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Abstract

Early water breakthrough can be a serious problem during waterflooding of heterogeneous reservoir formations. One possible remedy to this problem is to place a gel block in the high-permeability layer, thus diverting displacing brine into the less-permeable layers in order to sweep the remaining oil from these zones. In such a treatment, the gelant material must be placed in the correct location within the reservoir so that gel does not impair reservoir performance. In this paper, we study the dynamics of gel placement in heterogeneous (stratified) reservoir systems. The details of the gel placement are strongly affected by the level of communication between reservoir layers, which is characterized by the closeness of the system to vertical equilibrium (VE) conditions. We show that in viscous-stable injection of gelant in systems close to vertical equilibrium, considerable volumes of injected material can crossflow into the low-permeability layers, and subsequent gel formation can seriously reduce the performance of the continuing waterflood. Results from a range of experimental displacements in well characterized layered beadpacks are presented, along with supporting numerical simulations, which help to understand the mechanisms and benefits when performing gel treatments in reservoir systems with free crossflow. The central role of viscous crossflow in such systems is demonstrated. Since we consider only viscous forces in this work, the layered experimental packs are scaled only by the viscosity ratio (displacing to displaced), the geometry of the packs, the aspect ratio and the degree of vertical communication (closeness to VE). Thus the conclusions from the experimental and simulation results are directly applicable to similarly scaled viscous-dominated systems at the reservoir scale. Some analysis is also presented of the mechanism of disruption of slugs by viscous fingering in layered systems.

Introduction

The objective of gel treatments in injection wells is to reduce flow through fractures or high-permeability zones while diverting injected fluids into hydrocarbon-bearing strata. In oil production wells, the main objective of gel treatments is to reduce water production without significantly reducing oil production. Achieving these objectives may be impeded by the formation of gel material in less-permeable, oil-productive zones. If gel treatments are to improve sweep efficiency, a pathway must be available between the wellbore and mobile oil in the formation. This can sometimes be accomplished by mechanically isolating zones during the gel treatment. However, zone isolation will not be effective if extensive crossflow can occur between reservoir layers of contrasting permeability (or possibly, if flow can occur behind pipe).

Much of the previous work concerning gel placement has focused on gel treatments in reservoirs with no communication between zones.

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