Suitability Of Dynamic Flux Allocation For Simulation Of Stratfield Reservoirs
Authors: Koederitz Leonard , Simon Andrew 
Published: December 2007

Reservoir simulation is used to observe the change in production through varied recovery mechanisms. The most common type of simulator is the black oil or Beta type model. In studying recovery mechanisms, especially for stratified reservoirs, a cross-sectional model is often preferred to a full three-dimensional version due to minimal computer requirements (both in speed and storage) and ease of interpretation. Additionally, cross-sectional models have been used for homogeneous reservoirs to study the effects of gas-overriding and/or water underrunning. Usually this type of model is selected for a representative section of the field and no-flow boundaries are assumed to exist across the front and rear faces of the section; however, boundaries may not exist, or may be time-variant. One technique for circumventing this problem is to use an areal model to account for a net flux in the cross-sectional motel. This technique is severely limited by the presence of reservoir heterogeneity.

Another approach to this problem is the use of a dynamic flux allocation (DFA) scheme. This method maintains zonal integrity and provides for variations in production which would require additional areal simulation. In the simplest sense, DFA employs a gross three-dimensional model surrounding the section of interest. This technique is faster and easier to apply than the previously described techniques; it also has the advantage of being much faster than a three-dimensional study or an areal-to-cross-section simulation. Necessary gridding considerations to use this method are discussed and comparative cases are demonstrated.

Introduction

Numerical reservoir simulation has become a sophisticated reservoir engineering tool that has enabled the prediction and optimization of complex reservoir producing mechanisms under a variety of operating conditions. Since its inception in 1953 computer simulation of unsteady-state flow in porous media has been expanded to encampass a wide range of reservoir characteristics, hydrocarbon properties, and advanced recovery techniques. The most commonly used simulator for reservoirs that do not exhibit significant phase behavior effects is the black oil or Beta type model. Several authors have presented the mathematical basis of three dimensional, three phase black oil models, and a comprehensive treatment and extensive bibliography of the subject may be found in recent publications by Crichlow and Peaceman.

From an applied standpoint, numerous authors have discussed the problem of selecting the correct number of dimensions to adequately model reservoir performance in relation to the questions to be answered. performance in relation to the questions to be answered. Normally, the degree of reservoir stratification and significance of vertical flow has dictated whether a two or three dimensional model should be employed far the determination of reservoir performance. Due to the frequently excessive engineering and computer time requirements of finely gridded 3-D models, several alternatives to the rigorous treatment of three dimensional flow have been presented. These models have been primarily concerned with the adaptation of a 2-D areal simulator to account far vertical saturation discontinuity, either by a vertical equilibrium concept, the use of pseudo functions derived from reservoir permeability distribution or generated from simple 2-D cross-sectional studies.

The use of cross-sectional simulators will also often yield adequate results when applied to the study of reservoir mechanisms. This may be particularly manifest in highly stratified formations where areal attempts result in discontinuous history matches requiring extreme data manipulation, or in cases where the ability to provide a somewhat precise representation of areal conformance is not of great importance. As with areal models, a cross-sectional model is often preferred to a full three-dimensional version due to preferred to a full three-dimensional version due to minimal computer speed and storage requirements and ease of interpretation. However, to the authors' knowledge, very little information concerning the actual use of cross-sections has appeared in the literature.