Prediction of Miscibility

Abstract Recent studies of the mechanisms involved when a reservoir fluid isdisplaced miscibly by a light hydrocarbon mixture have resulted in thedevelopment of theoretical methods to predict the approximate conditionsnecessary in order that miscibility may occur. One of these methods employs theuse of a pseudo-ternary diagram and requires the calculations of the criticaltemperature and pressure of both the displacing and displaced phases. Preliminary and theoretical studies have been conducted in an attempt toverify the theoretical considerations involved in this particular method aswell as to modify the procedure by employing newer means of calculatingcritical conditions. In conjunction with this study a unique method of recording pressureprofiles as a function of time during an actual miscible displacement has beendevised. This paper presents and discusses some of the results obtained in thisproject. Introduction Miscible displacement of reservoir oil depends upon the formation of agraded saturation bank between the original oil in place and the injecteddisplacing fluid. This bank is all in a single phase with no interfaces presentand therefore they are no capillary forces to cause the oil to be retained inpore channels as residual oil. This situation can be achieved by employing oneor more different methods; namely solvent flooding, LPG slug process, highpressure gas drive or condensing gas drive. Three of the factors which influence the efficiency of miscible drives arethe reservoir temperature and pressure and the composition of the injectedfluid. A certain pressure for a given temperature must prevail to maintainmiscible conditions between oil and injected fluid. Proper requirements forpressure control of the reservoir must therefore be known in advance ofcommencement of a project for properly controlled miscible drive operations.The intermediate composition of the injected gas must also be controlled suchthat the composition will be a critical one and miscible drive can beachieved.