks with temperature.
Table 1 Sampling sites and rock types.28 Sample
Site
Rock type
JJ01 JJ02 JJ03 JJ04 JJ05 JJ06
Pielavesi Pielavesi Pielavesi Varpaisjärvi Varpaisjärvi Varpaisjärvi
Pyroxene diorite Mafic granulite Pyroxe norite Mafic granulite Enderbite Mafic granulite
Acknowledgments This research was supported by the Fundamental Research Funds for the Central Universities (No. 2015XKMS033) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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