Influence of shear and deviatoric stress on the evolution of permeability in fractured rock (original) (raw)
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Stress-dependent permeability of fractured rock masses: a numerical study
International Journal of Rock Mechanics and Mining Sciences, 2004
We investigate the stress-dependent permeability issue in fractured rock masses considering the effects of nonlinear normal deformation and shear dilation of fractures using a two-dimensional distinct element method program, UDEC, based on a realistic discrete fracture network realization. A series of "numerical" experiments were conducted to calculate changes in the permeability of simulated fractured rock masses under various loading conditions. Numerical experiments were conducted in two ways: (1) increasing the overall stresses with a fixed ratio of horizontal to vertical stresses components; and (2) increasing the differential stresses (i.e., the difference between the horizontal and vertical stresses) while keeping the magnitude of vertical stress constant. These numerical experiments show that the permeability of fractured rocks decreases with increased stress magnitudes when the stress ratio is not large enough to cause shear dilation of fractures, whereas permeability increases with increased stress when the stress ratio is large enough. Permeability changes at low stress levels are more sensitive than at high stress levels due to the nonlinear fracture normal stress-displacement relation. Significant stress-induced channeling is observed as the shear dilation causes the concentration of fluid flow along connected shear fractures. Anisotropy of permeability emerges with the increase of differential stresses, and this anisotropy can become more prominent with the influence of shear dilation and localized flow paths. A set of empirical equations in closed-form, accounting for both normal closure and shear dilation of the fractures, is proposed to model the stress-dependent permeability. These equations prove to be in good agreement with the results obtained from our numerical experiments.
International Journal of Rock Mechanics and Mining Sciences, 2013
The influence of in-situ stresses on flow processes in fractured rock is investigated using a novel modelling approach. The combined finite-discrete element method (FEMDEM) is used to model the deformation of a fractured rock mass. The fracture wall displacements and aperture changes are modelled in response to uniaxial and biaxial stress states. The resultant changes in flow properties of the rock mass are investigated using the Complex Systems Modelling Platform (CSMPþþ). CSMPþþ is used to model single-phase flow through fractures with variable aperture and a permeable rock matrix. The study is based on a geological outcrop mapping of a low density fracture pattern that includes the realism of intersections, bends and segmented features. By applying far-field (boundary) stresses to a square region, geologically important phenomena are modelled including fracture-dependent stress heterogeneity, the re-activation of pre-existing fractures (i.e. opening, closing and shearing), the propagation of new fractures and the development of fault zones. Flow anisotropy is investigated under various applied stresses and matrix permeabilities. In-situ stress conditions that encourage a closing of fractures together with a more pervasive matrix-dominated flow are identified. These are compared with conditions supporting more localised flow where fractures are prone to dilatational shearing and can be more easily exploited by fluids. The natural fracture geometries modelled in this work are not perfectly straight, promoting fracture segments that dilate as they shear. We have demonstrated the introduction of several realistic processes that have an influence on natural systems: fractures can propagate with wing cracks; there is the potential for new fractures to connect with existing fractures, thus increasing the connectivity and flow; blocks can rotate when bounded by fractures, bent fractures lead to locally different aperture development; highly heterogeneous stress distributions emerge naturally. Results presented in this work provide a mechanically rigorous demonstration that a change in the stress state can cause reactivation of pre-existing fractures and channelling of flow in critically stressed fractures.
Water Resources Research
Flow and transport properties of fractured rock masses are a function of geometrical structures across many scales. These structures result from physical processes and states and are highly anisotropic in nature. Fracture surfaces often tend to be shifted with respect to each other, which is generally a result of stress-induced displacements. This shift controls the fracture's transmissivity through the pore space that forms from the created mismatch between the surfaces. This transmissivity is anisotropic and greater in the direction perpendicular to the displacement. A contact mechanics-based, first-principle numerical approach is developed to investigate the effects that this shear-induced transmissivity anisotropy has on the overall permeability of a fractured rock mass. Deformation of the rock and contact between fracture surfaces is computed in three dimensions at two scales. At the rock mass scale, fractures are treated as planar discontinuities along which displacements and tractions are resolved. Contact between the individual rough fracture surfaces is solved for each fracture at the small scale to find the stiffness and transmissivity that result from shear-induced dilation and elastic compression. Results show that, given isotropic fracture networks, the direction of maximum permeability of a fractured rock mass tends to be aligned with the direction of the intermediate principal stress. This reflects the fact that fractures have the most pronounced slip in the plane of the maximum and minimum principal stresses, and for individual fractures transmissivity is most pronounced in the direction perpendicular to this slip.
Journal of Geophysical Research, 2011
1] We report on laboratory experiments designed to investigate the influence of pore pressure oscillations on the effective permeability of fractured rock. Berea sandstone samples were fractured in situ under triaxial stresses of tens of megapascals, and deionized water was forced through the incipient fracture under conditions of steady and oscillating pore pressure. We find that short-term pore pressure oscillations induce long-term transient increases in effective permeability of the fractured samples. The magnitude of the effective permeability enhancements scales with the amplitude of pore pressure oscillations, and changes persist well after the stress perturbation. The maximum value of effective permeability enhancement is 5 × 10 −16 m 2 with a background permeability of 1 × 10 −15 m 2 ; that is, the maximum enhanced permeability is 1.5 × 10 −15 m 2 . We evaluate poroelastic effects and show that hydraulic storage release does not explain our observations. Effective permeability recovery following dynamic oscillations occurs as the inverse square root of time. The recovery indicates that a reversible mechanism, such as clogging/unclogging of fractures, as opposed to an irreversible one, like microfracturing, is responsible for the transient effective permeability increase. Our work suggests the feasibility of dynamically controlling the effective permeability of fractured systems. The result has consequences for models of earthquake triggering and permeability enhancement in fault zones due to dynamic shaking from near and distant earthquakes. Citation: Elkhoury, J. E., A. Niemeijer, E. E. Brodsky, and C. Marone (2011), Laboratory observations of permeability enhancement by fluid pressure oscillation of in situ fractured rock,
Rock Mechanics and Rock Engineering, 2007
Permeability is a physical property in rocks of extreme importance in energy engineering, civil and environmental engineering, and various areas of geology. Early on, fractures in fluid flow models were assumed to be rigid. However, experimental research and field data confirmed that stress-deformation behavior in fractures is a key factor governing their permeability tensor. Although extensive research was conducted in the past, the three-dimensional stress-permeability relationships, particularly in the inelastic deformation stage, still remain unclear. In this paper, laboratory experiments conducted on large concrete blocks with randomly distributed fractures and rock core samples are reported to investigate fluid flow and permeability variations under uniaxial, biaxial and triaxial complete stress-strain process. Experimental relationships among flowrate, permeability and fracture aperture in the fractured media are investigated. Results show that the flowrate and stress/aperture exhibit “cubic law” relationship for the randomly distributed fractures. A permeability-aperture relationship is proposed according to the experimental results. Based on this relationship, stress-dependent permeability in a set of fractures is derived in a three-dimensional domain by using a coupled stress and matrix-fracture interactive model. A double porosity finite element model is extended by incorporating such stress-dependent permeability effects. The proposed model is applied to examine permeability variations induced by stress redistributions for an inclined borehole excavated in a naturally fractured formation. The results indicate that permeability around underground openings depends strongly on stress changes and orientations of the natural fractures.
Advances in Water Resources, 2020
The equivalent permeability of layered fractured rocks plays an important role in hydrocarbon recovery, underground energy storage, waste disposal management, groundwater hydrology, and subsurface contaminant transport. Borehole data contain some uncertainties/sampling bias during collection and interpretation. This sampling effect may lead to an inaccurate characterization of the fractured media. Studies show that long fractures with high permeability, which are rarely seen in borehole images, dominate the flow pattern and affect the overall permeability of the fractured system. This means that samples taken at any scale smaller than the scale of interest result in imprecise permeability upscaling. To understand this sampling problem, we have established an efficient sampling method to study the existence of the representative elementary volume (REV) in naturally fractured rocks. We selected a collection of outcrop data represented by discrete fracture and matrix (DFM) models in which fracture apertures are mechanically constrained. A finite-element-finite-volume approach is utilized to characterize the flow behavior of DFM models. Multiscale random sampling is combined with flow-based upscaling to determine the equivalent permeability tensor and its anisotropy by considering the variable orientation of the stress state and fracture roughness. Our findings indicate a convergence towards a scale-invariant equivalent permeability and fracture density with increasing sample size, and the equivalent permeability itself has a multimodal distribution. The spatial variation of the permeability tensor and the change in the degree of anisotropy with sample size reflect the inhomogeneity of the fracture patterns.
Stress-induced fluid flow anisotropy in fractured rock
Transport in Porous Media, 1990
Anisotropic stress states are common in the upper crust and result in fracture apertures being dependent on fracture orientation. Fractured rocks should therefore display an anisotropic permeability determined by the aperture, length, and orientation of those fractures remaining open. In this paper, a numerical study of this effect is made for a rock containing two orthogonal fracture sets subject to a uniaxial compressive stress applied perpendicular to one of the sets. With increasing compressive stress, the decreasing aperture of fractures orientated perpendicular to the stress axis leads to a decrease in permeability both parallel and perpendicular to the stress. For flow parallel to the stress direction, this is a consequence of the finite length of the fractures, flow in fractures perpendicular to the stress being required to connect fractures orientated parallel to the stress direction. As the number of fractures is decreased towards the percolation threshold, the average permeability tensor is found to become increasingly isotropic. This behaviour results from the highly tortuous nature of the flow paths just at the percolation threshold.
Evolution of Strength and Permeability in Stressed Fractures with Fluid–Rock Interactions
Pure and Applied Geophysics, 2015
We determine the evolution of frictional strength, strain weakening behavior and permeability in fractures subject to dissolution and precipitation. We establish these relations through slide-hold-slide experiments, with hold times from 10 to 3000 s, on split limestone core, under hydraulically open and closed conditions. Fracture friction and permeability are measured continuously throughout the experiments. The limestone displays velocity-strengthening behavior (stable slip) under incremented velocity steps of 1-6 lm/s. Frictional healing is observed to be time-and stress-dependent, showing higher gains in strength at both longer hold times and under lower effective stresses. Activation of healing is greater in wet samples than in dry samples. Flow-through experiments for flow rates in the range of 1-10 ml/ min are conducted to further investigate the role of flow and mineral redistribution in contributing to healing. These experiments show strength gains are lower at higher flow rates where advective mineral dissolution and redistribution is enhanced and cementation concomitantly limited. Concurrently measured permeability decreases throughout the slide-hold-slide sequences indicating that mean fracture aperture reduces during sliding. We combine models representing pressure solution and stress corrosion as models for the growth in fracture contact area and represent the observed timedependent behavior of strength gain and permeability evolution. The simulated results represent the observed strength gain at long hold times (*1000 s), but underestimate strengthening at short hold times. We conclude that the evolution of strength and permeability are significantly controlled by mechanisms of fluid-rock interactions and that the strengths and nature of feedbacks on these linkages are critical in understanding the mechanical and hydraulic behavior of faults.
Evolution of permeability in a natural fracture: Significant role of pressure solution
Journal of Geophysical Research, 2004
1] A mechanistic model is presented to describe closure of a fracture mediated by pressure solution; closure controls permeability reduction and incorporates the serial processes of dissolution at contacting asperities, interfacial diffusion, and precipitation at the free face of fractures. These processes progress over a representative contacting asperity and define compaction at the macroscopic level, together with evolving changes in solute concentration for arbitrarily open or closed systems for prescribed ranges of driving effective stresses, equilibrium fluid and rock temperatures, and fluid flow rates. Measured fracture surface profiles are applied to define simple relations between fracture wall contact area ratio and fracture aperture that represents the irreversible alteration of the fracture surface geometry as compaction proceeds. Comparisons with experimental measurements of aperture reduction conducted on a natural fracture in novaculite show good agreement if the unknown magnitude of microscopic asperity contact area is increased over the nominal fracture contact area. Predictions of silica concentration slightly underestimate the experimental results even for elevated microscopic contact areas and may result from the unaccounted contribution of free face dissolution. For the modest temperatures (20-150°C) and short duration (900 hours) of the test, pressure solution is demonstrated to be the dominant mechanism contributing to both compaction and permeability reduction, despite net dissolution and removal of mineral mass. Pressure solution results in an 80% reduction in fracture aperture from 12 mm, in contrast to a $10 nm contribution by precipitation, even for the case of a closed system. For the considered dissolution-dominated system, fracture closure rates are shown to scale roughly linearly with stress increase and exponentially with temperature increase, taking between days and decades for closure to reach completion.