3,089 research outputs found
A PDE-constrained optimization formulation for discrete fracture network flows
We investigate a new numerical approach for the computation of the 3D flow in a discrete fracture network that does not require a conforming discretization of partial differential equations on complex 3D systems of planar fractures. The discretization within each fracture is performed independently of the discretization of the other fractures and of their intersections. Independent meshing process within each fracture is a very important issue for practical large scale simulations making easier mesh generation. Some numerical simulations are given to show the viability of the method. The resulting approach can be naturally parallelized for dealing with systems with a huge number of fractures
A Review of Hydraulic Fracturing Simulation
Along with horizontal drilling techniques, multi-stage hydraulic fracturing has improved shale gas production significantly in past decades. In order to understand the mechanism of hydraulic fracturing and improve treatment designs, it is critical to conduct modelling to predict stimulated fractures. In this paper, related physical processes in hydraulic fracturing are firstly discussed and their effects on hydraulic fracturing processes are analysed. Then historical and state of the art numerical models for hydraulic fracturing are reviewed, to highlight the pros and cons of different numerical methods. Next, commercially available software for hydraulic fracturing design are discussed and key features are summarised. Finally, we draw conclusions from the previous discussions in relation to physics, method and applications and provide recommendations for further research
A multiscale flux basis for mortar mixed discretizations of reduced Darcy-Forchheimer fracture models
In this paper, a multiscale flux basis algorithm is developed to efficiently
solve a flow problem in fractured porous media. Here, we take into account a
mixed-dimensional setting of the discrete fracture matrix model, where the
fracture network is represented as lower-dimensional object. We assume the
linear Darcy model in the rock matrix and the non-linear Forchheimer model in
the fractures. In our formulation, we are able to reformulate the
matrix-fracture problem to only the fracture network problem and, therefore,
significantly reduce the computational cost. The resulting problem is then a
non-linear interface problem that can be solved using a fixed-point or
Newton-Krylov methods, which in each iteration require several solves of Robin
problems in the surrounding rock matrices. To achieve this, the flux exchange
(a linear Robin-to-Neumann co-dimensional mapping) between the porous medium
and the fracture network is done offline by pre-computing a multiscale flux
basis that consists of the flux response from each degree of freedom on the
fracture network. This delivers a conserve for the basis that handles the
solutions in the rock matrices for each degree of freedom in the fractures
pressure space. Then, any Robin sub-domain problems are replaced by linear
combinations of the multiscale flux basis during the interface iteration. The
proposed approach is, thus, agnostic to the physical model in the fracture
network. Numerical experiments demonstrate the computational gains of
pre-computing the flux exchange between the porous medium and the fracture
network against standard non-linear domain decomposition approaches
Overview of the numerical methods for the modelling of rock mechanics problems
Počeci numeričkih metoda sežu u rane 1960-e. Već tada je bilo jasno da numeričke metode mogu biti uspješno upotrijebljene za različita inženjerska i znanstvena područja, uključujući i primjenu u mehanici stijena. Ubrzan razvoj računala je omogućavao razvoj numeričkih metoda i rješavanje računalno zahtjevnijih sustava. Takav razvoj doveo je do velikog broja različitih metoda i pristupa koji se mogu svrstati u dvije skupine: metode kontinuuma i metode diskontinuuma. Određene zadaće zahtijevaju prednosti oba pristupa, što je dovelo do razvoja kombiniranih konačno-diskretnih metoda. Prvi cilj ovog rada je predstavljanje numeričkih metoda i pristupa koji se koriste za rješavanje zadaća u mehanici stijena, kao i kratko objašnjenje osnovnih teorijskih postavki svake od metoda. Drugi cilj je osvrt na primjenjivost pojedine metode u mehanici stijena.The numerical methods have their origin in the early 1960s and even at that time it was noted that numerical methods can be successfully applied in various engineering and scientific fields, including the rock mechanics. Moreover, the rapid development of computers was a necessary background for solving computationally more demanding problems and the development process of the methods in general. Thus, we have many different methods presently, which can be separated into two main branches: continuum and discontinuum-based numerical methods. Some problems require the strengths of both main approaches which brought the hybrid continuum/discontinuum methods. The first goal of this paper is to present the state of the art numerical methods and approaches for solving the rock mechanics problems, as well as to give the brief explanation about the theoretical background of each method. The second goal is to emphasise the area of applicability of the methods in rock mechanics
Conforming, non-conforming and non-matching discretization couplings in discrete fracture network simulations
Simulations of fluid flow in naturally fractured rocks have implications for
several subsurface applications, including energy storage and extraction, and
waste storage. We are interested in flow in discrete fracture networks, which
explicitly represent flow in fracture surfaces, but ignore the impact of the
surrounding host rock. Fracture networks, generated from observations or
stochastic simulations, will contain intersections of arbitrary length, and
intersection lines can further cross, forming a highly complex geometry. As the
flow exchange between fractures, thus in the network, takes place in these
intersections, an adequate representation of the geometry is critical for
simulation accuracy. In practice, the intersection dynamics must be handled by
a combination of the simulation grid, which may or may not resolve the
intersection lines, and the numerical methods applied on the grid. In this
work, we review different classes of numerical approaches proposed in recent
years, covering both methods that conform to the grid, and non-matching cases.
Specific methods considered herein include finite element, mixed and virtual
finite elements and control volume methods. We expose our methods to an
extensive set of test cases, ranging from artificial geometries designed to
test difficult configurations, to a network extruded from a real fracture
outcrop. The main outcome is guidances for choice of simulation models and
numerical discretization with a trade off on the computational cost and
solution accuracy
Numerical simulation of fracture pattern development and implications for fuid flow
Simulations are instrumental to understanding
flow through discrete fracture
geometric representations that capture the large-scale permeability structure of
fractured porous media. The contribution of this thesis is threefold: an efficient
finite-element finite-volume discretisation of the advection/diffusion
flow equations, a
geomechanical fracture propagation algorithm to create fractured rock analogues,
and a study of the effect of growth on hydraulic conductivity. We describe an
iterative geomechanics-based finite-element model to simulate quasi-static crack
propagation in a linear elastic matrix from an initial set of random
flaws. The
cornerstones are a failure and propagation criterion as well as a geometric kernel for
dynamic shape housekeeping and automatic remeshing. Two-dimensional patterns
exhibit connectivity, spacing, and density distributions reproducing en echelon crack
linkage, tip hooking, and polygonal shrinkage forms. Differential stresses at the
boundaries yield fracture curving. A stress field study shows that curvature can be
suppressed by layer interaction effects. Our method is appropriate to model layered
media where interaction with neighbouring layers does not dominate deformation.
Geomechanically generated fracture patterns are the input to single-phase
flow
simulations through fractures and matrix. Thus, results are applicable to fractured
porous media in addition to crystalline rocks. Stress state and deformation history
control emergent local fracture apertures. Results depend on the number of initial
flaws, their initial random distribution, and the permeability of the matrix. Straightpath
fracture pattern simplifications yield a lower effective permeability in comparison
to their curved counterparts. Fixed apertures overestimate the conductivity of
the rock by up to six orders of magnitude. Local sample percolation effects
are representative of the entire model
flow behaviour for geomechanical apertures.
Effective permeability in fracture dataset subregions are higher than the overall
conductivity of the system. The presented methodology captures emerging patterns
due to evolving geometric and
flow properties essential to the realistic simulation of
subsurface processes
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