Numerical Simulation of Naturally Fractured Reservoirs

Abstract

The most important and difficult aspect of modeling a naturally fractured reservoir is the correct calculation of the exchange of fluids between the matrix rock and the surrounding fractures. Several authors have published alternative techniques for handling this problem over the past few years. However, because each of these alternatives has some limitations, a new and more general technique has been developed. This new technique is used to simulate matrix/fracture exchange with special emphasis on the gravity forces included in the exchange terms. The exchange terms and the gravity forces within the exchange terms simulate the behavior of a single matrix block surrounded .by fractures that may contain several different fluids. The gravity forces are internally calculated as functions of saturation. This technique has been incorporated into a new three-dimensional (3D), threephase, fully implicit model for simulating fluid flow in a naturally fractured reservoir. The description of the porous medium might include highly fractured, microfractured, and nonfractured regions. Several examples explain the use of a new naturally fractured reservoir model and the essential differences between the new approach and those used in earlier naturally fractured reservoir models. IntroductionThe numerical simulation of naturally fractured reservoirs is a subject that has been described extensively in the literature of the oil industry for the last 25 years.In the initial work done, the primary goal of the simulation of fractured systems was to study pressure behavior during well tests. This was based primarily on the analytical solutions obtained by Barenblatt et at. l and Warren and Root Z for describing singlephase flow near a wellbore in a naturally fractured reservoir. Their work was based on the concept of a fractured continuum filled with noncontinuous matrix blocks. Warren and Root also delivered a one-dimensional (ID) radial model, which Kazemi 3 later extended to study two-dimensional (2D) flow during a well test. Further developments were made by Iffly et at.,4 Yamamoto et at.,5 Kleppe and Morse, 6 and others, but these models were used either to match laboratory results or to study the behavior of a single matrix block.More recently, more complex problems have been studied with full-field reservoir models. Our interest in this paper is the study of fractured reservoirs with full-field models.Examples are presented that show some of the differences between a new fractured reservoir model and other models described in the literature. Comparisons are also made that point out some of the complexities of fractured reservoir simulation and the potential problems with the use of a single-porosity approach to study various types of fractured reservoir behavior.One of the first papers to describe a fractured reservoir simulator that could be applied to full-field model studies was by Saidi 7 and was concerned with the simulation of the very highly fractured reservoirs in Iran. This model represented the fracture system as a ...

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

“…The gravity term was a function of fluid contact heights in the matrix block and its surrounding fracture. Litvak et al (1985) and Sonier et al (1988) used similar approaches, but improved the calculation of the fluid contact heights by including irreducible saturations. Thomas et al (1983) developed a 3D, three-phase, finite difference dual porosity model for simulating NFRs.…”

Fracture-matrix transfer function in fractured porous media

“…They use the term instead of in eqn. (7) and calculate it depending on the flow direction; if water flowing from the fracture to the matrix k rw calculated from: (8) and if oil flows from the fracture to the matrix system Kro is calculated from (9) For flow from the matrix to the fracture, unaltered relative permeability values are used. To include the effect of block coverage they multiply the matrix phase relative permeability value by the fracture phase saturation, i.e.…”

“…Sonier et al [8] presented another modification for eqn. (7) by assuming that the phase saturations within the fracture are the same in any portion of the grid cell and all matrix blocks have the same saturation.…”

Advanced Technology for Predicting the Fluid Flow Attributes of Naturally Fractured Reservoirs From Quantitative Geologic Data and Modeling

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