Abstract

When a reservoir oil is miscibly displacedfrom an isotropic horizontalreservoir by a less dense and less viscous solvent, the displacement process ispotentially unstable. Due to gravity the more dense reservoir oil will tend toslump under the less dense solvent thereby triggering unstable displacement. Aviscous finger of solvent overrides the oil and advances at an acceleratingrate along the top of the reservoir.

Molecular diffusion in the, case of thin reservoirs tends to create a favorablezone of graded viscosity While retaining miscibility, but, in a thick reservoirdiffusion may reduce the solvent slug composition to a level. where miscibilityis lost.

This paper describes a method of calculating, for the simple steady stateradial case, the Vertical profile of the solvent flood front at any particulartime step. Gravitational, viscous and diffusional effects are accounted for inthe analysis of the movement of the displacement front.

Introduction

Most field applications of horizontal miscible flooding of oil reservoirsinvolve the injection of a small bank of LPG solvent driven by gas. Theviscosity of the solvent is much lower than, that of the oil resulting in anunfavorable mobility ratio. Similarly, the mobility ratio -between the, duringgas and the solvent bank is unfavorable. The instability of the displacementprocess has been reported and discussed by severalauthors.1–6Perrine and Gay7derived an approximateequation for the motion of the override of free surface for the case of adisplacing phase continuously missile with the reservoir oil.

Under adverse mobility ratio conditions slight changes in the reservoir rockcharacteristics or gravitational forces can cause the formation of viscousfingers. Once a viscous finger is formed it will tend to grow at anaccelerating rate thereby resulting in rapid breakthrough of the displacingfluid into the producing wells and inefficient sweep of the reservoir. Typicalprofiles of the solvent oil and gas-solvent forms in an isotropic, single layered system are shown in Fig. 1.

The purpose of this paper is to describe a method of calculating the verticalprofiles of the solvent-oil and gas-solvent fronts, assuming that the front atany specified vertical level in the formation has advanced radially as shown bythe dotted lines on Fig. 1. This investigation will be confined to describingthe fronts AB and CD on Fig. 1 Irregularities in the radial displacement frontdue to viscous fingering, in the horizontal plane are not considered.

Density differences between displaced and displacing fluids causes gravityinduced tilting of the interfaces. A viscous finger of solvent overriding oilalong the top of the reservoir is thereby formed, followed by a viscous fingerof gas overriding solvent. These fingers grow at accelerating rates largelybecause of the differences in viscosity and density between the displaced anddisplacing fluids. During the early stages gravity forces play a major role inthe growth of the override viscous fingers, but as the flood progresses gravitybecomes less important and viscous forces predominate as the propagatingmechanism.

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