Crossflooding To Improve Waterflood Efficiency in Big Sinking Field, Kentucky
Authors: Vadgama Usman N. , Hinkle Bill B. 
Published: December 2007

Although the economic results of a 64-acre pilot crossflood project late in the life of this field are not spectacular, they show that the project is worth the bother. The expenditures for crossflooding were offset by lowered operating costs and increased oil recovery.

Introduction

In its search for an optimum oil recovery process, the oil industry has proposed and experimented with many different concepts with varying degrees of success. However, the most economical secondary recovery method has remained waterflooding., The performance of a waterflood depends mainly on the performance of a waterflood depends mainly on the ability of the water to displace the oil and upon the volumetric sweep efficiency. As the conditions are rarely ideal in an oil reservoir, in most cases a large percentage of oil is left behind after the waterflood percentage of oil is left behind after the waterflood reaches its economic limit. Many of the recently publicized improved recovery methods strive to increase publicized improved recovery methods strive to increase waterflood performance by adding different chemicals to the injection fluid to adjust viscosity or to reduce surface tension. Some of these methods are technically very successful; but because of the expensive chemicals involved and the complexity of the operation, they are not so attractive economically. Crossflooding, by increasing sweep efficiency, offers one practical approach to improving oil recovery. The technique involves a definite change in injection well pattern during the waterflood operation. It is simple pattern during the waterflood operation. It is simple to implement and requires a relatively small additional investment, In 1934, Muskat and Wyckoff presented a classical analysis of waterflood sweep efficiency at breakthrough for several injection patterns. Since then numerous papers have been published analyzing the effects of mobility ratio, permeability distribution, and other reservoir characteristics on the performance of waterflooding. These analyses indicate that, with the help of a five-spot flooding pattern, where the reservoir is homogeneous and there is unit mobility ratio, roughly 70 percent of the area can be swept. In practice, however, the expected sweep efficiency would be significantly less because of the unfavorable mobility ratio of oil and water in formation and because of inhomogeneities and vertical conformance of the reservoirs. The idea in crossflooding is to contact the unswept areas within the pattern and recover at least a part of the unswept oil by changing the injection pattern. In the past, different groups of engineers 3-6 have studied the crossflood technique theoretically as well as in the laboratory. Their conclusions, however, vary significantly one from another. We know of no publication giving large-scale field results. publication giving large-scale field results. Smith and Nobles, in a paper based on laboratory experiments with the help of electric resistance network and potentiometric studies indicate that, by a definite change in injection well pattern during a waterflood operation, the areal sweep efficiency can be increased to about 90 percent. Still and Crawford however, concluded that only a small amount of additional oil - a maximum of about 3 percent of pore volume - can be recovered by crossflooding. The additional recovery was considered to be too low to justify the additional investment necessary for crossflooding. Their results were based on tests with synthetic reservoir sand packs.

JPT

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