Evolution mechanism of the physical properties and thermal conductivity of thermal shock granite under chemical immersion
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摘要: 為了研究化學浸泡作用下熱沖擊花崗巖物理特性與導熱性能演化特征，對25～600 ℃范圍內不同溫度熱沖擊作用后的花崗巖試件開展了長期的酸性和中性溶液浸泡試驗，結合超聲檢測、核磁共振測試、熱常數分析和掃描電鏡試驗，定量表征了熱化改性花崗巖試件物理參數隨熱沖擊溫度的演化規律，建立了各物理參數之間的內在關聯性，揭示了物理性質變化的微觀機制。研究結果表明：隨著熱沖擊溫度的升高，熱化改性試件的體積逐漸增大，質量和密度逐漸降低，縱波波速呈線性下降，孔隙率呈冪函數遞增，導熱系數和熱擴散系數分別呈指數下降和線性下降；相同熱沖擊溫度下，熱化改性試件的體積增長率、縱波波速和導熱系數由大到小依次為未浸泡>水浸泡>酸浸泡，質量降低率和孔隙率從高到低依次為酸浸泡>水浸泡>未浸泡；孔隙率增大和導熱性能劣化均伴有縱波波速的下降，可通過測量縱波波速對孔隙率和導熱性能進行估測；熱化改性試件的孔隙結構對150～450 ℃范圍內的溫度更為敏感，固體顆粒骨架對450 ℃以上溫度更為敏感，顆粒骨架的劣化又將進一步引起孔隙結構的演化；熱化改性作用引起的微觀孔隙結構發育和物相轉變是導致物理性質變化的本質原因，其中以高溫熱沖擊起主導作用，研究發現300 ℃可作為產生強烈熱沖擊的溫度閾值。Abstract: To exploit the geothermal energy from low penetration rocks at a depth of 3–10 kilometers below the ground surface, an artificial geothermal system usually must be built. Thermal and chemical stimulation can be used as auxiliary means of hydraulic fracturing for artificial geothermal reservoir reconstruction, which is conducive to reducing the risk of earthquakes. However, thermal shock and chemical corrosion can also cause changes in physical parameters such as density, porosity, longitudinal wave velocity, and the thermal conductivity of high-temperature rock mass, which brings great uncertainty to the service life of a geothermal system. To study the evolution of the physical properties and thermal conductivity of thermal shock granite under chemical modification, long-term acid and neutral solution immersion tests were performed on granite specimens subjected to thermal shock at temperatures ranging from 25 ℃ to 600 ℃. Using ultrasonic testing, nuclear magnetic resonance, thermal constant testing, and scanning electron microscopy, the evolution of the physical parameters of thermal?chemical modified specimens with thermal shock temperature was quantitatively characterized, the internal correlation among physical parameters was established, and the microscopic mechanism of the change in physical properties was revealed. The results show that with increasing thermal shock temperature, the volume of thermal–chemical? modified specimens increases gradually, the mass and density decrease gradually, the longitudinal wave velocity decreases linearly, the porosity increases by a power function, and the thermal conductivity and thermal diffusivity decrease exponentially and linearly, respectively. At the same thermal shock temperature, the volume growth fraction, longitudinal wave velocity, and thermal conductivity of the modified specimens are in the order of non-immersion > water immersion > acid immersion, while the mass loss fraction and porosity are in the order of acid immersion > water immersion > non-immersion. The increase in porosity and the deterioration of thermal conductivity are accompanied by a decrease in longitudinal wave velocity, so the porosity and thermal conductivity can be estimated by measuring the longitudinal wave velocity. The pore structure of the modified specimens is more sensitive to temperatures in the range of 150–450 ℃, while the solid particle skeleton is more sensitive to temperatures above 450 ℃, and the deterioration of the particle skeleton will further cause the transformation of the pore structure. The thermal-chemical modification results in the development of pore structure and phase transformation, which are the fundamental reasons for the changes in the physical properties of granite. High-temperature thermal shock plays a leading role in the process of thermal-chemical modification, while chemical corrosion plays an auxiliary role. At the selected test temperature levels, 300 ℃ can be considered the temperature threshold for severe thermal shock.
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圖 1 熱化改性花崗巖試件表面形貌變化特征.(a)未浸泡；(b)蒸餾水浸泡；(c)鹽酸溶液浸泡
Figure 1. Change characteristics of the surface morphology of thermal-chemical-modified granite: (a) non-immersion; (b) immersion in distilled water; (c) immersion in HCl solution
圖 2 熱化改性花崗巖試件體積變化率隨溫度變化規律
Figure 2. Variation law of the volume change fraction of thermal-chemical-modified granite with temperature
圖 3 熱化改性花崗巖試件質量變化率隨溫度變化規律
Figure 3. Variation law of the mass change fraction of thermal-chemical-modified granite with temperature
圖 4 熱化改性花崗巖試件密度變化率隨溫度變化規律
Figure 4. Variation law of the density change fraction of thermal-chemical-modified granite with temperature
圖 5 熱化改性花崗巖試件縱波波速隨溫度變化規律
Figure 5. Variation law of the longitudinal wave velocity of thermal-chemical-modified granite with temperature
圖 6 熱化改性花崗巖試件孔隙率隨溫度變化規律
Figure 6. Variation law of the porosity of thermal-chemical-modified granite with temperature
圖 7 熱化改性花崗巖試件熱物理參數隨溫度變化規律. (a) 導熱系數隨溫度變化; (b) 熱擴散系數隨溫度變化
Figure 7. Variation law of the thermal physical parameters of thermal-chemical-modified granite with temperature: (a) thermal conductivity; (b) thermal diffusivity
圖 8 熱化改性花崗巖試件縱波波速與密度變化率關系
Figure 8. Relationship between the longitudinal wave velocity and the density change fraction of thermal-chemical-modified granite
圖 9 熱化改性花崗巖試件縱波波速與孔隙率關系
Figure 9. Relationship between the longitudinal wave velocity and the porosity of thermal-chemical-modified granite
圖 10 熱化改性花崗巖試件熱物理參數與縱波波速的關系. (a) 導熱系數; (b) 熱擴散系數
Figure 10. Relationship between the thermal parameters and the longitudinal wave velocity of thermal-chemical-modified granite: (a) thermal conductivity; (b) thermal diffusivity
圖 11 熱化改性花崗巖試件的T2譜分布特征. (a)不同浸泡條件；(b)不同熱沖擊溫度
Figure 11. T2 spectral distribution characteristics of thermal-chemical-modified granite: (a) different soaking conditions; (b) different thermal shock temperatures
圖 12 熱化改性花崗巖試件掃描電鏡圖像. (a) N-25; (b) N-600; (c) TW-600; (d) TH-600
Figure 12. SEM images of thermal-chemical-modified granite: (a) N-25; (b) N-600; (c) TW-600; (d) TH-600
久色视频表 1 熱化改性花崗巖試件基本物理參數
Table 1. Basic physical parameters of thermal-chemical-modified granite
Specimen
numberHeat-treatment
temperature /
℃Soaking
conditionBefore modification After modification D0/mm H0/mm V0/cm3 m0/g ρ0/(g?cm?3) Da/mm Ha/mm Va/cm3 ma/g ρa/(g?cm?3) N-U0 25 Non-immersion 49.40 99.60 190.80 503.46 2.639 N-U1 150 49.40 100.15 191.86 503.35 2.624 49.47 100.27 192.63 502.68 2.610 N-U2 300 49.90 100.20 195.86 512.00 2.614 50.00 100.35 196.94 511.13 2.595 N-U3 450 49.55 100.05 192.83 504.44 2.616 49.69 100.27 194.35 503.43 2.590 N-U4 600 50.20 100.20 198.22 516.52 2.606 50.48 100.81 201.66 515.00 2.554 TW-U0 25 Distilled water 49.20 100.10 190.21 500.29 2.630 49.20 100.09 190.15 500.08 2.630 TW-U1 150 49.10 100.10 189.44 500.51 2.642 49.16 100.20 190.09 499.65 2.628 TW-U2 300 49.10 100.20 189.63 498.28 2.628 49.18 100.32 190.50 497.12 2.610 TW-U3 450 49.30 100.20 191.18 503.94 2.636 49.42 100.37 192.39 502.21 2.610 TW-U4 600 49.20 100.20 190.40 501.94 2.636 49.54 100.71 194.02 498.95 2.572 TH-U0 25 HCl solution 49.20 100.20 190.40 500.67 2.630 49.17 100.15 190.07 499.20 2.626 TH-U1 150 49.40 100.20 191.95 506.79 2.640 49.44 100.27 192.40 504.90 2.624 TH-U2 300 49.40 100.20 191.95 506.31 2.638 49.45 100.28 192.49 504.12 2.619 TH-U3 450 49.50 100.30 192.92 506.47 2.625 49.57 100.42 193.70 503.47 2.599 TH-U4 600 49.40 100.30 192.14 506.88 2.638 49.66 100.73 195.00 500.97 2.569 
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