Abstract

This paper presents a practical and simple procedure for determining the Minimum Miscibility Pressure "MMP" required for the multi-contact miscible displacement of hydrocarbon systems by gas injection. The methodology is based on applying the Peng and Robinson Equation of State; in a modified form, in conjunction with a newly introduced "Miscibility Function". The mathematical form of the miscibility function is designed to provide with the necessary criteria for predicting the required injection pressure in miscible gas injection. The objective of this paper is to demonstrate how this miscibility function can be used to determine the necessary conditions required for miscible displacement of hydrocarbon systems by gas injection. The validity and the use of the proposed methodology are demonstrated by matching several experimentally measured minimum miscibility pressure values.

Introduction

The displacement efficiency of crude oil systems by gas injection is highly pressure dependent and miscible displacement is only achieved at pressures greater than a certain minimum. This minimum pressure is called the Minimum Miscibility Pressure "MMP". Slim-Tube displacement tests are commonly used to determine an MMP for a given crude oil. The minimum miscibility pressure is defined as the pressure of which the oil recovery vs. pressure curve (as generated from the slimtube test) shows a sharp change in slope, i.e. the inflection point. This minimum miscibility pressure is a strong function of temperature, composition of the crude oil system, and composition of the injection gas.

To facilitate screening procedures and to gain insight into the miscible displacement process, many correlations relating the MMP to the physical properties of the oil and the displacing gas have been proposed. Enick, et al1 pointed out that any correlation should:

1. account for each parameter known to affect the MMP, i.e. temperature, composition of the displacing and displaced fluid;

2. be based on thermodynamic or physical principles that affect the miscibility of fluids, and finally;

3. be directly related to the multiple contact miscibility process.

In general, all the published miscibility correlations can be divided into two categories; the first category deals with predicting the minimum miscibility pressure for pure and impure CO2; while the other category treats the MMP's of all other type of gases.

A) PURE AND IMPURE CO2 MMP:

It is well documented that the development of miscibility in a CO2 - crude oil displacement process is the result of extraction of some hydrocarbons from the oil by dense CO2. Orr and Silva2 stated that there is considerable evidence that the extraction of hydrocarbons from a crude oil is strongly influenced by the density of CO2. Improvement of extraction with the increase in CO2 density that accompanies increasing pressure accounts for the development of miscibility. The presence of impurities can affect the pressure required to achieve miscible displacement.

1. Orr and Silva2: The authors developed a methodology for determining the MMP for pure and contaminated CO2 - crude oil systems. Orr and Silva pointed out that the distribution of molecular sizes present in a crude oil has a significantly larger impact on the MMP than variations in hydrocarbon structure.

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