In this paper, certain fluid recovery mechanisms that occur in stratified systems are analysed using experimental floods in layered beadpacks. In particular, the roles of viscous and gravitational crossflow are studied in viscous stable and viscous slug type displacement processes. The results from these floods are then scaled to field processes using a combination of scaling theory and numerical simulation. The effects of smaller sub-layer heterogeneity (within the larger scale layering) on the fluid recovery mechanisms are also examined in some detail. It is found that, although the levels of smaller scale heterogeneity studied in this work does not largely affect the total recovery profiles, they do have some impact on the detailed levels of fluid crossflow in the system. However, it is stressed that the level of heterogeneity description required for a given system must depend both on the type of system, the details of the fluid recovery mechanism operating in that reservoir and also actual balance of forces (viscous/gravity etc.) that exists in that participle situation.
Introduction
One of the most common idealizations used by reservoir engineers to represent a complex heterogeneous formation is the layered reservoir model [e.g. 1–11]. There is, however, a good foundation in geology for this first approximation to certain reservoir formations, although it is known to be very simplified in some respects. For example, the next level of complexity that can be envisaged within such a model is to consider sub-layer heterogeneity; that is with permeability heterogeneity within each layer. It is also possible to imagine further transformation of such a layered system (at least in principle) into one where local length correlations within the sub-layers start to dominate over the stratification structure, thus leading to so-called laborynth and jig-saw type reservoir structures. Here, we consider physical systems - both reservoirs and experimental packs where the stratified model is still valid. However, we also wish to consider the extension of the stratified reservoir concept when sub-layer heterogeneity is present which, as we note above, is a first step towards modelling a realistic system. In particular, we wish to consider the question: how particular, we wish to consider the question: how important is it to describe this sublayer heterogeneity compared with the overall description of the larger scale stratification? In fact, without further qualification, this question is not very meaningful since we must consider it in the light of a particular fluid recovery process in that reservoir system. In other process in that reservoir system. In other words, the importance of any level of reservoir heterogeneity - including the larger scale layering - must pertain to the fluid displacement or oil recovery pertain to the fluid displacement or oil recovery mechanism that is operating. Indeed, not only is the fluid recovery process itself important in considering the relevance of questions about heterogeneity, but the balance of forces may also be very important. That is, the relative importance of viscous gravitational and capillary forces may also dictate the level of heterogeneity description that is necessary in order to model a given process. In the work presented in this paper, we examine some specific fluid flow processes in paper, we examine some specific fluid flow processes in layered systems as described below. The scaling from experiment to field systems is first considered and then the effects of heterogeneity are examined using numerical simulation.
Many experimental and simulation studies have appeared over the years which discuss various aspects of flow in layered systems.
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