Abstract

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

Introduction

Within the literature there are many correlations (1–4) available to predict CO2miscibility pressure, However, these correlation do notadequately reflect oil composition and oil characterization. From previousexperience in hydrocarbon miscible solvent design (5), at any temperature thecontribution of each hydrocarbon component is different so hat solventmiscibility is a function of the oil composition. To overcome the limitationsof the current approaches, a new technique is proposed for predicting CO2 miscibility pressureswhich uses a "tuned" equation of slate forprediction, The advantages of the technique are that cursory assessments onapplying CO2 can be made inexpensively and the number of slim tubetests needed to verify prediction can be minimized, This paper describes thenew technique and documents all slim tube test data obtained to date such asoil recovery, pressure drop, phase observations and the effluent gascompositions required to experimentally determine miscibility conditions.

Reservoir Fluids

Table 1 gives the five oil compositions form the four pools examined in thisstudy. The specific gravity and molecular weight of the C7+ fraction which were used in the equation of statecharacterization (6) are also shown in the table. The range of temperaturebeing tested is from 21 to 78 ° C and the bubble point pressure changes from3490 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 ternarydiagram construction technique (7) for predicting miscible solvent gascompositions. That technique is based on blending tow hydrocarbon streams, drygas (DG) and LPG, at a specified design pressure and reservoir temperature todetermine the correct solvent composition. Since either pure CO2 orCO2 that contact impurities represents only a single identifiablestream, it is more difficult to identify separate stream compositions needed toconstruct the pseudo ternary diagram using an approach similar to that forhydrocarbon miscible processes. Based on an examination of the problem, arecommended approach for selection of the second stream needed for ternarydiagram construction is as follows:

  1. As a logical choice, the CO2-rich stream (Either pure CO2 or CO2 with impurities) is selected to be equivalentto a dry gas stream. This includes and CI and N2 impurities in theCO2 stream.

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

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