Characterization of stress-dependent microcrack compliance and orientation distribution in anisotropic crystalline rocks

IF 1.8 3区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS Geophysical Prospecting Pub Date : 2024-08-09 DOI:10.1111/1365-2478.13593
Colin M. Sayers
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Abstract

Crystalline rocks in the subsurface are of interest for geothermal energy extraction, nuclear waste storage, and, when weathered or fractured, as aquifers. Compliant discontinuities such as microcracks, cracks and fractures may nucleate and propagate due to changes in pore pressure, stress and temperature. These discontinuities may provide flow pathways for fluids and, if fracturing extends to surrounding rocks, may allow escape of fluids to neighbouring formations. Monitoring such rocks using sonic logs, passive seismic, borehole seismic and surface seismic requires understanding of the propagation of elastic waves in the presence of such discontinuities. These may have an anisotropic orientation distribution as in situ stress may be anisotropic. As crystalline rock may display intrinsic anisotropy due to foliation and the preferential orientation of anisotropic minerals, quantification of the relative importance of intrinsic and microcrack-induced anisotropy is important. This may be achieved based on the stress sensitivity of elastic wave velocities. A method that allows both the orientation distribution of microcracks and the stress dependence of their normal and shear compliance to be estimated independently of the elastic anisotropy of the background rock is presented. Results are given for anisotropic samples of gneiss from Bukov in the Czech Republic and granite from Grimsel in Switzerland based on the ultrasonic velocity measurements of Aminzadeh et al. The microcrack orientation distribution is approximately transversely isotropic for both samples with a preferred orientation of microcrack normals perpendicular to foliation. This preferred alignment is stronger in the sample of gneiss than in the granite sample, and the normal and shear compliance of the microcracks decreases with increasing compressive stress. This occurs because the contact between opposing faces of the discontinuities grows with increasing compressive stress, and this results in a decrease in elastic anisotropy with increasing compressive stress. At low stress, the ratio of microcrack normal compliance to shear compliance is approximately 0.25 for the granite sample and 0.7 for the sample of gneiss. The normal compliance ZN for both samples decreases faster with increasing compressive stress than the shear compliance ZT, resulting in a decrease in ZN/ZT with increasing compressive stress.

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各向异性结晶岩中随应力变化的微裂缝顺应性和取向分布的表征
地表下的结晶岩在提取地热能、储存核废料以及风化或断裂后作为含水层方面都具有重要意义。由于孔隙压力、应力和温度的变化,微裂缝、裂纹和断裂等顺应性不连续体可能会成核和扩展。这些不连续性可为流体提供流动通道,如果断裂延伸到周围的岩石,则可使流体逸出到邻近的地层。利用声波测井、被动地震、井眼地震和地表地震监测这类岩石,需要了解弹性波在存在这些不连续面时的传播情况。由于原位应力可能是各向异性的,因此这些不连续面可能具有各向异性的方向分布。由于结晶岩可能会因褶皱和各向异性矿物的优先取向而显示出固有的各向异性,因此量化固有各向异性和微裂缝引起的各向异性的相对重要性非常重要。这可以根据弹性波速的应力敏感性来实现。本文提出了一种方法,可以独立于背景岩石的弹性各向异性来估算微裂缝的方向分布及其法向和剪切顺应性的应力依赖性。根据 Aminzadeh 等人的超声波速度测量结果,给出了捷克共和国 Bukov 片麻岩和瑞士 Grimsel 花岗岩各向异性样品的结果。片麻岩样品中的这种优先排列比花岗岩样品中的更强,微裂缝的法线和剪切顺应性随着压缩应力的增加而减小。出现这种情况的原因是,随着压缩应力的增加,不连续面之间的接触也会增加,从而导致弹性各向异性随着压缩应力的增加而减小。在低应力下,花岗岩样本的微裂缝法向顺应性与剪切顺应性之比约为 0.25,片麻岩样本约为 0.7。随着压缩应力的增加,这两种样本的法顺应性 ZN 比剪顺应性 ZT 下降得更快,从而导致压缩应力增加时 ZN/ZT 下降。
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来源期刊
Geophysical Prospecting
Geophysical Prospecting 地学-地球化学与地球物理
CiteScore
4.90
自引率
11.50%
发文量
118
审稿时长
4.5 months
期刊介绍: Geophysical Prospecting publishes the best in primary research on the science of geophysics as it applies to the exploration, evaluation and extraction of earth resources. Drawing heavily on contributions from researchers in the oil and mineral exploration industries, the journal has a very practical slant. Although the journal provides a valuable forum for communication among workers in these fields, it is also ideally suited to researchers in academic geophysics.
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Issue Information Simultaneous inversion of four physical parameters of hydrate reservoir for high accuracy porosity estimation A mollifier approach to seismic data representation Analytic solutions for effective elastic moduli of isotropic solids containing oblate spheroid pores with critical porosity An efficient pseudoelastic pure P-mode wave equation and the implementation of the free surface boundary condition
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