An Automatic, Unstructured Control Volume Generation System for Geologically Complex Reservoirs
Authors: Kocberber Sait 
Published: December 2007

Abstract
A new three-dimensional automatic grid generation system is developed that accurately represents sloping faults in heterogeneous reservoirs. This is achieved by combining automatic paving with automatic triangulation and tetrahedronization processes. The advancing front technique is used for automatic triangulation and tetrahedronization processes. First, a refined grid is obtained around sloping faults by paving a layer of control volumes whose boundaries align with faults and layer boundaries. This process assures that the control volumes do not span across layer interfaces and faults and hence do not contain permeability discontinuities. In regions where the reservoir is made of continuous stacked layers, the grid is made of Voronoi prisms. These two grids are then connected through automatic tetrahedronization and median bisection processes. The median bisectioning is necessary for reservoirs with anisotropic permeability or an unfavorable geometrical aspect ratio.

Introduction
An integral, and often time consuming part, of building a petroleum reservoir simulation model is the construction of a grid which forms the basis of a discretized representation of the reservoir. These grids have historically been regularly structured, often taking the form of orthogonal cartesian grids. More recently, however, such grids have been considered lacking in their ability to represent the increased geological realism now being put into reservoir models. These grids can conform to some areal geological features while retaining the local orthogonality demanded by the finite difference simulation engines. However, they fail to accurately represent such features as intersecting sloping faults, layer pinchouts, unconformities, and highly deviated wells. Moreover, the gridding process is manual and can be very time consuming. Voronoi and Control Volume Finite Element grids offer greater flexibility and have enjoyed increased usage in reservoir simulation. Although significant progress has been made, the accurate representation of geologically complex reservoirs in simulation models is still an active research and development topic. This paper describes our approach to the problem and presents some of our findings.
The following are the desirable characteristics that we were adhered to as much as possible in generating the automatic gridding system for the simulation of geologically complex reservoirs.A grid generation system should accurately represent the exterior boundary and heterogeneous internal structure of the reservoir. The exact geometry of major geological features should be honored.The produced grids should honor the well profile accurately. The grid around wells should be modular so that it can be activated when desired. The grid should have properly graded refinement that conforms to the natural flow pattern.The grid should consist of control volumes that, as much as possible, are locally orthogonal. Then the transmissibilities can be calculated by local finite differencing and the grid will be consistent with existing simulation technology.
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