Mechanical properties and fracture surface roughness of thermally damaged granite under dynamic splitting

In order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting,dynamic Brazilian splitting tests were conducted on granite samples after thermal treatment at 25,200,400,and 600℃.Results show that the dynamic peak splitting strength of thermally damaged granite samples increases with increasing strain rate,showing obvious strain‐rate sensitivity.With increasing temperature,thermally induced cracks in granite transform from intergranular cracks to intragranular cracks,and to a transgranular crack network.Thermally induced damages reduce the dynamic peak splitting strength and the maximum absorbed energy while increasing the peak radial strain.The fracture mode of the thermally damaged granite under dynamic loads is mode Ⅱ splitting failure.By using the axial roughness index Z2 a,the distribution ranges of the wedge‐shaped failure zones and the tensile failure zones in the fracture surfaces under dynamic Brazilian splitting can be effectively identified.The radial roughness index Z_(2)^(r)is sensitive to the strain rate and temperature.It shows a linear correlation with the peak splitting strength and the maximum absorbed energy and a linear negative correlation with the peak radial strain.Z_(2)^(r)can be used to quantitatively estimate the dynamic parameters based on the models proposed.

Granite thermal reservoirs in Lingshui area of Hainan Island and their significance to geothermal resources,China

Hainan Island located at the southernmost tip of the continental crust of the South China Plate,has high terrestrial heat-flow values,widely-distributed hot springs,and rich geothermal resources.Intensified researches on the origin and potentials of geothermal resources can promote Hainan Island's development into a clean energy island.To determine the geological conditions for the formation of geothermal resources in southern Hainan Island,we collected core samples of granites from the Baocheng batholith in southern Hainan Island and conducted systematic analysis in respect of petrology,geochronology,geochemistry,and petrophysical property.The results of this study are as follows.The Baocheng batholith in the southern Hainan Island has a crystallization age of 98.42±0.56 Ma,making it the product of magmatism in the early stage of the Late Cretaceous.It mainly consists of high-K calc-alkaline granites,which were intruded by intermediate-to-mafic veins.The Baocheng batholith has a high radioactive heat generation rate of 2.712-6.843μW/m^(3),with an average of 3.846μW/m^(3),a radioactive heat-flow value of 30.768 μW/m^(2)and a heat-flow contribution rate of 38.95%-43.95%.As shown by the results of their thermophysical property analysis,the granites have high thermal conductivity and can serve as highquality geothermal reservoirs.In combination with previous geological and geophysical data,the geothermal model of the Lingshui area was established in this study.The deep structure indicates the presence of high-conductivity and low-resistivity layers in the basement of the Baocheng batholith.It can be inferred thereby that asthenospheric upwelling may occur and that there exist two magma vents at depth in the batholith.Therefore,magmatic heat at depth and granites with high radioactive heat generation rate serve as the main heat sources in the Lingshui area.

Physico-mechanical properties of granite after cyclic thermal shock

Understanding rock mechanical behaviors after thermal shock is critically important for practical engineering application.In this context,physico-mechanical properties of Beishan granite,Gansu Province,China after cyclic thermal shock were studied using digital image correlation(DIC),acoustic emission(AE)monitoring,and microscopic observation.The results show that the peak strength and elastic modulus decreased gradually with increase in thermal shock cycle.However,the above two parameters showed no further changes after 10 thermal shock cycles.The loading stress ratio(i.e.the ratio of the current loading stress level to the peak stress in this state)corresponding to the occurrence of the uneven principal strain field and the local strain concentration zone on the surface of the granite specimen decreased with increase in thermal shock cycle.Three transformation forms of the standard deviation curves of the surface principal strain were found.For granite with fewer thermal shock cycles(e.g.no more than 2 cycles),the standard deviation curves exhibited approximately exponential growth in exponential form.With increase in thermal shock cycle,the S-shaped curve was dominant.After 10 thermal shock cycles,an approximate ladder-shaped curve was observed.It is displayed that AE activity was mainly concentrated around the peak strength zone of the granite specimen when the rock samples underwent fewer thermal shock cycles.With increase in thermal shock cycle,AE activity could occur at low loading stress levels.Microscopic observation further confirmed these scenarios,which showed that more microcracks were induced with increase in thermal shock cycle.The number of induced microcracks at the edge location of the granite specimen was significantly larger than that at the interior location.Finally,a continuum damage model was proposed to describe the damage evolution of the granite specimen after cyclic thermal shock during loading.

Numerical modeling of thermal breakthrough induced by geothermal production in fractured granite

It is well known that the complicated channeling of fluid flow and heat transfer is strongly related with the intricate natural fracture system.However,it is still challenging to set up the fracture network model which is strong heterogeneous.Compared with other methods(e.g.equivalent continuum model(ECM),discrete fracture model(DFM),and ECM-DFM),the fracture flow module in the COMSOL Multiphysics simulator is powerful in definition of fractures as the inner flow boundary existing in the porous media.Thus it is selected to simulate the fluid flow and heat transfer in the geothermal-developed fractured granite of Sanguliu area located at Liaodong Peninsula,Eastern China.The natural faults/fractures based on field investigation combined with the discrete fracture network(DFN)generated by the MATLAB are used to represent the two-dimensional geological model.Numerical results show that early thermal breakthrough occurs at the production well caused by quick flow of cold water along the highly connected fractures.Suitable hydraulic fracturing treatments with proper injection rates,locations,etc.can efficiently hinder the thermal breakthrough time in the natural fracture system.Large well spacing helps the long-term operation of geothermal production,but it is highly dependent on the geometrical morphology of the fracture network.The enhancement of reservoir properties at the near-well regions can also increase the geothermal production efficiency.The results in this study can provide references to achieve a sustainable geothermal exploitation in fractured granitic geothermal reservoirs or hot dry rocks at depth.

Influence of cooling speed on the physical and mechanical properties of granite in geothermal-related engineering

In deep-earth engineering,the high earth temperature can significantly affect the rock's mechanical properties,especially when the rock is cooled during the construction process.Accordingly,whether the cooling speed affects the mechanical and physical properties of rocks is worth to be investigated.The present study explored the influence of the cooling rate on the physical and chemical properties of granite heated at 25–800°C.The mechanical and physical properties involved in this study included uniaxial compression strength,peak strain,modulus,P-wave velocity,mass and volume,the change of which could reflect the sensitivity of granite to the cooling rate.Acoustic emission(AE)monitoring,microscopic observation,and X-ray diffraction(XRD)are used to analyze the underlying damage mechanism.It is found that more AE signals and large-scale cracks are accounted for based on the b-value method when the specimens are cooled by water.Furthermore,the microscopic observation by polarized light microscopy indicates that the density,opening degree,and connectivity of the cracks under water cooling mode are higher than that under natural cooling mode.In addition,the XRD illustrates that there is no obvious change in mineral content and diffraction angle at different temperatures,which confirms that the change of mechanical properties is not related to the chemical properties.The present conclusion can provide a perspective to assess the damage caused by different cooling methods to hot rocks.

Macro and micro mechanics behavior of granite after heat treatment by cluster model in particle flow code

In this paper, a cluster model in particle flow code was used to simulate granite specimens after heat treatment under uniaxial compression. The results demonstrated that micro-cracks are randomly distributed in the specimen when the temperature is below 300?C, and have partial coalescence when the temperature is up to 450?C, then form macro-cracks when the temperature is above 600?C. There is more inter-granular cracking than intra-granular cracking, and their ratio increases with increasing temperature.The micro-cracks are almost constant when the temperature decreases from 900?C to room temperature, except for quartz α–β phase transition temperature(573?C). The fracture evolution process is obviously affected by these cracks, especially at 600–900?C. Elevated temperature leads to easily developed displacement between the grains, and the capacity to store strain energy becomes weaker, corresponding to the plasticity of granite after heat treatment.

Changes in the thermodynamic properties of alkaline granite after cyclic quenching following high temperature action

During the development of hot dry rock,the research on thermal fatigue damage caused by thermal shock of cold and heat cycles is the basis that ensures the long-term utilization of geothermal resources,but there are not enough relevant studies at present.Based on this,the thermal damage tests of granite at different temperatures(250,350,450°C)and quenching cycles(1,5,10,15 cycles)were carried out.Preliminary reveals the damage mechanism and heat transfer law of the quenching cycle effect on hot dry rock.The results show that with the increase of temperature and cycles,the uneven thermal expansion of minerals and the thermal shock caused by quenching promote the crack development of granite,resulting in the decrease of P-wave velocity,thermal conductivity and uniaxial compressive strength of granite.Meanwhile,the COMSOL was used to simulate the heat transfer of hot dry rock under different heat treatment conditions.It concluded that the increase in the number of quenching cycles reduced the heat transfer capacity of the granite,especially more than 10 quenching cycles,which also reflects that the thermal fatigue damage leads to a longer time for the temperature recovery of the hot dry rock mass.In addition,the three-dimensional nonlinear fitting relationship among thermal conductivity,temperature and cycle number was established for the first time,which can better reveal the change rule of thermal conductivity after quenching thermal fatigue effect of hot dry rock.The research results provide theoretical support for hot dry rock reservoir reconstruction and production efficiency evaluation.

温度循环荷载下花岗岩微裂隙演化试验

花岗岩力学性能随环境温度的变化是岩石力学领域的热点问题,其微细观结构随温度的演化规律尚不清晰。基于现代化测试技术和实验设备设计了等幅值温度循环和峰值递增温度循环荷载,关注其作用下花岗岩微裂隙的衍生和扩展规律,并讨论了风化程度的影响。研究结果表明峰值温度比循环次数对裂隙网络发展的影响更大,峰值递增的温度循环可促进表面裂隙的衍生,而风化在一定程度上可减弱温度荷载的影响。本文研究可促进对温度作用下岩石力学性能演化的认知。

花岗岩侵位后的热应力时空演化及其影响因素

花岗岩与岩浆热液型矿床、油气成藏等有密切的成因关系。高温岩浆侵位到较冷的围岩中会形成岩浆热场和热应力,但热应力的大小和其影响范围尚缺乏系统研究。随着岩浆热耗散、与周围地层达到热平衡后,热应力会逐渐消失,因而数值模拟是定量研究岩浆热应力的常见方法之一。以往模拟岩浆热应力时往往采用岩石在常温下的线性热膨胀系数,但这与高温下岩石线性热膨胀系数存在较大差距。文章利用FLAC3D软件模拟花岗质岩浆侵位至上地壳范围内引起的热应力。求解物理方程包括热传导方程与线性热弹性本构方程,其中热场可通过温度差和线性热膨胀系数改变应力场,但应力场的变化不影响热场(即热场与应力场的单向耦合)。通过一系列数值模拟实验考察围岩岩性(花岗岩或碳酸盐岩)、杨氏模量、热学参数和岩浆侵位深度如何影响岩浆在上覆围岩产生的热应力。数值实验结果表明:岩石热传导系数通过传热快慢影响热应力的变化;围岩的杨氏模量越大,热应力也越大;由于花岗岩的平均杨氏模量大于碳酸盐岩,所以围岩为花岗岩时产生的热应力要高于碳酸盐岩;围岩无论是花岗岩还是碳酸盐岩,其在高温条件下的线性热膨胀系数比常温时高约1个数量级,产生的热应力最高可达100 MPa。花岗岩浆侵位后,围岩温度逐渐升高,对应的热应力不断增大;随着与岩浆房距离的增大,热应力不断减小,影响范围为岩浆房上方2 km以内;侵位深度浅的岩浆房冷却较快,其产生的热应力更有利于上覆围岩裂隙的形成和扩展。综合数值模拟结果可知,岩浆侵位所产生的热应力可影响岩体2 km内的应力场,这一局部存在且短瞬的热应力促使围岩破裂,为热液流体成矿提供运移通道或容矿空间。

珠江三角洲与周边典型地区岩石热物性及其地热效应的比较研究

珠江三角洲位于华夏板块南部,虽然已发现多个水热型地热田,但地热研究仍然较为薄弱.岩石热物性研究是地热研究中最为基础和重要的工作之一.本研究采集并测量了珠江三角洲与周边典型地区100块地表岩石的密度、热导率、热扩散率,以及50块岩浆岩的U、Th、K_(2)O含量,并分别计算了体积热容和放射性生热率.实测结果表明,碎屑岩中细砂岩的热导率最高(3.94±0.92 W·(m·K)^(-1)),页岩的热导率最低(2.68±0.55 W·(m·K)^(-1)).岩石热导率和热扩散率具有明显的正相关性.结合地层厚度建立了地层热物性柱,其中古近系的热导率最低,地层热物性与沉积环境有关.岩浆岩中玄武岩的热导率最低(2.17±0.13 W·(m·K)^(-1)),花岗岩平均热导率为3.87±0.59 W·(m·K)^(-1).花岗岩放射性生热率在0.36~14.23μW·m^(-3)之间,平均值为5.23±3.03μW·m^(-3),属于高产热花岗岩.结合前人对研究区岩石放射性元素含量的测试结果,我们发现,相对于K_(2)O,U、Th与生热率具有更强的相关性,并对产热的贡献更高.相对于花岗岩,玄武岩的生热率较低,K_(2)O对产热的贡献较高.新的资料为区域热流的确定和地热资源评价提供了新的视角.模拟和计算结果表明,三水盆地沉积层能对深部热量起到聚集和保存的作用,并在基底附近形成高温区(>150℃);上地壳中花岗岩体产生的热流超过45 mW·m^(-2),占地表热流的一半以上.盆地和花岗岩体均具有形成高温地热资源的地质条件.此外,酸性岩浆岩放射性生热率随时间逐渐降低的特征与幔源基性岩浆的混合作用有关.

不同冷却模式下高温花岗岩力学特性演变规律试验研究

【目的】探索相同高温状态下花岗岩在不同冷却模式(20℃水中热冲击冷却和20℃空气中自然冷却)下不同的力学特性。【方法】采用实验室自研热冲击破裂实验平台,对两种冷却模式下不同温度状态(25~600℃)花岗岩进行单轴抗压、巴西劈裂、变角剪切试验,测试其单轴抗压强度、抗拉强度、内聚力、内摩擦角宏观力学参数,研究不同冷却模式下高温花岗岩力学特性的演变规律。【结果】结果表明:1)冷却模式对单轴抗压强度、抗拉强度、内聚力的影响较大,对内摩擦角几乎无影响;2)相同高温状态下花岗岩力学强度在20℃水中热冲击冷却劣化更严重,且300℃为两种冷却模式出现差异的阈值温度;3)当加热温度为600℃时,热冲击冷却后花岗岩的单轴抗压强度、抗拉强度、抗剪强度(内聚力)与自然冷却模式下相比分别降低了27.6%、51.7%、33%,这说明冷却模式对花岗岩力学强度影响从大到小依次为抗拉强度、抗剪强度、抗压强度。【结论】研究结果可为相同工况下高温岩石力学工程岩体稳定性分析提供了一定的参考。

高温作用后的北山花岗岩时效变形特征研究

以甘肃高放废物重点预选场址的北山花岗岩为研究对象,开展不同热损伤和围压条件下的三轴分级蠕变试验,分析热力耦合作用下的北山花岗岩蠕变变形机理。试验结果表明,在单轴条件下,北山花岗岩的微裂纹密度随热处理温度升高而逐渐增加,进而导致轴向峰值应变持续增大。150℃和300℃温度荷载造成的轻微热开裂引发了裂纹尖端钝化,从而增强了热损伤花岗岩的长期强度;当热处理温度达到600℃后,石英相变产生的差异性热膨胀导致北山花岗岩的热裂纹密度显著增加,进而造成长期强度急剧降低。相比之下,在5 MPa和25 MPa围压的三轴蠕变试验中,静水压力能够愈合较低温度荷载造成的热开裂,使150℃和300℃热损伤花岗岩的轴向峰值变形和长期强度恢复至无损岩石水平。然而,上述静水压力未能完全愈合600℃热处理温度造成的热开裂,热损伤花岗岩的残余微裂纹导致轴向峰值应变增大,并造成了裂纹尖端的钝化效应,最终导致北山花岗岩的长期强度增大。

热-水-力耦合条件下花岗岩裂隙开度的变化

地质选址考察和地基分析对地下空间工程建设十分重要。以浙江舟山地区花岗岩为样本,在地质勘测的基础上对热-水-力耦合条件下岩体裂隙的开度进行了数值模拟分析与计算。结果表明:在热-水耦合条件下,花岗岩裂隙的开度随着时间的增加先迅速增大,然后趋于稳定,距裂隙入口20 mm处热应变增大明显,但热应变范围在10-5量级;在热-水-力耦合条件下,5 MPa围压下岩体裂隙变形程度较3 MPa围压下更大,裂隙的开度受耦合作用影响显著,裂隙的开度随着围压的增大而不断减小,导致花岗岩裂隙内部渗流能力降低。该研究结果可为花岗岩岩体裂隙稳定性研究提供参考。

南岭高分异花岗岩成岩与成矿

南岭是我国花岗岩研究程度最高的地区。特别是由于这一地区花岗岩与钨、锡、铌、钽、锆、铪、铜、钼、铅、锌、银、铋、锑、铀、锂、铍、稀土等金属成矿作用关系密切而受到国内外学术界关注。一般认为,南岭花岗岩及相关的成矿作用与花岗岩浆的高度结晶分异作用有关,但高分异作用发生的原因却并不明确。本文通过详细梳理南岭中生代燕山早期花岗岩的特征提出,这些花岗岩的结晶分异作用表现为岩浆房内晶粥体和残留岩浆的长期不断分凝。区内大面积的粗粒似斑状花岗岩为岩浆房早期结晶的堆晶体(主体),而晚期细粒花岗岩则为残留的高硅熔体(补体)。岩浆房发生充分结晶分异作用受控于两个因素:其一是岩浆房自身在演化过程中不断受到来自深部热的补给,使岩浆房内富含金属元素的残留熔体不断发生抽离,并向上运移。花岗岩体顶端的伟晶岩壳是保证抽离的熔体在近封闭环境下不断发生分异的另一个重要因素。这些特征使得南岭花岗岩的分异机制明显有别于喜马拉雅淡色花岗岩,后者以沿大型拆离断层就位并发生分异结晶为主要机制,两者可分别归类为热驱动分异和构造驱动分异类型,构成高分异花岗岩发生的两大重要机制。未来应结合锂资源的国家重大战略需求,对南岭地区高分异花岗岩,特别是晚期钨锡铌钽成矿花岗岩展开全面检查与评价,着重研究铁锂云母花岗岩及云英岩的岩石序列、矿物演变、金属元素富集和岩浆储存机制等,使南岭花岗岩与成矿作用研究再上新台阶。

局部高温诱导致裂非均质花岗岩机理研究

深部难钻地层钻速慢、钻井成本高的问题是现阶段钻井提速遇到的瓶颈问题,传统机械破岩难以大幅提速,亟需新的高效破岩方法。该文通过室内试验分析了局部高温作用下花岗岩的破碎模式,基于颗粒流离散元PFC2D建立了非均质花岗岩的等效岩体模型及局部高温诱导致裂花岗岩的热-力耦合模型,考虑了地层压力、液柱压力对于局部高温诱导花岗岩致裂的影响,再现了非均质花岗岩的微、宏观破裂过程,总结了温度、压力与岩石微观损伤的关系。实验结果表明:合理调节温度,在不产生熔融花岗岩的前提下,高温破碎花岗岩效果极为显著,破碎的岩屑呈片状,利于携岩。局部高温作用下花岗岩主要产生两个类型裂纹:晶内剪切裂纹和晶间拉伸裂纹;液柱压力对高温诱导裂纹的抑制作用特别明显,侧向压力能促进晶内剪切裂纹扩展;局部高温使花岗岩产生明显损伤的情况仅发生在浅地层,对深部地层作用甚微,建议结合PDC钻头切削或冲击;对花岗岩不同位置进行加热时,晶内剪切裂纹只存在于加热中心位置,而晶间拉伸裂纹扩展的范围更广,裂纹密度和分布与晶粒属性有关。研究结果有助于对高温热致裂岩石机理的深入了解以及为深部钻井提速提供新的思路。

热-水-力作用下圆孔花岗岩的动态损伤特征及结构模型

基于深层地热能开采时储能区井筒围岩所处的工程环境,采用高温加热、不同温度水浸泡、加热-循环次数及径向冲击加载的方法模拟井筒围岩经历的高温、遇水、循环采热及热冲击等造成的动力扰动等物理力学条件。同时,以不同内孔直径的同心圆孔岩样模拟深层地热井,采用分离式霍普金森压杆试验系统开展热-水-力作用下圆孔花岗岩的动态力学试验,并结合VIC-3D非接触应变测量及数值模拟分析技术监测冲击过程中圆孔岩样裂隙萌发、形成的历程和表面应变演化的规律,揭示热-水-力作用下圆孔岩样的动态损伤破坏机制。研究结果表明:径向冲击荷载作用下圆孔花岗岩先后经历弹性变形、塑性变形、结构失稳破坏3个典型阶段;内孔直径、加热温度、浸水温度、加热-浸水循环次数4因素都弱化了圆孔花岗岩抗外界荷载的能力,但未改变其整体的变形演化规律;圆孔花岗岩的破坏模式是动态拉伸破坏,先沿冲击方向由内孔壁向岩样外壁,再垂直冲击方向由岩样外壁向内孔壁萌发、贯通裂纹,形成近垂直的两组破裂面。最后,基于圆孔花岗岩的损伤变形特征及历程,在一定假设基础上,建立动态损伤结构模型,推演了结构方程,并结合试验结果确定了方程参数,通过对比分析发现,理论拟合曲线与试验曲线具有较好的一致性,验证构建的圆孔花岗岩动态损伤结构模型是合理的。研究成果不仅揭示了圆孔结构岩样的损伤破坏机制,也可为预测深部储能区地热井筒围岩损伤变形规律提供理论参考,具有一定的科学和工程实践意义。

基于声发射信息的热损伤花岗岩单轴压缩破裂机制及破裂前兆

为研究高温造成的热损伤对花岗岩在不同应力阶段声发射特征及破裂机制的影响,本文对25、200、400和600℃热损伤花岗岩开展了单轴压缩试验并进行了实时声发射监测,分析了不同应力阶段热损伤花岗岩峰值频率、上升时间/振幅-平均频率(RA-AF)数据分布特征以及能量集中度ρ的分布规律.结果表明:各温度热处理后花岗岩单轴压缩的应力阶段可以依据声发射发育特征分为:Ⅰ裂纹压密阶段、Ⅱ裂纹萌生及稳定发育阶段、Ⅲ裂纹非稳定发育阶段、Ⅳ峰后破坏阶段.花岗岩的热损伤越严重,声发射峰值频率越早产生中、高频破裂信号,且主频带分布范围越宽,破坏时超高频信号越少.各温度热损伤花岗岩声发射RA-AF数据分布特征可以表征各应力阶段产生的裂纹类型,热损伤花岗岩在压力作用下由压密至破坏过程中声发射RA-AF数据分布特征的变化说明剪切裂纹活动逐渐活跃,且热损伤温度越高,剪切裂纹越发育.能量集中度曲线的稳定发育阶段与突降阶段之间的突变点可以作为花岗岩单轴压缩条件下的破坏前兆.

热冲击对花岗岩动态断裂行为的影响研究

为了研究热冲击对加热花岗岩的影响,使用3种冷却方法来提供不同的冷却速率,对花岗岩试样施加不同程度的热冲击作用。通过分离式霍普金森压杆(SHPB)系统对热冲击处理后的花岗岩半圆盘中心直裂纹三点弯曲(NSCB)试样进行冲击断裂试验,并利用高速摄像机记录了试样的断裂模式。试验结果表明:随着试样温度和冷却速率的提升,试样的干密度和纵波波速显著下降,试样的孔隙率加大。试样的断裂韧度变化及破坏模式表明热冲击作用使花岗岩试样受到冲击力时抗裂纹扩展的能力下降,尤其在冲击载荷的加载率高于130 GPa·m^(0.5)/s时下降尤为明显。

江西赣县大地热流特征与热源机制研究

江西省赣县区位于江西省南部、属于我国东南高热流区,但赣县区内大地热流研究尚属空白,制约了区域地热背景的综合认知和进一步研究评价。本次研究基于钻孔测温及岩心样品热导率测试数据,通过分析校正得到赣县地区平均大地热流值为75.9 mW/m^(2),高于我国大陆地区平均热流值,反映了区域具有较高的热背景。通过放射性测试,得到研究区花岗岩体放射性生热率平均为5.68μW/m^(3),属于高放射性岩体,大面积分布的高产热花岗岩,可为区域地热背景提供稳定的附加热源,此外贯穿本区的深大断裂及其次生断裂给热水提供了深循环通道加热,因此推测研究区地热资源的热源机制为“地下水深循环加热+高产热花岗岩体生热”的模式。

热冲击花岗岩力学响应及损伤特征显微CT试验研究

在深部地热资源开发过程中,通常利用低温流体的冲击作用诱导高温岩石热破裂来提高储层的渗透特性。为了揭示热冲击作用下岩石的损伤破裂机理,对高温加热后的花岗岩(20℃、150℃、300℃、450℃、600℃和750℃)进行了自然冷却和水冷却处理,并对处理后的花岗岩开展了波速测试、单轴压缩试验和CT扫描试验,探讨了热冲击作用对花岗岩纵波波速、抗压强度、弹性模量等力学参数以及细观结构损伤的影响。研究结果表明:①随着热处理温度升高,花岗岩的纵波波速、抗压强度与弹性模量逐渐减小,峰值应变逐渐增加,且相比于自然冷却,水冷却作用后岩石的波速与力学性质劣化更显著。②通过CT扫描试验,获得了不同加热温度与热处理方式作用下花岗岩的孔裂隙结构空间分布特征,可以直观反映岩石细观结构的热损伤程度。当热处理温度小于等于450℃时,花岗岩扫描切片中的热致裂纹数量较少,裂隙连通性较差;超过450℃后,花岗岩内部微裂纹快速发育和扩展,并逐渐有形成裂隙网络的趋势,且水冷却对花岗岩的细观损伤致裂效果更明显。③基于三角网格离散技术,结合椭球模型重构算法和裂隙张量计算理论,对热冲击后花岗岩的三维裂隙场进行定量表征,并建立了裂隙组构张量与峰值强度的关系,进一步揭示了热冲击花岗岩细观结构对其力学性质的影响机理。