Permian Basin Oil Recovery Conference, 7–8 May, Midland, Texas

Abstract

Theoretical and potentiometric model studies have been made of the effect of nonuniform lateral permeabilities on pattern sweep efficiency and production capacity in waterflooding and gas cycling programs.

It is shown that a difference in directional permeability by a factor of three may result in a sweep efficiency of only 43 per cent for a five-spot pattern or a sweep of 79 or 38 per cent for a direct line-drive square pattern depending on the direction of the line-drive flood. Changes in the pattern conductivity varied from about 0.8 to 1.34 over this same permeability variation, depending on the pattern used.

It is suggested that field measurements be made to determine the possible magnitude and extent of the directional permeability phenomenon early in the field development and certainly prior to the initiation of any fluid injection program.

Introduction

Irregularities in reservoir sand properties have long been a major difficulty to anyone attempting to explicitly describe the field characteristics of oil production. In particular, it is well known that vertical and lateral permeabilities often differ appreciably; however, the existence of large regions with lateral permeability variation is not widely recognized. A number of years ago, extensive studies were conducted by the Secondary Recovery Research Laboratory of the Pennsylvania Grade Crude Oil Association primarily on the Bradford Field. Johnson and Hughes reported a permeability trend in the northeast-southwest direction. They indicated that flow in the preferred direction may be 25 to 30 per cent greater than in the northwest-southeast direction. They also reported that similar effects may be found in other nearby fields. The origin of the permeability variation has been discussed by Griffith. Results of laboratory tests on limestone cores have led some to believe that directional permeabilities may differ by factors of ten to 100 or more. Certain areas of the Spraberry are believed to have effective directional permeabilities greater than 100 to one.

In order to prevent the reduction of sweep efficiency which would normally result from flooding or cycling in such reservoirs, well spacing could be varied from the customary square to a diamond shaped pattern. Johnson and Hughes presented a method for determining the desired shape. Another method will be discussed in detail in the next section.

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