Enriched Gas Displacement: Design Of Solvent Compositions
Authors: Wu R.S. , Batycky J.P. , Harker B. , Rancier D. 
Published: March 2010

It is essential for a successful hydrocarbon miscible project that the design of the miscible solvent composition be accurate so as to balance technical and economic constraints. When a single solvent is under consideration, laboratory experiments alone are adequate. But when a range of solvents must be considered or when the compositions of the make-up streams can vary, a reliable predictive method becomes invaluable.

A methodology has been developed for accurate and rapid design of a miscible solvent composition. Using an adequately "tuned" equation-of-state for predicting phase compositions at a design pressure and temperature, the proposed technique presents a unique method for constructing a pseudoternary diagram that incorporates both solvent and oil properties. Unlike other procedures which require specification of a solvent composition to generate the phase behaviour for each solvent being examined, this method requires only that the composition of LPG and dry gas streams which combine to form the solvent be specified, thus allowing the final composition to be determined explicitly. The proposed procedure has been applied in three pools. The success of this new technique for predicting solvent gas compositions has been verified experimentally.

Background

In its simplest form, a ternary diagram can be used to uniquely represent the phase behaviour resulting when three components are combined in various proportions, either experimentally or using equation-of-state calculations. Because reservoir fluids are comprised of numerous components, it is necessary for the purpose of visual representation to group components so that the ternary diagram represents the phase behaviour of three pseudocomponents. The components are grouped according to the physical properties of each component at the reservoir temperature and operating pressure. For a miscible hydrocarbon process, the pseudocomponents are generally dry gases, intermediates or LPGs and heavy ends. While the resulting diagrams can be useful for developing qualitative understanding of process behaviour, their application in specific situations requires that they must be constructed for that specific need. This paper is intended to indicate limitations inherent in and a suitable method for developing pseudotenary representations of the enriched gas or hydrocarbon miscible gas process. The proposed construction method is both accurate and rapid. A typical pseudoternary diagram is pictured in Figure 1.

Although the grouping of components into pseudocomponents is somewhat arbitrary, the location of phase boundaries and tie lines joining equilibrium gas and liquid compositions are accurately determined either experimentally or analytically. The graphical pseudoternary representation merely enables the calculation to be plotted for visualization purposes.

The solvent is assumed to be obtained by blending specific dry gas and LPG liquid streams which are proportioned to satisfy a miscibility criterion with a given reservoir oil at reservoir temperature and a design pressure. For simplicity, with reference to Figure 1, the LPG components (basically intermediates) will all be contained in the upper vertex and the dry gas stream will be contained in the lower left vertex. Thus, all possible solvents that can be determined by blending dry gas and LPG are represented by compositions lying on the left side axis of the diagram.