Effect Of Multiple-Contact Miscibility On Solvent Slug Size Determination

Abstract

Abstract It is a common practice to apply the dispersion theory to simulate the idealized miscible displacement process. In applying this theory, first-contact miscibility is assumed to calculate the solvent slug size required for a successful miscible flood. For a multiple-contact miscible process such as condensing drive however, additional solvent is required because:the composition of the reservoir oil needs to be enriched to a level such that miscibility between the solvent and the enriched-oil' can be achieved;a significant amount of solvent is required to offset dilution effects and ensure that at least a thin bank of "enriched-oil" (in the solvent-'enriched-oil?-reservoir oil mixing zone) exists to maintain miscibility throughout the displacement. Despite the importance of these factors, they are seldom considered while calculating solvent slug size requirements. The main objectives of this paper are to:explain clearly how multiple-contact miscibility (condensing drive only) affects solvent slug size requirements;develop a procedure and provide standard formulas using the concept of dispersion and phase behaviour, to determine solvent slug size required for multiple-contact miscible floods. An example was used to illustrate the calculation procedures. Introduction Miscible displacement of oil may be achieved with a solvent that is in first-contact miscible with the reservoir oil, or a solvent that either condenses or vapourizes intermediate-molecular- weight components into or out of the oil, thereby establishing multiple-contact miscibility (MCM). To ensure that this behaviour will exist in the reservoir, phase behaviour studies and laboratory tests are conducted to decide on the solvent composition. The next step in the design process is to calculate the amount of solvent that must be injected to maintain miscibility for the duration of the displacement process. Generally, conventional methods of slug size determination for all of these processes account only for the effects of diffusion and dispersive mixing. However, the field performance of projects using the MCM process have indicated that large safety factors must be applied to account for other factors besides the dilution of the solvent 50 that the technical success of a project is ensured. This paper investigates the effect of multiple-contact miscibility (condensing process only) on solvent slug size determination. The processes of first-contact and multiple-contact miscible displacements, as well as the conventional method of slug size determination using diffusion and dispersion theory are reviewed. Then a simplified model of the MCM process is presented to help calculate the additional solvent required to sufficiently enrich the oil through a condensation process such that miscibility will be maintained throughout the life of a project. The effect that the degree of the enrichment of the solvent has on this calculation is also discussed, and an example of the calculation method using hypothetical data is included. Review of Miscible Displacement Processes First-Contact Miscibility (FCM) Two fluids are determined to be in first-contact miscible if they can be mixed in any proportion and result in only one phase.