American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.

Abstract

Poor sweep efficiency in waterfloods can cause many barrels of recoverable oil to be bypassed and left in place. Sweep efficiency is a function of variations in permeability both horizontally and vertically. Many types of diverting agents have been used in an attempt to control paths taken by injected fluids, but all have been extremely limited in application. Recently, however, high concentration solutions of certain polyacrylamides have proved highly effective in controlling both horizontal and vertical flood patterns by permeability adjustment. Although their use involves only a short-term treatment, normally without special well preparation or workover equipment, results have been long lasting. This paper describes the use of these polymer materials to alter flood patterns and presents case histories demonstrating horizontal and vertical adjustment.

Introduction

In waterflooding, many barrels of recoverable oil are bypassed as injected water seeks out high-permeability zones or streaks through which it flows more easily. This is often apparent on injection profile jobs in which one or more sections over the vertical extent of the formation accept most of the water, while others accept little or none. If such a condition continues to the economic life of a waterflood, it can cause great quantities of oil to remain in place, locked in low-permeability streaks sandwiched between higher permeability streaks that already are watered out. Another condition that quite frequently exists is poor areal distribution of injected water. In this situation, a flood front advances unequally in a horizontal direction and may actually reach one of the offset producers with little or no lateral traverse toward the others. This condition is not apparent on injection logs and usually is determined by production well response over a period of time. In either case, a material or method that diverts fluid from the higher permeability sections and achieves both a uniform vertical and horizontal injection fluid pattern could result in the recovery of many additional barrels during the life of a waterflood project. In this paper the term "sweep efficiency" denotes the efficiency or uniformity, both horizontal and vertical, with which the flood front advances. This should not be confused with "displacement efficiency", which refers to the amount or percentage of oil displaced by the displacing medium.

To accomplish effective diversion, a suitable material should be injected during the middle or later life of the flood. At this time, the zones into which the material is injected would still contain recoverable oil. For this reason, strict requirements are imposed on the diverting agent. These are that [1] it must reduce the effective permeability to water of the more permeable zones, so that both horizontal and vertical distribution of the injected water is improved; [2] it must be mobile, so that oil remaining in the more permeable zones will continue to be displaced; [3] it must penetrate deeply and be long lasting, so that it is penetrate deeply and be long lasting, so that it is not bypassed; and [4] it must not reduce the effective permeability to oil.

Many materials have been used for diversion in injection wells in the past, but none has fulfilled all the above requirements. Solids, for example, are effective only near the wellbore and accomplish no diversion out in the formation. The more recently devised silicate acid gels are deeply penetrating, but they set up into permanent plugs and cannot be used unless the section is to be abandoned.

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