An experimental and numerical investigation of compositional displacements in heterogeneous reservoirs

Abstract

Compositional displacements in porous media occur when there is mass transfer between multiple fluids. A good example of such flows is gas injection to an oil reservoir for enhanced oil recovery. Injected gas can be miscible with oil at first contact or at multiple contacts or can stay nearly immiscible with oil. This depends on how reservoir pressure compares with so-called minimum miscibility pressure. Miscible gas injections often achieve high microscopic recovery and occur compositionally, creating zones of varying interfacial tension between oil and gas along the mixing zone. However, the macroscopic efficiency of gas injection at reservoir scale remains much lower, being controlled by gravity segregation, viscous fingering and reservoir heterogeneity.

The purpose of this thesis is to investigate the combined effects of driving forces on non-equilibrium compositional displacements in dual- and triple-permeability porous mediums. Glass-bead packs of different uniform sizes were used to construct a quasi 2-D porous media. An analog ternary fluid system (isooctane, isopropanol and brine) was adapted to allow controlling interfacial tension between the phases at ambient laboratory conditions. Recovery of phases, compositional analysis, pressure drop and snapshots of saturation distributions during each experiment are reported in this thesis. Experimental observations were interpreted by black-oil numerical simulations.

This thesis reveals that heterogeneity has more pronounced effect on recovery under immiscible conditions than miscible conditions. This is caused mainly by capillary forces dominating the flow in immiscible conditions. In miscible floods, viscous and gravity forces govern the flow. This affects recovery moderately although the patterns of displacement fronts are changed significantly. Several recent numerical studies have suggested that, in gravity drainage projects, multi-contact displacements may yield higher recovery than first-contact miscible floods. Relevant experimental results reported in this thesis confirm this observation.

Force-balance scaling criteria (i.e., capillary, gravity and Bond numbers) are used to diagnose flow regimes during displacements. Laboratory results demonstrate that these criteria successfully describe low- and high-rate immiscible, low-rate multi-contact miscible and high-rate miscible displacements. Experimental results also suggest that, in miscible displacements, flow profiles follow the patterns depending on the mobility ratio and critical rate, as reported in the literature.