A Practical Approach to Defining Reservoir Heterogeneity
Authors: Alpay O. Allen 
Published: December 2007

Here is a method for evaluating the physical and textural variations in reservoir rocks to construct a flow model using core descriptions, permeability profiles, and well logs. The method is a practical and permeability profiles, and well logs. The method is a practical and versatile alternative to existing complex analytical techniques. It can be used with or without mathematical simulators.

Introduction
In conventional engineering practice, mathematical expressions describing the flow of fluids in porous media are applied to reservoirs via various simplifying assumptions regarding the physical nature of the reservoir rock and its contents. These well known assumptions usually consider the reservoir rock to be either entirely homogeneous and isotropic, or allow for some simple types of anisotropy in certain directions.
It is generally recognized that idealization of rock characteristics in flow calculations is unrealistic in the face of actual physical and textural conditions encountered in geologic formations. The experience gained from several decades of fluid injection and recovery operations has left no doubt that an overly simplified concept of the reservoir rock is hardly suitable for a valid representation or appraisal of reservoir behavior. However, dealing effectively with the characterization of physical and textural variation in a reservoir has been an elusive problem because the reservoir portion that can be investigated through boreholes is usually insignificant in comparison with the bulk of the reservoir. Moreover, it is extremely difficult to construct a precise mathematical analog of the detailed reservoir rock variability even when abundant information is available. Still, if the evaluation of reservoir performance in various engineering operations is to be reasonably realistic, performance calculations must include adequate provisions for the extent of control imposed on flow by the reservoir rock. Consequently, before he can proceed with his projections the practicing engineer is frequently faced with the need for (1) a valid description of certain physical and textural variants in the reservoir, such as porosity, permeability, stratification, and continuity; and permeability, stratification, and continuity; and (2) means for enumerating such descriptions. Numerous investigations of reservoir heterogeneity and its characterization have been made in recent years. Most of the methods proposed, however, either are too involved mathematically to be practical in routine applications, or require specific information on the reservoir that is not always easily available. Therefore, it appears that few of them have had wide use outside research or academic circles. Meanwhile, as new and more complicated recovery techniques are being introduced by the industry, the need for a thorough understanding of reservoir anatomy and for a means of characterizing its variability in an easily comprehensible and practical way is becoming increasingly urgent. practical way is becoming increasingly urgent. The method presented here is intended to fulfill those needs. Its objective is to describe the conditions of physical and textural variation in the reservoir that actually interfere with the flow of fluids, by relying on the investigation of the sedimentary-stratigraphic makeup of the reservoir rock. Although the method may appear to be intended primarily for fluid-injection recovery applications, primarily for fluid-injection recovery applications, it can be used in all phases of reservoir depletion and development with or without mathematical models.
JPT
P. 841