A Comparative Study of RBA, P-x, Multicontact And Slim Tube Results
Authors: Thomas F.B. , Zhou X.L. , Bennion D.B. , Bennion D.W. 
Published: March 2010

Abstract
The Rising Bubble Apparatus (RBA) is beginning to gain acceptance as an efficient method to Measure Minimum Miscibility Pressure (MMP) and Minimum Miscibility Composition (MMC)(1,2). Its application has been limited in the past to a few oils, This paper presents applications of RBA technology to a more broad cross-section of reservoir fluids with a variety of injection gases including N2, CO2 and hydrocarbons. The accuracy of the REA is discussed and compared to more conventional P-x diagram, multi-contact phase behavior and slim tube results on a number of oil-solvent systems.
Results indicate that the REA can be used to estimate MMP. It has performed well compared to the slim tube testing performed in this work. The RBA usually provides conservative estimates compared to slim tube procedures. From a time and cost perspective, the RBA is significantly more efficient than the slim tube. The limitations of the REA are that there is some subjectivity associated with the MMP interpretation and very little quantitative information (changes in composition, IFT. displacement efficiency) is provided.

Introduction
Miscible flooding was invented in the 1950's. The first contact miscible (FCM) process was described by Koch(3) and Hall et al.(4). The high pressure vaporizing miscible process was discussed by Whorton et al.(5,6&7) and the condensing mechanism was described by Stone et al.(8) and by Kehn(9). These different processes have been implemented in reservoirs throughout the world.
Before applying these techniques in the field, laboratory data is usually generated to determine phase behavior effects, influence of water saturation, micro-scale heterogeneities and overall process efficiency. Several methods have been developed for determining phase behavior, three of which will next be discussed.

Standard Tests
Over the past forty years, three laboratory tests have been used repeatedly for inferring miscibility limits. These are the ternary diagram, pressure-composition diagram (P-x) and the slim tube test. They are ubiquitous in the miscible flooding literature(10) their operation and properties are not discussed herein. Recently their ability to predict miscibility has received some criticism and the References(11, 12, 13, 14, 15) review some of these concerns. Along with the enhancements in these laboratory tests, a novel design incorporating aspects of the visual cell and slim tube test was developed by Christiansen and Kim(l6). The Rising Bubble Apparatus (RBA) combines the size of the visual cell with the dynamic nature of the slim tube test. It therefore represents a hybrid of the two and all indications are that it makes miscibility determinations much more efficient than how they were previously performed. Huang(17) describes a study based on slim tube technology, whereas Poettmann(18) discusses RBA operation.
This paper discusses a series of oil-solvent systems where slim tube data, P-x diagrams and multi-contact data are available along with RBA testing, The next section compares the results of these tests, comments on and explains the differences.
FIGURE 1: Conventional MMP plot. (Available in full paper)

Comparative Analysis of Slim Tube, P-x Diagram, Multi-contact and RBA Data Sets
Before proceeding to report results for different oil systems from the different experimental apparatus, one term requires some attention.