Mechanical properties and damage evolution of granite under high temperature thermal shock
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摘要: 在干熱型地熱資源開發過程中，高溫巖石面臨遇水冷卻引起的熱沖擊損傷問題。為了研究高溫花崗巖在熱沖擊作用后的力學特性和損傷演化規律，開展了25～600 ℃范圍內不同溫度熱沖擊作用下花崗巖試件的單軸壓縮試驗，獲得了熱沖擊花崗巖試件的應力?應變關系；提出了一種考慮初始熱沖擊損傷與加載期間試件微元破裂損傷相結合的熱?力耦合損傷本構模型，并對統計損傷本構模型的相關參數進行了理論求解；考慮熱沖擊損傷引起的孔隙結構劣化效應，引入壓密系數對熱沖擊花崗巖的本構關系進行了修正；通過試驗應力?應變曲線對模型的有效性進行了對比和驗證，討論了溫度水平對熱沖擊花崗巖單軸壓縮損傷演化規律的影響。研究結果表明，隨著熱沖擊溫度的升高，花崗巖試件的初始熱損傷不斷增大，應力?應變曲線具有明顯的非線性壓密階段；引入壓密系數修正的統計損傷本構模型能夠更加準確地表征熱沖擊花崗巖在初始加載階段的非線性壓密特征；在熱沖擊溫度較低時，損傷變量演化曲線上升較為陡峭，隨著熱沖擊溫度的升高，曲線上升速率逐漸變緩并由非線性向線性轉變。Abstract: Hot dry rock (HDR) is an underground rock with high temperatures (usually above 180 °C), low porosity, and low permeability. The extraction of geothermal energy from HDR generally requires the stimulation of man-made reservoirs. In the enhanced geothermal system (EGS) project, high-pressure water is usually injected into the deep HDR reservoir from the injection well, and the artificial fracture network is stimulated via fracking. The ultimate goal is to enhance fluid flow and heat exchange between injection and production wells. During this period, thermal shock induced by the injected cold water, also known as thermal stimulation, leads to thermal fracture of the HDR, which contributes to the formation of fractures near the injection well. However, this process results in a series of rock damage problems to the high-temperature rock mass, such as borehole collapse and microseismicity. To analyze the mechanical properties and damage evolution of high-temperature granite after thermal shock, the uniaxial compression test of granite specimens at different temperatures in the range of 25 °C–600 ℃ was conducted, and the stress–strain relationship of the specimens was obtained. Based on the theory of damage mechanics, a thermal–mechanical coupled damage constitutive model considering the combination of the initial thermal shock damage and the microelement fracture damage during loading was proposed, and the relevant parameters of the statistical damage constitutive model were theoretically solved. Furthermore, given the effect of pore structure deterioration caused by thermal shock, the constitutive relationship of thermal shock granite was modified by introducing a compaction coefficient. The statistical damage constitutive model was also verified by the experimental results. The influence of temperature on the damage evolution of thermal shock granite under uniaxial compression was discussed. Results showed that with the increase in thermal shock temperature, the initial thermal damage of the granite specimen increases continuously, resulting in a nonlinear compaction stage in the stress–strain curve. The statistical damage constitutive model modified by the compaction coefficient can accurately characterize the nonlinear compaction characteristics of thermal shock granite specimens in the initial loading stage. When the thermal shock temperature is low, the damage variable evolution curve rises steeply. However, with the increase in the thermal shock temperature, the increase rate of the curve gradually slows down and changes from nonlinear to linear. The research results not only help elucidate the deterioration process of the mechanical properties of thermal shock granite but also provide important theoretical guidance for the construction of accurate numerical calculation models and engineering scheme demonstrations.
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Key words:
- geothermal /
- thermal shock /
- granite /
- damage evolution /
- constitutive model
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圖 1 不同溫度熱沖擊后花崗巖試件表面形貌變化
Figure 1. Surface morphology changes in granite specimens after thermal shock at different temperatures
圖 3 熱沖擊花崗巖試件單軸壓縮應力?應變曲線
Figure 3. Uniaxial compressive stress–strain curve of granite specimens after thermal shock
圖 4 熱沖擊花崗巖單軸應力?應變曲線與本構模型理論曲線對比. (a) 25 ℃; （b）150 ℃; （c）300 ℃; （d）450 ℃; （e）600 ℃
Figure 4. Comparison between the uniaxial stress–strain curve of granite and the theoretical curve of the statistical damage constitutive model: (a) 25 ℃; （b）150 ℃; （c）300 ℃; （d）450 ℃; （e）600 ℃
圖 5 單軸加載期間熱沖擊花崗巖試件損傷變量演化特征
Figure 5. Evolution characteristics of the damage variables of granite specimens after thermal shock during uniaxial loading
久色视频表 1 熱沖擊花崗巖試件單軸壓縮統計損傷本構模型參數
Table 1. Parameters of the statistical damage constitutive model of granite specimens after thermal shock under uniaxial compression
Specimen Temperature/℃ Peak strength/MPa Peak strain/% Elastic modulus/GPa Poisson’s ratio m S0 n N-U0 25 235.48 0.43 58.86 0.26 11.95 260.50 200 N-U1 150 230.47 0.41 60.79 0.27 13.20 249.91 100 N-U2 300 217.07 0.50 53.58 0.22 4.67 309.34 5 N-U3 450 184.44 0.67 44.72 0.30 2.06 351.84 0.3 N-U4 600 86.70 1.19 18.79 0.71 1.06 194.58 0.05 
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