花岗岩干热岩体裂隙的大位移动态剪切-渗流特征
Characteristics of large-displacement based dynamic shearing-flow through the fractures in the granitic hot dry rock mass
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摘要: 水力剪切是干热岩地热、页岩气等深部能源开采的又一重要技术手段,水力剪切在提高储层渗透性的同时还会诱发地震。水力剪切增透和注入诱发地震的本质均是节理裂隙/断层的剪切-渗流。为深入研究不同诱因下,节理裂隙/断层岩体的剪切-渗流特性演化特征,利用自主研发的高围压大位移实时剪切渗流装置,研究了花岗岩干热岩体裂隙在剪切蠕变、剪应力增加、水压力增加和位移速率增加4种诱因引起的滑动过程中的渗流特征,并利用现有的速率-状态方程分析了诱发地震过程中裂隙/断层的摩擦特性。结果表明:(1)在三轴静水压力下,花岗岩裂隙内的注水压力与注水流量之间呈线性关系,裂隙的等效张开度与渗透率随法向应力的增加呈负指数衰减特征。(2)剪切蠕变、注水压力、剪切应力速率和剪切位移速率4种诱因所引起的裂隙/断层的黏滑均导致裂隙/断层的渗透性显著增加,且滑移变形和滑移速率越大,渗透率增幅越大。4种诱因中,剪切蠕变引起的渗透率增幅最小,剪切应力速率引起的渗透率增幅最大。(3)在剪切-渗流过程中,速率-状态摩擦定律仍然可以用于分析裂隙/断层的稳定性特征,随着黏滑次数的增多,剪切应力降、b值(即摩擦因数随时间或变形的降幅)及滑移速率等摩擦参数在规则黏滑中以对数形式增长,断层滑移的不稳定程度增加;而在混沌黏滑中,上述参数符合朗格缪尔方程,并趋向于0,断层滑移的不稳定程度降低。Abstract: Hydro-shearing is an important technical method for deep energy extraction, such as hot dry rock geothermal energy and shale gas extraction. In addition to improving reservoir permeability, the hydro-shearing will also induce earthquakes. The essence of both hydro-shearing induced permeability enhancement and injection-induced seismicity are shearing-seepage through the geological joints or faults. In order to study the evolution of shearing-flow properties of joints or faults due to different derived conditions, we employed a high confining pressure and real-time large-displacement shearing-flow setup designed by ourselves to study the fluid flow characteristics of man-made fracture in the granitic hot dry rock mass during shearing-slip which were derived by shearing creep, increasing shearing stress, increasing water-injected pressure and increasing displacement rate, and further analyzed the friction characteristics of fracture or fault in the process of induced seismicity using the rate-state equation. The results indicate that:(1) Under the triaxially hydrostatic stresses, there existed linear relationship between water-injected pressure and injection rate in the granitic fracture, and the equivalent opening and permeability of the fracture showed negative exponential decay with the increment of normal stress.(2) The stick-slip of fracture or fault caused by shearing creep, water-injected pressure, shearing stress rate and shearing displacement rate could induce a significant increment in the permeability. The greater the slip deformation and slip rate, the greater the permeability enhancement. Among the four derivations, the permeability increment caused by shearing creep was the smallest as shearing stress rate induced enhancement was the largest.(3) During the shearing flow process, the rate-state friction law could still be used to analyze the stability of fracture or fault. With the increment of stick-slip times, the friction parameters such as shearing stress drop, b value(That is the drop of frictional coefficient with time and/or displacement) and slip rate increased in the form of logarithmic function in the regular stick-slip which represented the increment of instability of fault slip. In the chaotic stick-slip, the above parameters conformed to the Langmuir equation and tended to zero which indicated that the instability of fault slip decreased.