Abstract

Many oil reservoirs in Alberta and Saskatchewan have a bottom-water cone which leads to poor efficiency under a waterflood. A possible means of improving waterflood performance in these reservoirs would be to precede the waterfloor with a slug of a mobility control agent. This possibility was extensively investigated in this study using a laboratory flow model, with polymer as a mobility control agent.

Twenty four experimental runs were conducted using polymer in various slug sizes. Core floor experiments were conducted to study the effect of permeability contrast, relative oil-water layer thickness, oil viscosity, and slug size. Oils of viscosities ranging from one to 200 mPa.s were used as different displacement tests. The blocking mechanism for polymer was investigated in detail. The flow stability and displacement pattern is discussed for polymer and the waterflood that followed. It is show that in order to have effective mobility control polymer slugs do not have to be displaced as an entity. Even though the waterflood following the polymer slug injection is highly unstable, the improvement over conventional waterflood is remarkable. However, the ultimate oil recovery depends on variables such as permeability contrast, relative oil-water layer thickness, oil viscosity, injection time interval. It is also shown that the injection of polymer as a slug is more efficient than alternating it with water. The effect of slug size is discussed and an optimum slug size is proposed. It is shown that polymer injection is particularly effeicent in cases for which waterflood performance is very poor. For instance, in the case of a thick (hb / ho=1) and very permeable (ko/ kw = 0.06) bottom-water zone, about 2 % IOIP was recovered by a waterflood, whereas by the use of polymer as a mobility control agent, more than 45% IOIP was recovered.

Also presented are numerical simulation results for polymer flow in an oil reservoir with a bottom-water zone. Experimental evidence suggest that polymer reduces the effective permeability to water without altering that to oil. Experimental investigations also show that the flow mechanism with polymer is dominated by effects like instability, mechanical entrapment, etc. In order to stimulate these effects the concept of 'selective resistance factor' is introduced for the waterflood following a polymer slug a model for the effective permeability to water is developed. This information was used in numerically simulating polymer blockage in the framework of a three-phase, two-dimensional reservoir simulator. Good agreement was obtained between numerical and experimental observations.

Introduction

Many reservoirs in Alberta and Saskatchewan contain a high water saturation zone underlying the oil reservoir. Such reservoirs show very poor performance under a conventional waterflood. There is a need to develop a technique that would improve the performance of a waterflood in such reservoirs. This problem has received attention for more than two decade and several techniques have been proposed in the literature 1–2 to control the mobility of water or gas. One of these techniques is the use of polymer solutions as a slug.

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