Abstract

In many light or moderately viscous oil reservoirs in Saskatchewan and Alberta, a high inactive water saturation zone of varying thickness and extent ("bottom-water zone") occurs in communication with the oil zone above. As a result, the primary production period is short, and water breakthrough occurs very early in the life of the reservoir. Later, during the secondary recovery stage, such a zone can have an adverse effect on the waterflood efficiency. This Paper addresses the problem of waterflooding such reservoirs.

This study was directed towards reducing water mobility in the bottom-water zone for more efficient oil displacement. A commercial simulator (CMG/IMAX) was used to conduct this study. Polymer in various concentrations was used as a blocking agent in the bottom-water zone and as a mobility control agent in the oil zone. Different strategies were investigated to reduce the water mobility in the bottom-water zone and improve the vertical sweep efficiency. The variables examined were: permeability ratio, oil viscosity, relative waterto- oil layer thickness, polymer concentration, injection rate and injection point, as well as the effect of vertical and horizontal injection and production well combinations.

The results showed that minimizing crossflow between layers by blocking the bottom-water zone could increase oil recovery. It was also found that for an unfavorable mobility ratio, as the injection rate increases the ultimate oil recovery increases. The injection of a polymer solution had a favorable impact on waterflood performance. Moreover, the worse the conventional waterflood performance was, the more effective the polymer was as a mobility and blocking control agent. The use of horizontal wells showed slightly better oil recovery over vertical wells in a conventional waterflood of reservoirs under bottom-water conditions. In addition, some certain well combinations (horizontal production and vertical injection) gave better oil recovery due to the increase in the swept area.

Introduction

The efficient and economic recovery of oil from reservoirs under bottom-water conditions is recognized as a formidable task. High water cuts and rapidly decreasing oil rates early in the production life of such reservoirs have in many instances prompted their suspension or abandonment at very low levels of oil recovery. Reservoir characteristics and rock and fluid properties combine to yield the single most important parameter (mobility ratio) in a waterflood. A number of chemicals such as polymers, emulsions, biopolymers, foam and carbon dioxideactivated silica gel have been used to control the mobility ratio.

One of the oldest techniques to control mobility of water in waterflooding is the use of polymers. This control agent was shown to be effective in the early sixties by Pye1. He performed numerous field and laboratory studies of polymer flooding using polyacrylamide solutions. It was observed experimentally that the viscosity of the water-soluble polymer solutions measured in the formation sample departed markedly from that obtained using a viscometer. He quantified the unusual departure of the measured values from the expected response as the resistance factor. It was assumed that the permeability was constant.

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