New Approach in Gas Injection Miscible Processes Modelling in Compositional Simulation
Authors: Shtepani E. , Thomas F.B. , Bennion D.B. 
Published: March 2010

A number of factors must be considered in the design of miscible displacement processes. This paper discusses a new approach that relates primarily to the laboratory and modelling studies that precede the EOS-based compositional simulation of reservoir performance during the vapourizing/condensing gas drive process.

The phase behaviour of a solvent/oil system and determination of miscibility conditions by various special PVT experiments (including the swelling test, RBA, slim tube test, and the continuous multiple-contact experiment) are reviewed along with their importance in building an accurate EOS model to be used in compositional simulation. In addition to experimental PVT data, a special core flow test design for measuring the relative permeabilities to generated fluids by forward/reverse multiple contact experiments is discussed.

Based on laboratory PVT and SCAL data, a novel interfacial tension-dependent model of relative permeability and capillary pressure data is presented along with the advantages if incorporated into commercial EOS-based compositional simulation software packages.

Introduction

Mass transfer between the gas and oil components dominates the displacement characteristics of miscible or near-miscible floods. The overall displacement efficiency of any oil recovery displacement process can be considered conveniently as the product of microscopic and macroscopic displacement efficiencies. In equation form:

where E = overall displacement efficiency, ED = microscopic displacement efficiency, and EV = macroscopic (volumetric) displacement efficiency.

Microscopic displacement relates to the displacement or mobilization of oil at pore scale. That is, ED is a measure of effectiveness of the displacing fluid in moving (mobilizing) the oil at those places in the rock where the displacing fluid contacts the oil. ED is reflected in the magnitude of the residual oil saturation, Sor, in the regions contacted by the displacing fluid.

Macroscopic displacement efficiency relates to the effectiveness of the displacing fluid(s) in contacting the reservoir in a volumetric sense. EV is a measure of how effectively the displacing fluid sweeps out the volume of a reservoir, both areally and vertically, as well as how effectively the displacing fluid moves the displaced oil toward production wells. It is desirable in an Enhanced Oil Recovery (EOR) process that the values of ED and Ev, and consequently E, approach 1. An idealized EOR process would be one in which the displacing fluid removed all oil from the pores contacted by the fluid (Sor →0). Several physical/chemical interactions occur between the displacing fluid and the oil that can lead to efficient microscopic displacement (low Sor). These include miscibility between the fluids, decreasing the interfacial tension (IFT) between the fluids, oil volume expansion, and reducing oil viscosity. The maintenance of a favourable mobility ratio between displaced and displacing fluids also contributes to better microscopic displacement efficiency. EOR processes are thus developed with consideration of these factors. The goal of an acceptable EOR fluid is to maintain favourable interaction(s) as long as possible during the flooding process.

In enhanced recovery operations, oil entrapment occurs due to complex interactions between viscous, gravity, and capillary forces.