Development of More-Efficient Gas Flooding Applicable to Shallow Reservoirs

Abstract

ABSTRACT:The objective of this research is to widen the applicability of gas flooding to shallow oil reservoirs by reducing the pressure required for miscibility using gas enrichment and increasing sweep efficiency with foam. Task 1 examines the potential for improved oil recovery with enriched gases. Subtask 1.1 examines the effect of dispersion processes on oil recovery and the extent of enrichment needed in the presence of dispersion. Subtask 1.2 develops a fast, efficient method to predict the extent of enrichment needed for crude oils at a given pressure. Task 2 develops improved foam processes to increase sweep efficiency in gas flooding. Subtask 2.1 comprises mechanistic experimental studies of foams with N 2 gas. Subtask 2.2 conducts experiments with CO 2 foam. Subtask 2.3 develops and applies a simulator for foam processes in field application.Regarding Task 1, significant research was accomplished in the last three and one-half years with over 11 journal and conference publications, many of which have been repeatedly referenced by other researchers. All goals, except one, were accomplished for both Task 1.1 and Task 1.2. The remaining goal on the development of accurate CO 2 MMP correlations is near completion and will be published in a future paper. Task 1.1 focused on the effect of dispersive processes on oil recovery and how to enrich gas so that the MMP can be reduced. Simulation studies (Solano et al., 2001; 4 Stalkup, 1990) show that oil recovery for enriched gas drives depends on the amount of dispersion in reservoir media, but the value of dispersion, expressed as dispersivity, at the field scale, is unknown. This research investigated three types of dispersion in permeable media to obtain realistic estimates of dispersive mixing at the field scale. These types are echo dispersivity, which is unaffected by fluid flow direction, transmission dispersivity, which is caused by layering in permeable media, and local dispersivity, which is the mixing observed at a point in the reservoir as tracer flows past.We analyzed a collection of single-well tracer test (SWTT) data to estimate echo dispersivity at the SWTT scale. The estimated echo dispersivities closely match a published trend with length in dispersivities obtained from groundwater tracer tests. This unexpected result-it was thought that transmission dispersivity should be greater than echo dispersivity-is analyzed with numerical simulation. The agreement between the SWTT echo dispersivities and the field trend suggests that the field data are measuring local dispersivities, which are on the order of 2 ft. All dispersivities appear to grow with length.The interaction of phase behavior and flow, in the presence of dispersive mixing, was examined for 1-D four-component and twelve-component enriched-gas displacements. Numerical and analytical solutions demonstrated that oil recovery may be substantially increased by gas enrichment above the minimum enrichment for miscibility (MME). For the twelve-component oil, the increase in recovery (displ...