Impact of fracture development on the effective permeability of porous rocks as determined by 2-D discrete fracture growth modeling (original) (raw)
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Discontinuities in effective permeability due to fracture percolation
Mechanics of Materials, 2018
Motivated by a triaxial coreflood experiment with a sample of Utica shale where an abrupt jump in permeability was observed, possibly due to the creation of a percolating fracture network through the sample, we perform numerical simulations based on the experiment to characterize how the effective permeability of otherwise low-permeability porous media depends on fracture formation, connectivity, and the contrast between the fracture and matrix permeabilities. While a change in effective permeability due to fracture formation is expected, the dependence of its magnitude upon the contrast between the matrix permeability and fracture permeability and the fracture network structure is poorly characterized. We use two different high-fidelity fracture network models to characterize how effective permeability changes as percolation occurs. The first is a dynamic two-dimensional fracture propagation model designed to mimic the laboratory settings of the experiment. The second is a static three-dimensional discrete fracture network (DFN) model, whose fracture and network statistics are based on the fractured sample of Utica shale. Once the network connects the inflow and outflow boundaries, the effective permeability increases non-linearly with network density. In most networks considered, a jump in the effective permeability was observed when the embedded fracture network percolated. We characterize how the magnitude of the jump, should it occur, depends on the contrast between the fracture and matrix permeabilities. For small contrasts between the matrix and fracture permeabilities the change is insignificant. However, for larger contrasts, there is a substantial jump whose magnitude depends non-linearly on the difference between matrix and fracture permeabilities. A power-law relationship between the size of the jump and the difference between the matrix and fracture permeabilities is observed. The presented results underscore the importance of fracture network topology on the upscaled properties of the porous medium in which it is embedded.
Evolution of Fracture Aperture under Combined Effect of Stress and Flow
International Journal of Chemical Engineering and Applications
Fracture apertures may decrease or increase by different mechanical and chemical mechanisms when the fractures are subject to stress and flow. A model is presented to describe evolution of fracture aperture mediated by dissolution and precipitation. The model accounts for the fact that dissolved minerals carried by flowing water along the fracture can not only diffuse into and out of the adjacent rock matrix but also at first diffuse into the stagnant water zone existing in part of the fracture plane and then from there into and out of the rock matrix adjacent to it. This simple model allows us to gain some insights into which processes and mechanisms have the larger impact on the fracture aperture under different circumstances. The analytical solution in Laplace domain is used to study fracture closure/opening rate in a pseudo steady state procedure. It is found that the times involved for any changes in fracture aperture are very much larger than the times needed for concentrations of dissolved minerals to reach steady state in the rock matrix, the stagnant water zone and the flow channel. Moreover, it is shown that diffusion into the rock matrix, which acts as a strong sink or source for dissolved minerals, clearly dominates the rate of concentration change and consequently the rate of evolution of the fracture aperture.
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.
Permeability of a random array of fractures of widely varying apertures
Transport in Porous Media, 1987
We modelize a fractured rock by a random array of plane cracks of finite extent having a very broad distribution of apertures (or of hydraulic conductances). If the rock is permeable, the flow will essentially take place along a 'subnetwork' made of the less resistant cracks. Using an analogy with the treatment of variable range transport in semiconductors, we evaluate the homogenization length and the permeability of this disordered network. This evaluation makes use of the notion of the critical bonds which are the weakest cracks among the good ones necessary for percolation; the remaining weaker bonds make a negligible contribution to the permeability. The method is applicable to other examples of transport in very heterogeneous macroscopic random materials.
A B S T R A C T Modeling of fluid flow in naturally fractured reservoirs is often done through modeling and upscaling of discrete fracture networks (DFNs). The two-dimensional fracture geometry required for DFNs is obtained from subsurface and outcropping analog data. However, these data provide little information on subsurface fracture aperture, which is essential for quantifying porosity and permeability. Apertures are difficult to obtain from either out-cropping or subsurface data and are therefore often based on fracture size or scaling relationships, but these do not consider the orientation and spatial distribution of fractures with respect to the in situ stress field. Using finite-element simulations, mechanical aperture can be modeled explicitly, but because changes in fracture geometry require renewed meshing and simulating, this approach is not easily integrated into subsurface DFN modeling workflows. We present a geometrically based method for calculating the shear-induced hydraulic aperture, that is, an aperture of up to 0.5 mm (0.02 in.) that can result from shear displacement along irregular fracture walls. The geometrically based method does not require numerical simulations, but it can instead be directly applied to DFNs using the fracture orientation and spacing distributions in combination with an estimate of the regional stress tensor and orientation. The frequency distribution of hydraulic aperture from the geometrically based method is compared with finite-element models constructed from five real fracture networks , digitized from outcropping pavements. These networks cover a wide range of possible geometries and spatial distributions. The geometrically based method predicts the average hydraulic aperture and equivalent permeability of fractured porous media with error margins of less than 5%.
Experimental Measurements of Stress and Chemical Controls on the Evolution of Fracture Permeability
Transport in Porous Media, 2013
We explore how fracture permeability in confined tight carbonates evolves due to flow of reactive fluids. Core plugs of the Capitan Massive Limestone are saw-cut to form a smooth axial fracture that is subsequently roughened to control the fracture surface topography. Either distilled water or distilled water-ammonium chloride solutions are circulated through these plugs, where fracture roughness, inlet fluid pH, and confining stresses are controlled. Throughout the experiment we measure the fluid flow rate and chemical composition of the effluent fluid. Mass balance, conducted on the effluent fluid mass and on dissolved mineral components, independently constrains the mineral mass removal. We use an idealized lumped parameter model of asperity supported fractures undergoing simultaneous stress corrosion cracking-induced diffusion and free-face dissolution to infer theoretical rates of aperture loss or gain. This model incorporates the roles of confining stress, fracture contact area, and composition and reactivity of the permeating fluid while identifying zones of diffusion-dominated mass transfer within the fracture. These theoretical rates of aperture strain are compared to those inferred from the experimentally determined permeability evolution and permeating fluid mineral mass balance. By measuring in regimes of both increasing and decreasing permeability we quantitatively constrain the transition between fracture-gaping and fracture-closing modes of behavior. We parameterize this transition in permeability evolution by the ratio of mechanically to chemically controlled dissolved mass fluxes. The transition from regimes of closing to regimes of gaping occurs at unity (χ ≈ 1) when stress and chemically driven mass fluxes are theoretically equal.
Effect of Fracture Permeability on Connectivity of Fracture Networks
2009
Many open pit mines are located in fractured rock systems where water flow paths are complex and difficult to predict. These flow paths are typically controlled by a small subset of fractures that are permeable and interconnected. Most models of flow in fractured rock systems are based on a network of interconnected fractures that are all assumed to be permeable. However this assumption is rarely observed in natural rocks where a significant fraction of the fractures within a connected cluster could be impermeable. Thus in studying fracture flow systems, we need to consider the permeability status (i.e. permeable or impermeable) of individual fractures in addition to the fracture network's connectivity. Primary percolation clusters based on connectivity alone can be generated according to the fracture density, and probability density functions of fracture length and fracture orientation. These primary clusters, potentially including impermeable clusters, may not all conduct water. Hence percolation clusters need to be refined so that they comprise only open fractures. The density of these refined clusters can then be linked to the hydraulic conductivity, providing a more realistic representation of the natural system. Here we use numerical simulations to examine the effect (on connectivity and permeability) of removing a portion of fractures that are assumed to be impermeable. A discrete fracture network model is applied to formulate an analytical relation between two potentially measurable quantities of fractured rock systems, i.e., scan-line density of all fractures within core samples or boreholes and scan-line density of conductive fractures intercepted by boreholes.
Effects of stress on the two-dimensional permeability tensor of natural fracture networks
Geophysical Journal International, 1996
The effects of stress on the 2-D permeability tensor of natural fracture networks were studied using a numerical method (Universal Distinct Element Code). On the basis of three natural fracture networks sampled around Dounreay, Scotland, numerical modelling was carried out t o examine the fluid flow in relation to the variations in burial depth, differential stress and loading direction. It was demonstrated that the permeability of all the networks decreased with depth due to the closure of aperture.
A numerical model simulating reactive transport and evolution of fracture permeability
International Journal for Numerical and Analytical Methods in Geomechanics, 2006
A numerical model is presented to describe the evolution of fracture aperture (and related permeability) mediated by the competing chemical processes of pressure solution and free-face dissolution/precipitation; pressure (dis)solution and precipitation effect net-reduction in aperture and free-face dissolution effects netincrease. These processes are incorporated to examine coupled thermo-hydro-mechano-chemo responses during a flow-through experiment, and applied to reckon the effect of forced fluid injection within rock fractures at geothermal and petroleum sites. The model accommodates advection-dominant transport systems by employing the Lagrangian-Eulerian method. This enables changes in aperture and solute concentration within a fracture to be followed with time for arbitrary driving effective stresses, fluid and rock temperatures, and fluid flow rates. This allows a systematic evaluation of evolving linked mechanical and chemical processes. Changes in fracture aperture and solute concentration tracked within a wellconstrained flow-through test completed on a natural fracture in novaculite (Earth Planet. Sci. Lett. 2006, in press) are compared with the distributed parameter model. These results show relatively good agreement, excepting an enigmatic abrupt reduction in fracture aperture in the early experimental period, suggesting that other mechanisms such as mechanical creep and clogging induced by unanticipated local precipitation need to be quantified and incorporated. The model is applied to examine the evolution in fracture permeability for different inlet conditions, including localized (rather than distributed) injection. Predictions show the evolution of preferential flow paths driven by dissolution, and also define the sense of permeability evolution at field scale.