Practical Gridding Algorithms for Discrete Fracture Modeling Workflows

Mallison, Bradley T.; Hui, Mun-Hong; Narr, Wayne

doi:10.3997/2214-4609.20144950

Cited by 34 publications

(30 citation statements)

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“…Addressing this limitation, Mallison et al (2010) presented new gridgeneration algorithms for simulating fluid flow in NFRs. Moreover, Mallison et al (2010) argue that there is inherent uncertainty in the precise position and geometry of fractures in an NFR and hence chose not to impose an exact agreement of fracture geometry between the geologic and flow-simulation models. Instead, their gridgeneration algorithms are designed to capture only geometric features that are larger than the specified grid resolution.…”

Section: Chevron Discrete Fracture Modelsmentioning

confidence: 99%

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

et al. 2013

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Summary Many naturally fractured reservoirs around the world have depleted significantly, and improved-oil-recovery (IOR) processes are necessary for further development. Hence, the modeling of fractured reservoirs has received increased attention recently. Accurate modeling and simulation of naturally fractured reservoirs (NFRs) is still challenging because of permeability anisotropies and contrasts. Nonphysical abstractions inherent in conventional dual-porosity and dual-permeability models make them inadequate for solving different fluid-flow problems in fractured reservoirs. Also, recent technologies for discrete fracture modeling may suffer from large simulation run times, and the industry has not used such approaches widely, even though they give more-accurate representations of fractured reservoirs than dual-continuum models. We developed an embedded discrete fracture model (DFM) for an in-house compositional reservoir simulator that borrows the dual-medium concept from conventional dual-continuum models and also incorporates the effect of each fracture explicitly. The model is compatible with existing finite-difference reservoir simulators. In contrast to dual-continuum models, fractures have arbitrary orientations and can be oblique or vertical, honoring the complexity of a typical NFR. The accuracy of the embedded DFM is confirmed by comparing the results with the fine-grid, explicit-fracture simulations for a case study including orthogonal fractures and a case with a nonaligned fracture. We also perform a grid-sensitivity study to show the convergence of the method as the grid is refined. Our simulations indicate that to achieve accurate results, the embedded discrete fracture model may only require moderate mesh refinement around the fractures and hence offers a computationally efficient approach. Furthermore, examples of waterflooding, gas injection, and primary depletion are presented to demonstrate the performance and applicability of the developed method for simulating fluid flow in NFRs.

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Section: Chevron Discrete Fracture Modelsmentioning

confidence: 99%

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

et al. 2013

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“…In a similar manner, Kappa (2014) applied an approach to model exactly the fracture geometry by placing fixed Voronoi cell centers around fractures without explicitly defining simulation gridblocks for fractures. Also, Mallison et al (2010) tried an force-based optimization algorithm (Holm et al 2006;Persson and Strang 2004) to generate triangular grids for fractures, for which they could not precisely honor the geometry of fracture line segments. Instead, fractures are approximated by edges of the triangular mesh, and some fractures might be snapped together in situations where the input fractures do not intersect.…”

Section: Mesh Generation For Complex Fracture Networkmentioning

confidence: 99%

“…First of all, intersection information helps the mesh generation algorithms to better conform to the input of complex fracture geometry and therefore will ensure good mesh quality. Second, for those approaches (Branets et al 2008;Kappa 2014;Karimi-Fard et al 2003) where no fracture gridblocks are explicitly defined, the intersection points are still needed to compute the inter-cell transmisibilities; for those approaches (Mallison et al 2010;Mun-Hong Hui 2008) where fractures are not described exactly as straight line segments, the intersection points are considered as background information to guide the mesh generation algorithms. In this study, not only do we compute intersection points, but also define the intersections into five types which offer us additional freedom to manipulate fracture connectivity through intersections.…”

Section: Compute Intersections Of the Connected Fracture Networkmentioning

confidence: 99%

Optimization-Based Unstructured Meshing Algorithms for Simulation of Hydraulically and Naturally Fractured Reservoirs with Variable Distribution of Fracture Aperture, Spacing, Length and Strike

Sun

Schechter

2014

SPE Annual Technical Conference and Exhibition

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Multi-stage hydraulically fractured wells are applied widely to produce unconventional resource plays. In naturally fractured reservoirs, hydraulic fracture treatments may induce complex fracture geometries which cannot be modeled accurately and efficiently using Cartesian and corner-point grid systems or standard dual porosity approaches. The interaction of hydraulic and naturally occurring fractures almost certainly plays a role in ultimate well and reservoir performance. Current simulation models are unable to capture the complexity of this interaction. Generally speaking, our ability to detect and characterize fracture systems is far beyond our capability of modeling complex natural fracture systems. In order to evaluate production performance in these complex settings using numerical simulation, fracture networks require advanced meshing and domain discretization techniques. This paper investigates these issues by developing natural fracture networks using fractal-based techniques. Once a fracture network is developed we demonstrate the feasibility of gridding complex natural fracture behavior using optimization-based unstructured meshing algorithms. It can then be demonstrated that natural fracture complexities such as variable aperture, spacing, length and strike can be simulated. This new approach is a significant step beyond the current method of dual porosity simulation which essentially negates the sophisticated level of fracture characterization pursued by many operators.We use currently established code for fractal discrete fracture network (FDFN) models to build realizations of naturally fractured reservoirs in terms of stochastic fracture networks. From outcrop, image log and core analysis, it is possible to extract fracture fractal parameters pertaining to aperture, spacing, length distribution, including center distribution as well as a fracture strike. Then these parameters are used as input variables for the FDFN code to generate multiple realizations of fracture networks mimicking fracture clustering and randomly distributed natural fractures. After incorporating hydraulic fractures, complex fracture networks are obtained for further reservoir domain discretization.In order to discretize the complex fracture networks, a new mesh generation approach is developed to conform to non-orthogonal and low-angle intersections of extensive-clustered discrete fracture networks with non-uniform aperture distribution. Optimization algorithms are adopted to reduce highly skewed cells, and to ensure good mesh quality around fracture tips, intersections and regions of extensive fracture clustering. Moreover, Local Grid Refinement (LGR) is implemented with a pre-defined distance function to control cell sizes and shapes around and far away from fractures. Natural fracture spacing, length, strike and aperture distribution are explicitly gridded thus introducing a new simulation approach that is far superior to dual porosity simulation. Finally initial sensitivity studies are performed to demonstrate both the ca...

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“…In the discrete-fracture method, rock-matrix and fracture elements coincide at the interface, so an unstructured grid is used to honour the explicit fracture geometry(see Mallison et al (2010); Mustapha et al (2011)). Also, the matrix cells near the fracture are small enough to conform to the complex fracture-network geometry.…”

Section: Introductionmentioning

confidence: 99%

Control-Volume Distributed Multi-Point Flux Approximation Coupled with a Lower-Dimensional Fracture Model

Ahmed

Edwards

Lamine

et al. 2015

SPE Reservoir Simulation Symposium

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A novel cell-centred control-volume distributed multi-point flux approximation (CVD-MPFA) finitevolume formulation is presented for discrete fracture-matrix simulations in three-dimensions. The grid is aligned with the fractures and barriers which are then modelled as lower-dimensional interfaces located between the matrix cells in the physical domain. The three-dimensional(3D) pressure equation is solved in the matrix domain coupled with a two-dimensional(2D) pressure equation solved over fracture networks via a surface CVD-MPFA formulation. The CVD-MPFA formulation naturally handles fractures with anisotropic permeabilities on unstructured grids. Matrix-fracture fluxes are expressed in terms of matrix and fracture pressures and must be added to the lower-dimensional flow equation (called the transfer function). An additional transmission condition is used between matrix cells adjacent to low permeable fractures to link the velocity and pressure jump across the fractures. Numerical tests serve to assess the convergence and accuracy of the lower-dimensional fracture model for highly anisotropic fractures having different apertures and permeability tensors. A transport equation for tracer flow is coupled via the Darcy flux for single and intersecting fractures. The lower-dimensional approach for intersecting fractures avoids the more restrictive CFL condition corresponding to the equi-dimensional approximation with explicit time discretisation. Lower-dimensional fracture model results are compared with equi-dimensional model results. Fractures and barriers are efficiently modelled by lower-dimensional interfaces which yield comparable results to those of the equi-dimensional model. Highly conductive fractures are modelled as lower-dimensional entities with continuous pressure across these leading to reduced local degrees of freedom for the cluster of cells. Moreover, we present 3D simulations involving geologically representative complex fracture networks.

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Practical Gridding Algorithms for Discrete Fracture Modeling Workflows

Cited by 34 publications

References 0 publications

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

Optimization-Based Unstructured Meshing Algorithms for Simulation of Hydraulically and Naturally Fractured Reservoirs with Variable Distribution of Fracture Aperture, Spacing, Length and Strike

Control-Volume Distributed Multi-Point Flux Approximation Coupled with a Lower-Dimensional Fracture Model

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