Evaluation Of Miscibility From Slim Tube Tests
Authors: Wu R.S. , Batycky J.P. 
Published: March 2010

For a technically and economically successful miscible project, it is important that the solvent composition be as lean as possible for a given design pressure or that the operating pressure be as low as possible for a given solvent composition. In applying slim tube testing to assess miscibility, oil recovery by itself has historically been considered a sufficient criterion. This paper emphasizes that analyzing other test data, such as effluent gas compositions and pressure drop, are likely more reliable because there can be significant experimental errors associated with evaluating recovery.

It is demonstrated that the ambient laboratory condition effluent gas compositions accurately reflect the solvent/oil mixture phase behaviour. In addition, the slim tube pressure drop data can be used to verify miscibility. Thus, by measuring these parameters, one can readily correlate miscibility conditions.

It is also demonstrated that on accurately tuned equation of state together with a reliable prediction technique can significantly reduce the number of slim tube displacement tests required to quantify the miscibility conditions.

Introduction

Economics and solvent supply dictate the selection of miscible gas as an enhanced oil recovery technique.

Miscible solvent design criteria, based mainly on oil recovery levels obtained from slim tube displacement tests, have been described in the literature(1,2) for evaluating miscibility. The purpose of this paper is to emphasize that other measured data such as atmospheric flash condition effluent gas compositions and pressure drops are reliable indicators of miscibility and must be included.

There are generally two approaches to miscibility design. The first requires determining solvent compositions for a given reservoir pressure and is typically applied to hydrocarbon-based processes. The second approach is to predict a minimum miscibility pressure for a specific solvent composition. This technique is appropriate when considering carbon dioxide, nitrogen, or methane as solvents.

An equation of state (EOS), can be used as a predictive tool for miscibility design(3). To use an EOS, it is required that the equation be tuned to accurately match reservoir gas-liquid phase behaviour. That is, the experimentally measured bubble point pressure, (Pb), gas-oil ratio, (GOR), and atmospheric flashed gas and liquid compositions of either a bottom-hole or a recombined test separator reservoir fluid sample must be accurately represented by an EOS. The EOS can then be readily used as basis to predict miscibility conditions.

To verify an EOS prediction, two types of laboratory tests can be conducted: static vapour-liquid equilibria (VLE) measurements and/or dynamic slim tube displacement tests(l). Although interpreting VLE test data is usually straightforward, slim tube displacement tests are often misinterpreted by industry. Hence, it is necessary to review the physics occurring during displacement testing, so as to interpret the data from both the fluid phase behaviour and fluid displacement points of view.

Terminology

Reference to the ternary phase diagram in Figure 1 enables a review of the terminology. This diagram illustrates the phase behaviour existing at a single temperature and pressure. Components are grouped at the three vertices according to whether they are light ends, intermediates, or heavy ends.