Improving Waterflood Performance in Oil Reservoirs with Bottomwater

Abstract

ABSTRACT This research addresses the problem of waterflooding a medium gravity oil-bearing formation with a water leg, and offers recommendations for process selection. In many reservoirs the presence of a bottom water zone results in very poor areal and vertical sweep efficiencies. However, waterflooding still remains the most widely used oil recovery technique for these reservoirs. Waterflood performance in such reservoirs can be improved greatly with effective methods of partially plugging the bottom-water zone. Some of these methods were developed in this study with a large flow model, by using a series of mobility control agents. Polymer, emulsion, biopolymer, air, silica gel and foam, in various slug sizes, were used as mobility control agents for waterflooding an oil saturated reservoir model with a bottom-water layer. More than 80 model experiments were conducted to study the effect of permeability contrast, water-oil layer thickness ratio, oil viscosity, slug size, etc. Also, a series of runs were conducted with an artificial barrier of different lengths. Oils of viscosities ranging from one to 200 mPa.s were used in the displacement tests. A qualitative comparison is made to show the relative merits of the various mobility control agents. The blocking mechanism for each mobility control agent is investigated in detail and a comparison is made. It is shown that the choice of one mobility control agent over others depends on variables such as permeability contrast, oil-water layer thickness ratio, oil viscosity, injection interval (as a function of time). The effect of slug size is discussed for various mobility control agents and an optimum slug size is proposed. It is shown that for polymer, emulsion, foam and silica gel the poorer the conventional waterflood performance is, the more efficient the mobility control agents are. For instance, in the case of bottom water zone as thick and permeable as the oil zone, only about 15% 10IP may be recovered by a waterflood, whereas by the use of polymer, emulsion, foam or silica gel as mobility control agent, as much as 70% of the IOIP may be recoverable.