Factors Affecting Waterflood Pattern Performance and Selection

Abstract

Abstract A review is given of the results of early work showing sweep efficiency and steady-state injection rates for various types of patterns which may be used for waterflooding programs. This early work indicated that the sweep efficiency may be near 72 per cent for the five-spot pattern, 56 per cent for the direct, line-drive square pattern and may range from 45 to 90 per cent for the nine-spot pattern. The results were calculated on the basis that the mobility ratio was one, the reservoir was uniform and homogeneous, steady-state conditions existed, and gravity and capillary effects could be neglected. In actual flooding operations it is found that all reservoirs are heterogeneous, steady-state conditions do not exist, the mobility ratio is not one, and gravity and capillary phenomena undoubtedly play a role. Theoretical calculations show that rock heterogeneities can cause the sweep efficiency of both the five-spot and direct, line-drive square patterns to vary from near zero to 100 per cent. Actual laboratory data have been obtained to show that the oil recovery by water flooding in depleted reservoirs results in the production of substantially the same quantity of oil for the five-spot, direct, line-drive square pattern and nine-spot pattern for a wide range of initial fluid saturations. The experimentally observed sweep efficiency of the five-spot pattern is shown to depend on the initial connate-water saturation and oil saturation. Introduction It is believed quite likely that many of the decisions affecting the selection of the pattern to be used in water flooding are based largely on information and conclusions which were made many years ago. The purpose of this paper is to review this early work and include recent work showing various factors which affect the performance of pattern-type floods. In the mid 1930's Muskat and his associates presented the first analytical solutions showing the sweep efficiency and injectivity for the five-spot, direct, line-drive, staggered, seven-spot and nine-spot patterns.