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1.
Field observations of highly porous and permeable sandstone in the Orange area (S-E Basin, France) show that networks of shear-enhanced compaction bands can form in a contractional regime at burial depths of about 400 m ± 100 m. These bands show equal compaction and shear displacements, are organized in conjugate and densely distributed networks, and are restricted to the coarse-grained (mean grain diameter of 0.6 ± 0.1 mm) and less porous (porosity of 26 ± 2%) sand layers. The bands are crush microbreccia with limited grain comminution and high grain microfracture density. They show reductions of permeability (mD) ranging from 0 to little more than 1 order of magnitude. They show no control on the alteration products related to meteoric water flow, which suggests that these shear-enhanced compaction bands have no or only negligible influence on subsurface fluid flow. Their selective occurrence and small (20%) reduction in transmissibility in densely populated layers prevented them from compartmentalizing the sandstone reservoirs. A comparison with compaction-band populations in the Navajo and Aztec sandtsones (western U.S.) emphasizes the role of burial depth and the presence of chemical compaction processes for the sealing potential of deformation bands. 相似文献
2.
The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis within the bands differ and can be expressed as functions of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear-enhanced (reverse) compaction bands; (2) a regularly spaced less dense network of reverse compactional shear bands; and (3) a localized network of normal shear bands. The field data show that strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands. 相似文献
3.
Use of acoustic emissions to investigate localization in high-porosity sandstone subjected to true triaxial stresses 总被引：4，自引：3，他引：1
Acoustic emissions were used to investigate the evolution of damage and strain localization in Castlegate sandstone specimens subjected to true triaxial states of stress, where the intermediate principal stress ranged from equal to minimum compression to equal to maximum compression. Three failure modes were observed: shear band formation at low mean stresses, compaction localization at intermediate stresses, and no localization at high mean stresses. For shear bands, the onset of localization consistently occurred prior to peak stress, while compaction localization initiated at the beginning of a stress plateau. The band angle (defined as the angle between the band normal and the maximum compression direction) determined by fitting a plane through the localized acoustic emission events corresponded well with the shear band angle expressed on the specimen jacket. As expected, the band angle decreased with increasing mean stress. The theoretically predicted dependence of band angle on deviatoric stress state was not confirmed; however, data scatter due to natural variations in material could obscure such a trend. Each failure mode displayed a unique acoustic emission rate response; therefore, this rate response alone can be used to determine the failure mode and the onset of localization in Castlegate sandstone. 相似文献
4.
Aboozar Garavand Yuri Pavlovich Stefanov Yuri Leonidovich Rebetsky Rustam Alfredovich Bakeev Artem Veniaminovich Myasnikov 《国际地质力学数值与分析法杂志》2020,44(6):823-850
In many wellbore stability analyses, the ability to forecast both the occurrence and extent of plastic deformation and failure hinges upon a fundamental understanding of deformation mode and failure mechanism in the reservoir rock. This study focuses on analyzing plastic zones, localized deformations, and failures around a borehole drilled overbalanced or underbalanced through a highly porous rock formation. Based on several laboratory experiments, porous rocks are prone to deform under both shear-induced dilation and shear-enhanced compaction mechanisms depending on the stress state. The shapes of the deformation and failure patterns around the borehole are shown, depending on the initial stress state and the local stress paths. The inquiry of the local stress paths in the near-wellbore zone facilitates the understanding of the reasons for different types of failure mechanisms, including the mixed-mode and the plastic deformation structures. The modification of the 2D plane strain condition by imitating third stress in the numerical scheme helps us bring the stress paths closer to the real state of loading conditions. Our modeling reveals that the transition from isotropic to anisotropic stress state is accompanied by an increase in the deviatoric part of effective shear tensor that leads to the development of inelastic deformation, degradation, and subsequent rock failure. Particular interest is devoted to the modeling of strain localization especially in compaction mode around a wellbore and computing the amount of stress concentration at the tips of dog-eared breakouts. Stress concentration can result in a change in irreversible deformation mode from dilatancy to compaction, elucidating the formation of the shear-enhanced compaction phenomenon at the failure tips in the direction of the minimum horizontal stress. 相似文献
5.
砂岩三轴循环加卸载条件下的渗透率研究 总被引：6，自引：0，他引：6
渗透率是地下工程的流-固耦合分析中的一个关键因素。对多孔红砂岩进行了三轴压缩试验,在不同变形阶段实施了轴向应力循环加卸载,并在试验全过程中测量轴向渗透率,得到了试样破坏全过程的渗透率演化规律。从平均应力和循环加卸载对渗透率的影响等两方面进行了深入分析,结果表明,（1）随着轴向变形的增加,初始压密阶段和弹性变形试样渗透率均匀减小;进入塑性变形阶段,渗透率与轴向变形的曲线逐渐趋于水平,低围压条件下渗透率略有增加。（2）轴向加载使骨架颗粒被压缩,引起孔隙减小,造成渗透率减小;采用经验公式定量描述了渗透率和平均应力之间的关系。（3）轴向应力循环加卸载过程中,骨架颗粒的不可恢复变形引起渗透率产生不可恢复现象。（4）峰值后渗透率只发生少许突跳,说明对于多孔砂岩,孔隙和裂隙对渗透率的影响相当,且渗透率的突跳程度随着围压的升高而降低。 相似文献
6.
Various mechanisms can affect the permeability of dense unconsolidated sands: Volumetric dilation can lead to permeability increase, whereas strain localization in shear bands may increase or decrease the permeability depending on the state of compaction and on the level of grains breakage inside the band. To investigate these various mechanisms, an experimental study has been performed to explore the effect of different factors such as grain size and grain shape, confining pressure, level of shear, stress path, and formation of one or several shear bands on the permeability of dense sands under triaxial loading. The experimental results show a reduction of permeability during the consolidation phase and during the volumetric contraction phase of shear loading, which can be related to the decrease of porosity. The experimental results also show that, depending on the confining pressure, the permeability remains stable or decreases during the volumetric dilation phase despite the increase of total porosity. This permeability reduction is attributed to the presence of fine particles, which result from grains attrition during pre-localization and grains breakage inside the shear band during the post-localization phase. 相似文献
7.
Permeability changes have been studied under deviatoric stresses for chalk cores and under both hydrostatic- and deviatoric stresses for sandstone cores at room temperature. To avoid end effects in the triaxial cell, caused by friction between the axial steel pistons and the sample, the cell was modified to have pressure outlets from the mid-section of the sample with pressure tubes connected to the outside of the cell for pressure recording. Both permeabilities over the mid-section and over the total core were determined during the action of stresses. The chalk cores with permeability in the range of 1–3 × 10− 15 m2 and porosity of about 40–45% were flooded with methanol, while the sandstone cores with permeability values varying from 8 to 100 × 10− 15 m2 and porosity of about 30% were flooded with a mineral oil. Major observations were:
- (1) For the chalk cores, 4 out of 8 samples showed a mid-section permeability with a factor of 1.2 to 1.4 higher than the overall permeability, the remaining 4 samples did not show differences in permeability values taking into account the error on measurements.
- (2) For the sandstone samples, the mid-section permeability was a factor of 1.2 to 2.4 higher than the overall permeability.
- (3) In all cases during the deviatoric phase, the change in permeability was rather small, even if the tests were run beyond the yield point.
- (4) The permeability generally decreased with increasing hydrostatic stresses.
Keywords: Permeability; Stress; End effects; Chalk; Sandstone 相似文献
8.
Internal erosion is one of the most common failure modes of embankment dams or foundations, and the simplest and most effective preventive measure is to build a cutoff wall. The soil at the bottom of the cutoff wall is usually under complex stress states. The deeper the cutoff wall, the higher is the stress. In this study, the effects of stress conditions on the evolution of internal erosion were investigated in sandy gravel foundations containing a suspended cutoff wall using a newly developed stress-controlled erosion apparatus. Three series of erosion tests were conducted on gap-graded soil under different confining stresses, different deviatoric stresses, and different confining and deviatoric stresses. The results of these tests are as follows: (1) The discharge and permeability decrease with an increase in the confining stress, but the critical hydraulic gradient increases. (2) In the second series of erosion tests, the specimen is compressed under low deviatoric stress; the specimen undergoes shear expansion under high deviatoric stress. (3) In the third series of erosion tests, the confining and deviatoric stresses synchronously change, and therefore, their combined effect on the evolution of internal erosion is complicated. Under low stress, the soil is compressed in the early stage of the experiment, and its structure may change during internal erosion. When the stress level is high, the specimen also undergoes shear expansion, and the degree of expansion is controlled by both confining and deviatoric stresses. 相似文献
9.
We present a multiscale investigation on the initiation and development of compaction bands in high-porosity sandstones based on an innovative hierarchical multiscale approach. This approach couples the finite element method and the discrete element method (DEM) to offer direct, rigorous linking of the microscopic origins and mechanisms with complex macroscopic phenomena observed in granular rocks such as strain localization and failure. To simulate compaction band in granular cementitious sandstone, we adopt a bonded contact model with normal and tangential interparticle cohesions in the DEM and propose a dual-porosity structure consisting of macro-pores and interstitial voids for the representative volume element to mimic the typical meso-structure of high-porosity sandstones. In the absence of particle crushing, our multiscale analyses identify debonding and pore collapses as two major contributors to the formation of compaction bands. The critical pressures predicted by our simulations, corresponding to surges of debonding and pore collapse events, agree well with the estimations from field data. The occurrence patterns of compaction band are found closely related to specimen heterogeneity, porosity and confining pressure. Other deformation band patterns, including shear-enhanced compaction bands and compactive shear bands, were also observed under relatively low confining pressure conditions with a rough threshold at \(0.55P^{*}\) (\(P^{*}\) is the critical pressure) on the failure envelop. Key microscopic characteristics attributable to the occurrence of these various deformation patterns, including fabric anisotropy, particle rotation, debonding and pore collapse, are examined. Shear-enhanced compaction bands and pure compaction bands bear many similarities in terms of these microscopic characteristics, whereas both differ substantially from compactive shear bands. 相似文献
10.
Modeling of damage,permeability changes and pressure responses during excavation of the TSX tunnel in granitic rock at URL,Canada 总被引：1，自引：0，他引：1
Jonny Rutqvist Lennart Börgesson Masakazu Chijimatsu Jan Hernelind Lanru Jing Akira Kobayashi Son Nguyen 《Environmental Geology》2009,57(6):1263-1274
This paper presents numerical modeling of excavation-induced damage,
permeability changes, and fluid-pressure responses during excavation of a test tunnel associated with the tunnel
sealing experiment (TSX) at the Underground Research Laboratory (URL) in Canada. Four
different numerical models were applied using a wide range of approaches to model damage
and permeability changes in the excavation disturbed zone (EDZ) around the tunnel.
Using in situ calibration of model parameters, the modeling could reproduce observed
spatial distribution of damage and permeability changes around the tunnel as a
combination of disturbance induced by stress redistribution around the tunnel
and by the drill-and-blast operation. The modeling showed that stress-induced
permeability increase above the tunnel is a result of micro and macrofracturing
under high deviatoric (shear) stress, whereas permeability increase alongside
the tunnel is a result of opening of existing microfractures under decreased mean
stress. The remaining observed fracturing and permeability changes around the periphery
of the tunnel were attributed to damage from the drill-and-blast operation. Moreover,
a reasonably good agreement was achieved between simulated and observed
excavation-induced pressure responses around the TSX tunnel for 1 year following its
excavation. The simulations showed that these pressure responses are caused by
poroelastic effects as a result of increasing or decreasing mean stress, with
corresponding contraction or expansion of the pore volume. The simulation results
for pressure evolution were consistent with previous studies, indicating that the
observed pressure responses could be captured in a Biot model using a relatively
low Biot-Willis’ coefficient, α ≈ 0.2,
a porosity of n ≈ 0.007, and a relatively
low permeability of k ≈ 2 × 10−22 m2,
which is consistent with the very tight, unfractured granite at the site. 相似文献
11.
In this paper, several mechanical deformation curves of limestone are reviewed, and the effects of temperature, confining pressure, and fluid are discussed. Generally, Mohr–Coulomb is used for limestone brittle fracture. The characteristic of low temperature cataclastic flow and the conditions and constitutive equations of intracrystal plastic deformation such as dislocation creep, diffusion creep, and superplastic flow are discussed in detail. Specifically, from the macroscopic and microscopic view, inelastic compression deformation (shear-enhanced compaction) of large porosity limestone is elaborated. Compared with other mechanics models and strength equations, the dual porosity (macroporosity and microporosity) model is superior and more consistent with experimental data. Previous research has suffered from a shortage of high temperature and high pressure limestone research; we propose several suggestions to avoid this problem in the future: (1) fluid-rock interaction research; (2) mutual transition between natural conditions and laboratory research; (3) the uniform strength criterion for shear-enhanced compaction deformation; (4) test equipment; and (5) superplastic flow mechanism research. 相似文献
12.
Lúcia Carvalho Coelho Antonio Claudio Soares Nelson Francisco F. Ebecken Jos Luis Drummond Alves Luiz Landau 《国际地质力学数值与分析法杂志》2006,30(14):1477-1500
High porosity and low permeability limestone has presented pore collapse. As fluid is withdrawn from these reservoirs, the effective stresses acting on the rock increase. If the strength of the rock is overcome, pore collapse may occur, leading to irreversible compaction of porous media with permeability and porosity reduction. It impacts on fluid withdrawal. Most of reservoirs have been discovered in weak formations, which are susceptible to this phenomenon. This work presents a study on the mechanical behaviour of a porous limestone from a reservoir located in Campos Basin, offshore Brazil. An experimental program was undergone in order to define its elastic plastic behaviour. The tests reproduced the loading path conditions expected in a reservoir under production. Parameters of the cap model were fitted to these tests and numerical simulations were run. The numerical simulations presented a good agreement with the experimental tests. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
13.
不同应力状态下地层渗透系数的变化及其对流体运移影响的数值模拟研究 总被引：15，自引：3，他引：12
本文选择地层渗透系数受应力影响而变化作为研究流体运移的基础。压实固结和构造作用所产生应力对渗透系数影响的有限元数学模拟研究表明,平均有效应力与介质渗透性能呈负相关的变化趋势,应力驱动流体由应力高值区向应力低值区运移。 相似文献
14.
Compaction bands induced by borehole drilling 总被引：1，自引：1，他引：0
Drilling experiments in rock blocks subjected to pre-existing true triaxial far-field stresses simulating real in situ conditions
often result in localized failure around the created borehole, which brings about the formation of borehole breakouts. In
weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like) openings extending along
a plane perpendicular to the maximum applied-stress direction. Scanning electron microscopes images of failed boreholes strongly
suggest that these breakouts are compaction bands that have been emptied to different extents. The bands form as a result
of the stress concentration accompanying the creation of the borehole. The evacuation of the compaction bands is brought about
by the circulating drilling fluid flushing out debonded and often fragmented grains from within these bands (Haimson and co-workers,
2003–2007). The objective of this paper is to predict the conditions under which compaction bands are formed around boreholes.
To this end, a new analytical model is formulated that enables prediction of the stress field around emptied and filled compaction
bands, the various factors affecting the breakouts lengths, and their final length. Good agreement of the developed analytical
model with experimental results obtained by Haimson and co-workers (Haimson and Klaetsch in Rock physics and geomechanics
in the study of reservoirs and repositories, vol 284, pp 89–105, 2007; Haimson and Kovachich in Eng Geol 69:219–231, 2003;
Klaetsch and Haimson in Mining and tunneling innovation and opportunity, University of Toronto press, pp 1365–1371, 2002;
Sheets and Haimson in Proceedings, paper ARMA/NARMS 04-484, 2004) is demonstrated. The presented study is of practical relevance:
boreholes are often drilled deep into weak porous sandstone formations for the purpose of extracting oil and gas, and the
question of borehole stability is crucial. In addition, borehole breakouts are often used to estimate the state of stress
in the Earth’s crust, and our new formulation will help improve these estimates. 相似文献
15.
Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory‐scale drilling tests in some high‐porosity quartz‐rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture‐like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production. This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture‐like breakouts and to identify the role of far‐field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre‐existing far‐field stresses. Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture‐like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture‐like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far‐field stresses. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
16.
Mechanisms of the opening of back-arc systems are analyzed. Limited focal mechanisms of intraplate earthquakes are used to determine the stress regime of an overriding plate. Preliminary analyses show that compressive deviatoric stresses exist in the plate except near the spreading center. Based on this observation “trench suction” does not appear to be the primary force that drives back-arc spreading, since it will result in tensional deviatoric stresses within the overriding plate. Even though “continental pull” is able to satisfy the stress requirements, it does not appear to be a likely mechanism either because of the initiation and subsequent symmetric spreading difficulty associated with such a mechanism. The mechanism we favor is the one that involves the induced convective current in the mantle wedge immediately above the slab. Calculations show that the induced flow is able to generate sufficient stress to break up the overriding lithosphere if the tectonic stresses of the region are favorable. Both trench suction and continental pull may help to provide such a favorable tectonic stress regime. 相似文献
17.
Compaction bands are localized failure patterns that appear in highly porous rock material under the effect of relatively high confining pressure. Being affected mainly by volumetric compression, these bands appear to be almost perpendicular to the most compressive principal stress of a stress state at the so-called “cap” of the yield surface (YS). In this study, we focus on the mechanism that leads to the onset of compaction bands by using a viscoplasticity model able to describe the post-localization response of these materials. The proposed constitutive framework is based on the overstress theory of Perzyna (1966) and the anisotropic clay plasticity model of Dafalias (1986), which provides not only the necessary “cap” of the YS, but introduces a rotational hardening (RH) mechanism, thus, accounting for the effect of fabric anisotropy. Following the analysis of Veveakis and Regenauer-Lieb (2015), we identify the compaction bands as “static” cnoidal wave formations in the medium that occur at a post-yield regime, and we study the effect of rotational and isotropic hardening on their onset. Moreover, we determine a theoretical range of confining pressures in triaxial compression tests for the compaction bands to develop. Under the assumption of coaxiality between stress and anisotropy tensors, the results show that the isotropic hardening promotes compaction localization, whereas the RH has a slightly negative effect on the onset of compaction localization. 相似文献
18.
泌阳凹陷核三下亚段砂岩成岩作用及储集性 总被引：12，自引：1，他引：12
泌阳凹陷东部核三下亚段砂岩储层由辨状河三角洲成因的长石砂岩及岩屑长石砂岩组成。主要成岩作用包括压实、石英和长石次生加大、晶粒状方解石胶结、溶解、自生绿泥石等，演化程度已达到晚成岩Ｂ亚期。成岩作用使砂岩原生孔隙体系发生强烈变化。各种成岩作用对储集性有不同影响，孔隙度的减少主要与压实及碳酸盐胶结作用有关；石英和长石次生加大及自生粘土衬边经常使喉道堵塞，对渗透率危害较大；溶解作用形成的次生孔隙及喉道对砂 相似文献
19.
《Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy》2001,26(1-2):15-20
We induced borehole breakouts in a 25%-porosity Berea sandstone by drilling 23 mm diameter holes into 152×152×229 mm blocks subjected to constant true triaxial far-field stresses. BSen5 consists of large quartz grains (0.5 mm) cemented mainly by sutured grain contacts. Breakouts in BSen5 are demonstratively different from those observed in granite, limestone, and lower porosity sandstones. Rather than the typically short ‘V’-shaped breakouts, BSen5 displays long fracture-like tabular slots, which counterintuitively, develop orthogonally to σH. These breakouts originate at the points of highest compressive stress at the borehole wall, along the σh spring line. Micrographs of BSen5 breakouts show an apparent compaction band created just ahead of the breakout tip in the form of a narrow layer of grains that are compacted normal to σH. The compaction band characteristics are nearly identical to those observed in the field. The mechanism leading to fracture-like breakouts is seen as anti-dilatant, and related directly to grain debonding and porosity reduction accompanying the formation of the compaction band. Some compacted grains at the borehole wall are expelled as a result of the line of tangential loading and the radial expansion of adjacent grains. The circulating drilling fluid flushes out the remaining compacted loose grains at the borehole-rock interface. As the breakout tip advances, the stress concentration ahead of it persists, extending the compaction band, which in turn leads to additional grain removal and breakout lengthening. By extrapolation, this process may continue for considerable distance (at least several times the wellbore diameter) in field situations, leading potentially to substantial sand production. 相似文献
20.
利用岩石薄片、物性等分析资料，对库车坳陷东部依奇克里克构造带低渗透储层特征及其成因进行了研究。依奇克里克构造带中下侏罗统储层岩石类型均为岩屑砂岩、含长石岩屑砂岩。岩石学总体特征为成分成熟度低，结构成熟度较高，岩石成分成熟度由北向南逐渐变好。储集空间类型在东西向也有所差异，东部吐格尔明地区主要为次生-原生孔隙型，西部依南地区主要为裂缝-次生孔隙型。储集性能平面非均质性较强，以低孔、低渗、特低孔、特低渗储层为主，东部吐格尔明地区明显优于西部依南地区。影响本区储集性能的主要因素有压实作用、沉积环境、岩石学特征、溶蚀作用及构造作用，但最主要的影响因素是埋藏压实作用，构造挤压作用的叠加加强了本区西部储层压实作用，但裂缝发育又使储集性能有所改善。 相似文献