Analysis Of One-Dimensional Rich Gas Displacements With An Equation Of State Simulator
Authors: Sibbald L.R. , Novosad Z. , Costain T.G. 
Published: March 2010

Injection of enriched natural gas is widely applied in Alberta as a means to effect a miscible displacement process for improved oil recovery. Many such gas miscible projects are ongoing. The level of natural gas enrichment required for dynamic miscibility between solvent and reservoir fluid is most often determined in one dimensional laboratory displacements, i.e. slim tube tests. In order to best translate results from these idealized displacements into successful field applications, the mechanism of the mass transfer responsible for the high oil recovery achieved in these rich gasfloods must be understood. The variety of interpretations presented in the recent literature indicates that, in fact, the displacement process is not well understood.

In this paper the authors outline the results of a study undertaken to characterize miscibility development for a typical rich gasflood system using an equation of state simulator. The simulator confirms that a frontal or "forward contact" mechanism, not the traditional condensing or "swept zone contact" mechanism is the pertinent process for this system. Also, the comparison of simulated 1-D displacements (slim tube experiments) and multiple phase contacts (analogous to rising bubble apparatus tests) indicates that the same basic mechanism applies to either miscibility testing technique. The insights gained from the study give increased confidence in the ability to make a meaningful interpretation of laboratory miscibility experiments.

It is felt that investigation of the miscibility development process is necessary for the achievement of a better understanding of miscible displacement in the reservoir.

Introduction

There has been much written recently regarding the mechanisms leading to the development of miscibility in rich gas/crude oil systems. The papers by Zick(l) and Novosad, et al(2.3) challenged the long-held concept that miscibility development involved the condensing process(4) for all rich gas drives. Since these contributions, there has been a renewed study of the field using compositional simulators(5,8). Each of these studies revealed that miscibility could develop by a process other than the condensing mechanism.

This topic was also the subject of this investigation which used an equation of state (EOS) simulator. An EGS simulator allows the examination of the effects of a model displacement process (e.g. slim tube) on the phase behaviour that describes miscibility development. The basic goal of this work was to examine this process for the case of an Alberta crude oil. Of special interest was the nature of the process in the situation of a l-D displacement of oil by solvent. Also, the developed phase behaviour predicted by simulations of 1-D displacements was compared to that from batch-type or discrete multiple phase contact calculations.

Although some examination of the effects of discretization and the choice of relative permeability relationships was undertaken, a full investigation of the potential of simulation to predict slim tube results was not made. The purpose of the study was examination of trends in miscibility development phase behaviour and associated component transfer as functions of rich gas solvent enrichment level. The insights that this type of investigation reveals have important relevance to the field of solvent design for rich gas miscible floods.