A Generalized Methodology for Minimum Miscibility Pressure

Abstract

Abstract This paper presents a generalized methodology for predicting the MMP required for the multi-contact miscible displacement of crude oil systems by gas injection. The approach is based on applying the Peng and Robinson Equation of State; in a modified form, in conjunction with a newly introduced "Miscibility Function". The miscibility function is designed to provide accurate predictions of the MMP. It is the objective of this paper 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 is illustrated by matching several experimentally measured minimum miscibility pressure values. Introduction The displacement efficiency of oil by gas 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 the MMP for a given crude oil. The minimum miscibility pressure is defined as the pressure at which the oil recovery vs. pressure curve (as generated from the slim-tube test) shows a sharp change in slope, i.e. the inflection point. 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 al. pointed out that, ideally, any correlation 1) should account for each parameter known to affect the MMP; 2) should be based on thermodynamic or physical principles that affect the miscibility of fluids, and finally; 3) should be directly related to the multiple contact miscibility process. The MMP's correlations fall into two categories. The first category is dedicated to pure and impure CO2 while the other category treats the MMP's of other gases. The following section briefly reviews some of these correlations. A) Pure and Impure CO2 MMP It is well documented that the development of miscibility in a CO2/crude oil displacement is the result of extraction of some hydrocarbons from the oil by dense CO2. Orr and Silva 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.Orr and Silva: 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. The carbon- number distributions of the crude oil system are the only data needed to use the correlation. The authors introduced a weighted composition parameter that is based on partition coefficients of C2 through C37 fractions. The specific steps of the method are summarized below:From the chromatographic or the simulated compositional distribution of the crude oil; omit C1 and all the nonhydrocarbon components from the oil composition.