The propagation of compaction bands in porous rocks based on breakage mechanics (original) (raw)
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A theoritical approach to the study of compaction bands in porous rocks
2011
The formation and propagation of compaction bands in high porosity sandstones is theoretically investigated in this paper using a new constitutive model based on the recently developed continuum breakage mechanics theory [1,2]. This model possesses a micromechanics-based link between the evolving grain size distribution (gsd) and the macroscopic stress strain relationship, through an internal variable called Breakage. This is an advanced feature over many existing plasticity based models in the literature, helping to faithfully track the evolving gsd and its related physics (e.g. permeability reduction). A localization analysis based on the acoustic tensor [3] is performed to determine both the onset and orientation of compaction bands due to grain crushing. It is shown that the model used is able to capture well both the material behaviour and formation of compaction band experimentally observed. An enhancement using rate-dependent regularization is applied to the model to deal with instability issues in the analysis of Boundary Value Problems. Based on the regularised model, the formation and propagation of compaction bands due to grain crushing is analysed through a numerical experiment on a porous rock specimen under triaxial loading condition. Good agreement between numerical predictions and experimental observations demonstrates the capability of the new model.
Grain crushing and pore collapse are the principal micromechanisms controlling the physics of compaction bands in porous rocks. Several constitutive models have been previously used to predict the formation and propagation of these bands. However, they do not account directly for the physical processes of grain crushing and pore collapse. The parameters of these previous models were mostly tuned to match the predictions of compaction localization; this was usually done without validating whether the assigned parameters agree with the full constitutive behavior of the material. In this study a micromechanics-based constitutive model capable of tracking the evolving grain size distribution due to grain crushing is formulated and used for a theoretical analysis of compaction band formation in porous rocks. Linkage of the internal variables to grain crushing enables us to capture both the material behavior and the evolving grain size distribution. On this basis, we show that the model correctly predicts the formation and orientation of compaction bands experimentally observed in typical high-porosity sandstones. Furthermore, the connections between the internal variables and their underlying micromechanisms allow us to illustrate the significance of the grain size distribution and pore collapse on the formation of compaction bands.
Mechanical Compaction of Porous Sandstone
Oil & Gas Science and Technology
Compaction mécanique des grès poreux -Pour de nombreux problèmes de tectonique et d'ingénierie de réservoir, la capacité à prévoir à la fois la fréquence, l'ampleur de la déformation inélastique et les ruptures repose sur une compréhension fondamentale de la phénoménologie et de la micromécanique de compaction dans les roches-réservoirs. Cet article présente les résultats de recherches récentes sur la compaction mécanique des grès poreux. On insiste plus particulièrement sur la synthèse des données de laboratoire, la caractérisation microstructurale quantitative de l'endommagement, ainsi que sur les modèles théoriques basés sur un contact élastique et sur la mécanique de la rupture. Les attributs mécaniques de la compaction sur des échantillons initialement secs et saturés ont été étudiés sous des chargements hydrostatiques et non hydrostatiques dans une large gamme de pression. Les sujets spécifiques étudiés ici incluent : la comparaison des données d'émission acoustique et mécanique avec une théorie de la plasticité ; le contrôle microstructural du début et du développement de la compaction ; l'écrouissage et l'évolution spatiale de l'endommagement lors de la compaction ; enfin, l'effet affaiblissant de l'eau sur le seuil de compaction et l'évolution de la porosité.
Tectonophysics, 2011
The study of localized deformation in porous sandstones at the laboratory scale can yield valuable insights into the internal structures and mechanisms of shear zones and compaction bands that might impact on flow at a reservoir scale. Herein, we report results of a laboratory study of shear and compaction band formation in a porous sandstone using a range of full-field experimental techniques: acoustic emissions, ultrasonic tomography, X-ray tomography, and 3D volumetric digital image correlation, plus thin section and Scanning Electron Microscope observations. The two main mechanisms involved in shear and compaction band formation, grain breakage (damage) and porosity reduction (compaction), are both well captured by the combination of all these laboratory techniques. The combined use of these techniques demonstrated the processes of shear and compaction band generation and the associated strain components that developed in the laboratory, and potentially also increased understanding of the naturally developed equivalents. The physical mechanisms of shear and compaction involved seem to be similar, but at the laboratory scale they show differences in the proportions and the order of occurrence in time.
Effects of pre-existing faults on compaction localization in porous sandstones
Tectonophysics, 2018
The formation of deformation bands can significantly modify the strength and transport properties of porous sedimentary rocks. Among the different types of deformation bands, compaction bands exhibit porosity reduction with little to no shear displacement. Natural compaction bands have previously been reported and studied in only a few areas. They often coexist with faults and other localized deformation structures. We mapped the geometrical relation between compaction bands, shear bands and faults in Lower Cretaceous porous sandstone at Makhtesh Katan, Israel. To understand the effect of pre-existing faults on the formation of compaction bands, we conducted deformation experiments on pre-faulted Bentheim sandstones. These experiments produced compaction bands consistently intersect the pre-existing fault. To gain better mechanical understanding of the observed band geometry, we also carried out three-dimensional (3D) numerical simulations with the input elastic moduli and yield strength well-constrained from the deformation experiments. We demonstrated that the formation of deformation bands is dictated by stress concentrations associated with the pre-existing fault. Frictional slip along the heterogeneous fault plane can produce a local stress concentration that would be responsible for further localized damage and the development of deformation zones. When fault slip is restricted (a possible result of high confinement), compaction bands initiate at high stress concentration sites resulting from geometrical irregularities of the fault. Finally, using a plane-strain twodimensional (2D) linear-elastic model with the geometry of the faults mapped in the outcrop, we were able to provide a mechanical explanation of the distribution for deformation bands observed at the Makhtesh Katan study area.
Experimental Study of Localised Deformation in Porous Sandstones
This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time.
Geophysical Journal International, 2004
We address the gradual transition from brittle failure to cataclastic flow under increasing pressures by a new model, incorporating damage rheology with Biot's poroelasticity. Deformation of porous rocks is associated with growth of two classes of internal flaws, namely cracks and pores. Cracks act as stress concentrations promoting brittle failure, whereas pores dissipate stress concentrations leading to distributed deformation. The present analysis, based on thermodynamic principles, leads to a system of coupled kinetic equations for the evolution of damage along with porosity. Each kinetic equation represents competition between cracking and irreversible porosity change. In addition, the model correctly predicts the modes of strain localization such as dilating versus compacting shear bands. The model also reproduces shear dilatancy and the related change of fluid pressure under undrained conditions. For triaxial compression loading, when the evolution of porosity and damage is taken into consideration, fluid pressure first increases and then decreases, after the onset of damage. These predictions are in agreement with experimental observations on sandstones. The new development provides an internally consistent framework for simulating coupled evolution of fracturing and fluid flow in a variety of practical geological and engineering problems such as nucleation of deformation features in poroelastic media and fluid flow during the seismic cycle.
Geophysical Research Letters, 2011
In order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks, the authors carried out a series of rock fracture tests under triaxial compression in the laboratory. The detailed spacetime distribution of acoustic emission due to microcracking was used to examine pre-failure damage and failure behavior in Berea sandstone, which has a porosity of 20% and a permeability of 100 mD. The pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions. Experimental results indicate that drainage conditions play a governing role in deformation and fracture. The well-established dilatancyhardening effect can be greatly suppressed by dilatancydriven fluid flowing under good drainage conditions. Fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process. Furthermore, good drainage conditions have the potential to enlarge the nucleation dimension and duration, thereby improving the predictability of the final catastrophic failure. In addition, compaction bands, which were observed in porous rocks under higher confining pressure, were also observed at low confining pressure (corresponding to a depth of 1 km) in undrained tests. These results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role, such as hydrocarbon reservoirs, enhanced geothermal systems, geological storage of CO 2 .
Time-dependent compaction band formation in sandstone
Compaction bands in sandstone are laterally extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earth's upper crust. To better understand the time dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence, background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10 À8 to 10 À5 s À1 ). We find that despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry and thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.