单轴应力下带钻孔花岗岩注入高温蒸汽破坏特征研究
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摘要
大型水力压裂是干热岩地热能开发中人工储留层建造的最有效手段,其核心力学问题为高温、高压下岩石的水力破岩机制。通过单轴应力下带钻孔花岗岩注入高温蒸汽破坏试验,研究固-热耦合作用下花岗岩的水力破岩机制。结果表明:高温对花岗岩破裂有很大的促进作用,热效应导致强度弱化,降低破裂压力。高速率注入430℃和350℃蒸汽破坏试验中,破裂压力比常温水压裂至少降低58%;低速率注入400℃和450℃蒸汽破坏试验中,花岗岩破裂压力比常温水压裂降低75%。注蒸汽破坏过程可分为热破裂损伤和宏观裂缝扩展两个阶段。高温蒸汽产生的热应力在钻孔周围随机发生热破裂,随着注入蒸汽时间的增加,热破裂范围由钻孔附近逐渐向远处扩展,热破裂分布密度增大,为宏观裂缝的产生提供便利条件。初始宏观裂缝首先出现在钻孔两侧,沿着最终形成的宏观裂缝轨迹扩展,直到试样破坏。与常温水压裂相比,低速率注蒸汽破坏是一个缓慢的延性拉破坏过程,裂缝相对钻孔不对称扩展,宽度小于水力压裂裂缝宽度。
Large-scale hydraulic fracturing is the most effective way to construct the artificial reservoir in the development of hot dry rock geothermal energy. The key issue is to reveal the hydraulic fracture mechanism of rock under high temperature and pressure. The high-temperature-vapour-driven failure experiments were carried out on granite under uniaxial stress, and the preliminary results on the failure mechanism were obtained under thermo-mechanical coupling. Results indicate that high temperature can greatly cause the failure of granite by weakening the strength and reducing the breakdown pressure. Compared with the breakdown pressure of hydraulic fracturing at room temperature, they were decreased by at least 58% at the high injection rates of 430℃ and 350℃ vapours, while they were reduced by 75% at the low injection rates of 400℃ and 450℃ vapours in the vapour driven failure experiments. The process of high-temperature-vapour-driven failure can be divided into two stages, namely, thermal fracture damage and macrocrack propagation. In the thermal cracking stage, the thermal stress resulted in thermal cracks randomly around the borehole. With the increase of vapour injection time, the thermal cracking range gradually extended, and the microcracking density enhanced, which facilitated the fracture propagation. In the second stage, the fractures were primarily generated on both sides of the borehole and then propagated along the final trace of macrocracks until the failure of the specimen. Compared with hydraulic fracturing at room temperature, the failure induced by vapour at a low injection rate was a slow ductile tensile failure. The fracture extended asymmetrically along the borehole, and the width was smaller than that of hydraulic fracturing at room temperature.
Large-scale hydraulic fracturing is the most effective way to construct the artificial reservoir in the development of hot dry rock geothermal energy. The key issue is to reveal the hydraulic fracture mechanism of rock under high temperature and pressure. The high-temperature-vapour-driven failure experiments were carried out on granite under uniaxial stress, and the preliminary results on the failure mechanism were obtained under thermo-mechanical coupling. Results indicate that high temperature can greatly cause the failure of granite by weakening the strength and reducing the breakdown pressure. Compared with the breakdown pressure of hydraulic fracturing at room temperature, they were decreased by at least 58% at the high injection rates of 430℃ and 350℃ vapours, while they were reduced by 75% at the low injection rates of 400℃ and 450℃ vapours in the vapour driven failure experiments. The process of high-temperature-vapour-driven failure can be divided into two stages, namely, thermal fracture damage and macrocrack propagation. In the thermal cracking stage, the thermal stress resulted in thermal cracks randomly around the borehole. With the increase of vapour injection time, the thermal cracking range gradually extended, and the microcracking density enhanced, which facilitated the fracture propagation. In the second stage, the fractures were primarily generated on both sides of the borehole and then propagated along the final trace of macrocracks until the failure of the specimen. Compared with hydraulic fracturing at room temperature, the failure induced by vapour at a low injection rate was a slow ductile tensile failure. The fracture extended asymmetrically along the borehole, and the width was smaller than that of hydraulic fracturing at room temperature.
引文
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[18]ZHAO Z H.Thermal influence on mechanical properties of granite:a micro cracking perspective[J].Rock Mechanics and Rock Engineering,2016,49:747-762.
[19]方新宇,许金余,刘石,等.高温后花岗岩的劈裂试验及热损伤特性研究[J].岩石力学与工程学报,2016,35(增刊1):2687-2694.FANG Xin-yu,XU Jin-yu,LIU Shi,et al.Research on splitting-tensile tests and thermal damage of granite under post-high temperature[J].Chinese Journal of Rock Mechanics and Engineering,2016,35(Suppl.1):2687-2694.
[20]李正伟,张延军,张驰,等.花岗岩单裂隙渗流传热特性试验[J].岩土力学,2018,39(9):3261-3269.LI Zheng-wei,ZHANG Yan-jun,ZHANG Chi,et al.Experiment on convection heat transfer characteristics in a single granite fracture[J].Rock and Soil Mechanics,2018,39(9):3261-3269.
[2]HOFMANN H,BABADAGLI T,ZIMMERMANN G.Hot water generation for oil sands processing from enhanced geothermal systems process simulation for different hydraulic fracturing scenarios[J].Applied Energy,2014,113:524-547.
[3]武晋文,赵阳升,万志军,等.中高温三轴应力下鲁灰花岗岩热破裂声发射特征的试验研究[J].岩土力学,2009,30(11):3331-3336.WU Jin-wen,ZHAO Yang-sheng,WAN Zhi-jun,et al.Experimental study of acoustic emission characteristics of granite thermal cracking under middle-height temperature and triaxial stress[J].Rock and Soil Mechanics,2009,30(11):3331-3336.
[4]武晋文,赵阳升,万志军,等.高温均匀压力花岗岩热破裂声发射特性实验研究[J].煤炭学报,2012,37(7):1111-1117.WU Jin-wen,ZHAO Yang-sheng,WAN Zhi-jun,et al.Experimental study of acoustic emission of granite due to thermal cracking under high temperature and isostatic stress[J].Journal of China Coal Society,2012,37(7):1111-1117.
[5]ZHAO Yang-sheng,FENG Zi-jun,ZHAO Yu,et al.Experimental investigation on thermal cracking,permeability under HTHP and application for geo thermal mining of HDR[J].Energy,2017(132):305-314.
[6]赵阳升,孟巧荣,康天合,等.显微CT试验技术与花岗岩热破裂特征的细观研究[J].岩石力学与工程学报,2008,27(1):28-34.ZHAO Yang-sheng,MENG Qiao-rong,KANG Tian-he,et al.Micro-CT experimental technology and meso-investigation on thermal fracturing characteristics of granite[J].Chinese Journal of Rock Mechanics and Engineering,2008,27(1):28-34.
[7]张渊,张贤,赵阳升.砂岩的热破裂过程[J].地球物理学报,2005,48(3):656-659.ZHANG Yuan,ZHANG Xian,ZHAO Yang-sheng.Process of sandstone thermal cracking[J].Chinese Journal of Geophysics,2005,48(3):656-659.
[8]万志军,赵阳升,董付科,等.高温及三轴应力下花岗岩体力学特性的实验研究[J].岩石力学与工程学报,2008,27(1):72-76.WAN Zhi-jun,ZHAO Yang-sheng,DONG Fu-ke,et al.Experimental study on mechanical characteristics of granite under high temperatures and triaxial stresses[J].Chinese Journal of Rock Mechanics and Engineering,2008,27(1):72-76.
[9]ZHAO Y S,FENG Z J,ZHAO Y,et al.THM(thermo-hydro-mechanical)coupled mathematical model of fractured media and numerical simulation of a 3Denhanced geothermal system at 573 K and buried depth6000-7000 M[J].Energy,2015(82):193-205.
[10]ZHANG Y J,GUO L L,LI Z W,et al.Electricity generation and heating potential from enhanced geothermal system in Songliao Basin,China:Different reservoir stimulation strategies for tight rock and naturally fractured formations[J].Energy,2015(93):1860-1885.
[11]ZENG Y C,SU Z,WU N Y.Numerical simulation of heat production potential from hot dry rock by water circulating through two horizontal wells at desert peak geothermal field[J].Energy,2013(56):92-107.
[12]郭亮亮.增强型地热系统水力压裂和储层损伤演化的试验及模拟研究[D].吉林:吉林大学,2016.GUO Liang-liang.Test and model research of hydraulic fracturing and reservoir damage evolution in enhanced geothermal system[D].Jilin:Jilin University,2016.
[13]TOMAC I,GUTIERREZ M.Coupled hydrothermo-mechanical of hydraulic fracturing in quasi-britter rocks using BPM-DEM[J].Journal of Rock Mechanics and Geotechnical Engineering,2017,9:92-104.
[14]ZHOU C B,WAN Z J,ZHANG Y,et al.Exepermental study on hydraulic fracturing of granite under thermal shock[J].Geothermics,2018,71:146-155.
[15]杨天鸿,谭国焕,唐春安,等.非均匀性对岩石水压致裂过程的响[J].岩土工程学报,2002,24(6):724-728.YANG Tian-hong,TAN Guo-huan,TANG Chun-an,et al.Influence of heterogeneity on hydraulic fracturing in rocks[J].Chinese Journal of Geotechnical Engineering,2002,24(6):724-728.
[16]郤保平,赵阳升.600℃内高温状态花岗岩遇水冷却后力学特性试验研究[J].岩石力学与工程学报,2010,29(5):892-898.XI BAO-ping,ZHAO Yang-sheng.Experimental research on mechanical properties of water-cooled granite under high temperatures within 600℃[J].Chinese Journal of Rock Mechanics and Engineering,2010,29(5):892-898.
[17]WANG X Q,SCHUBNEL A,FORTIN J,et al.Physical properties and brittle strength of thermally cracked granite under confinement[J].Journal of Geophysical Research:Solid Earth,2013(118):6099-6112.
[18]ZHAO Z H.Thermal influence on mechanical properties of granite:a micro cracking perspective[J].Rock Mechanics and Rock Engineering,2016,49:747-762.
[19]方新宇,许金余,刘石,等.高温后花岗岩的劈裂试验及热损伤特性研究[J].岩石力学与工程学报,2016,35(增刊1):2687-2694.FANG Xin-yu,XU Jin-yu,LIU Shi,et al.Research on splitting-tensile tests and thermal damage of granite under post-high temperature[J].Chinese Journal of Rock Mechanics and Engineering,2016,35(Suppl.1):2687-2694.
[20]李正伟,张延军,张驰,等.花岗岩单裂隙渗流传热特性试验[J].岩土力学,2018,39(9):3261-3269.LI Zheng-wei,ZHANG Yan-jun,ZHANG Chi,et al.Experiment on convection heat transfer characteristics in a single granite fracture[J].Rock and Soil Mechanics,2018,39(9):3261-3269.