A new equation of sate-based technique for predicting minimum miscibility pressures for CO2 cured old systems has been developed. To verify the technique, a total of 33 slim tube displacement tests were conducted. Reservoir fluids from four different pools were included in the testing. Slim tube displacement test results included measurement of oil recovery, pressure drop, phase observations and effluent gas compositions, all of which were required to infer the experimental miscibility conditions. The application of the new technique successfully matched the experimental results over a wide range of reservoir temperature, 21.1 ° c to 78.3 °c, and fluid bubble point pressures, 3490 kPA to 17330kPa.


Within the literature there are many correlations (1–4) available to predict CO2miscibility pressure, However, these correlation do not adequately reflect oil composition and oil characterization. From previous experience in hydrocarbon miscible solvent design (5), at any temperature the contribution of each hydrocarbon component is different so hat solvent miscibility is a function of the oil composition. To overcome the limitations of the current approaches, a new technique is proposed for predicting CO2 miscibility pressureswhich uses a "tuned" equation of slate for prediction, The advantages of the technique are that cursory assessments on applying CO2 can be made inexpensively and the number of slim tube tests needed to verify prediction can be minimized, This paper describes the new technique and documents all slim tube test data obtained to date such as oil recovery, pressure drop, phase observations and the effluent gas compositions required to experimentally determine miscibility conditions.

Reservoir Fluids

Table 1 gives the five oil compositions form the four pools examined in this study. The specific gravity and molecular weight of the C7+ fraction which were used in the equation of state characterization (6) are also shown in the table. The range of temperature being tested is from 21 to 78 ° C and the bubble point pressure changes from 3490 to 17,330 kPa for Oil-1 and Oil-4 crude respectively.

New Prediction Technique

The procedure used in this new technique is an extension of the pseudo ternary diagram construction technique (7) for predicting miscible solvent gas compositions. That technique is based on blending tow hydrocarbon streams, dry gas (DG) and LPG, at a specified design pressure and reservoir temperature to determine the correct solvent composition. Since either pure CO2 or CO2 that contact impurities represents only a single identifiable stream, it is more difficult to identify separate stream compositions needed to construct the pseudo ternary diagram using an approach similar to that for hydrocarbon miscible processes. Based on an examination of the problem, a recommended approach for selection of the second stream needed for ternary diagram construction is as follows:

  1. As a logical choice, the CO2-rich stream (Either pure CO2 or CO2 with impurities) is selected to be equivalent to a dry gas stream. This includes and CI and N2 impurities in the CO2 stream.

  2. The second stream, which in the hydrocarbon miscible process is the LPG composition. Is made up of intermediates in the oil itself.

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