Equilibrium Phase Compositions of CO2/Hydrocarbon Mixtures Part 1: Measurement by a Continuous Multiple-Contact Experiment

A new experimental technique that simultaneously measures compositions and densities of two phases in equilibrium is described. Because it operates continuously, the experiment can be performed more rapidly than conventional static-equilibrium measurements. Details of the experimental apparatus are reported, and results of test displacements for two simple CO2/hydrocarbon systems are compared with static-equilibrium phase composition measurements for the same systems. A simple analysis of the operation of the experiment is used to assess the experimental error that results from the continuous nature of the experiment and to suggest ways to reduce that error. Application of the experimental technique to a CO2/crude oil system is reported in a companion paper. Introduction It is by now well documented that phase behavior of CO2/crude oil mixtures has an important impact on CO2-flood-displacement efficiency. As a result, a variety of reservoir simulators applicable to CO2 flooding attempt to calculate compositions and fluid properties of phases that occur during the displacement process. However, the equations of state (EOS) or K-value correlations used to calculate the distribution of components between phases in those simulators often do not yield predictions accurate enough phases in those simulators often do not yield predictions accurate enough to be used without some experimental verification. The correlation parameters that describe the pseudocomponents used to represent the crude parameters that describe the pseudocomponents used to represent the crude oil are rarely known well enough to guarantee accurate a priori predictions. Usually, some sort of adjustment of input parameters is predictions. Usually, some sort of adjustment of input parameters is required to achieve an acceptable match of experimental phase-behavior data. Volumetric data, obtained from observations in a visual cell of binary mixtures of CO2 with crude oil at various pressures, are often used to make that adjustment. A few investigators have also reported measurements of phase compositions, though problems with sampling and analysis of high-pressure mixtures make such experiments difficult to perform. Phase-composition data provide a more rigorous test of the perform. Phase-composition data provide a more rigorous test of the predictions of a phase-behavior calculation than volumetric data and hence predictions of a phase-behavior calculation than volumetric data and hence are desirable. In addition, simultaneous fluid property data would be useful for testing predictions of phase densities and viscosities and for direct application in design of some processes--for instance, those that rely on gravity segregation to reduce the adverse impact of viscous instabilities. Unfortunately, published examples of simultaneous measurements of phase compositions and fluid properties are rare. The time and equipment required to make the mixtures, obtain samples and analyze them, and finally determine their viscosities and densities are sufficient to make such measurements unattractive for routine support of field projects. projects. SPEJ p. 272