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Control-volume distributed multi-point flux approximation coupled with a lower-dimensional fracture model / R. Ahmed; M.G. Edwards; S. Lamine; B.A.H. Huisman; M. Pal
Journal of Computational Physics, Volume: 284, Pages: 462 - 489
Swansea University Author: Edwards, Michael
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A novel cell-centered control-volume distributed multi-point flux approximation (CVD-MPFA) finite-volume formulation is presented for discrete fracture-(rock)matrix flow simulations. The grid is aligned with the fractures and barriers which are then modeled by lower-dimensional interfaces located be...
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A novel cell-centered control-volume distributed multi-point flux approximation (CVD-MPFA) finite-volume formulation is presented for discrete fracture-(rock)matrix flow simulations. The grid is aligned with the fractures and barriers which are then modeled by lower-dimensional interfaces located between rock matrix cells in the physical domain. The n D (n-dimension) pressure equation in the rock matrix is coupled with the (n−1)D pressure equation in the fractures, leading to the first reduced dimensional flux-continuous CVD-MPFA formulation. This formulation naturally handles fractures efficiently on unstructured grids. Matrix-fracture fluxes are expressed in terms of matrix and fracture pressures, resulting in a transfer function, which is added to the lower-dimensional flow equation. An additional transmission condition is used between matrix cells separated by low permeable fractures to couple the velocity and pressure jump across the fractures. Numerical tests serve to assess the convergence and accuracy of the lower-dimensional fracture model for lower anisotropic fractures having a range of apertures and permeability tensors. A tracer flow transport equation is solved for problems with single and intersecting fractures. A lower-dimensional mass balance for intersecting fracture cells circumvents the more restrictive CFL condition corresponding to standard equi-dimensional approximation with explicit time discretization. Lower-dimensional fracture model results are compared with hybrid-grid and equi-dimensional model results. Fractures and barriers are efficiently modeled by lower-dimensional interfaces which yield comparable results to those of the equi-dimensional model. Highly conductive fractures are modeled as lower-dimensional entities without the use of locally refined grids that are required by the equi-dimensional model, while pressure continuity across fractures is built into the model, without depending on the extra degrees of freedom which must be added locally by the hybrid-grid method. The lower-dimensional fracture model also yields improved results when compared to those of the hybrid-grid model for fractures with low-permeability in the normal direction to the fracture where pressure is discontinuous. In addition, transient pressure simulation involving geologically representative complex fracture networks is presented.
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