Design Of Miscible Solvents For A Rich Gas Drive-Comparison Of Slim Tube And Rising Bubble Tests
Authors: Novosad Z. , Sibbald L.R. , Costain T.G. 
Published: March 2010

Abstract
This study addresses miscible solvent design for the planned hydrocarbon flood in the Virginia Hills Beaverhill Lake Unit in north central Alberta. The development a/miscibility between reservoir fluid and a series of increasingly rich solvents was studied in a rising bubble apparatus. In order to establish the validity of this relatively new technique of solvent design, the rising bubble test observations were compared to results of conventional slim tube tests previously conducted with the same soh1ents. A very good correlation was found between the minimum solvent enrichment/or miscibility determined by the two techniques. The rising bubble method is shown to be not only much faster but also superior in some respects to slim tube testing.

Introduction
The Virginia Hills Beaverhill Lake reservoir, a carbonate reef formation in north central Alberta, was discovered in 1957 and has been under water flood since 1963. Of the original estimated 60 × 106 m3 oil in place, about one-third has been recovered to date. Current plans are for implementation of a miscible enriched gas flood, with solvent injection targeted for 1990(1). The relatively high reservoir temperature (100 °C) and intermediate pressure (20 MPa to 27 MPa) are common to several Alberta pools where miscible gas injection is currently underway. The Virginia Hills development is a joint venture project with Shell Canada as the operator.
The annual cost of the solvent (a blend of dry gas and natural gas liquids streams) required by the project is estimated to exceed $30 million. It is thus important to design the solvent containing the minimum amount of expensive natural gas liquids (NGL) yet be miscible so as to maximize the oil recovery. As the composition of the NGL stream available from the pipeline is expected to vary throughout the miscible flood project life, solvent enrichment levels have to be determined for a variety of NGL stream compositions. Given the time consuming and costly nature of the slim tube tests (and our unwillingness to rely entirely on minimum solvent enrichment predicted with the equation of state), the rising bubble apparatus (RBA) tests present an attractive alternative. However, while the slim tube test is universally accepted by the industry, the validity of this more recently emerged method of solvent design has to be established. A systematic comparison of the two techniques was therefore undertaken, the results of which are presented in this paper.
Equation of state (EOS) calculations were used throughout the study to interpret, in qualitative terms, the oil-solvent phase behaviour and its trends.

Background - Mass Transfer Mechanism in Virginia Hills Miscible Rich Gas Drive
Table 1 shows the composition of the Virginia Hills recombined reservoir fluid (VH fluid). This is a light but somewhat waxy crude containing about 3070 (wt) asphaltenes. The compositions of typical dry gas and NGL streams available for blending the solvent are also shown in Table 1. Throughout this paper, the solvents are referred to by their NGL to dry gas molar ratio; 75/25 solvent thus contains 750/0 NGL and 250/0 dry gas.