Swept Areas After Breakthrough in Vertically Fractured Five-Spot Patterns

Abstract

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 73–77. Abstract Potentiometric model data have been obtained to estimate the effect of vertical fractures on the areas swept after breakthrough in water flooding and miscible displacement programs such as gas cycling where the mobility is near one. The data are presented for the case of the five-spot pattern in which the center well is fractured various lengths and orientations. The data indicate that for 10-acre spacing, fractures extending over 200 ft in either direction from the fractured well may result in reductions in sweep efficiencies from 72 to approximately 34 per cent. However, the area swept after breakthrough may be quite large and only 10 or 12 per cent less than would be obtained if the reservoir were not fractured. For the specific case when the volume of fluid injected is equivalent to 100 per cent of the pattern volume, the swept area may vary from 80 to 88 per cent, depending on the length of the fracture. The former value is that which occurs when the breakthrough or sweep efficiency was only 34 per cent and the latter figure of 88 per cent is that which is obtained if the reservoir were unfractured. It is pointed out that although the sweep efficiency may be very low in vertically fractured five-spot patterns, the area swept at low water-oil ratios may be only 5 to 10 per cent less than those achieved if the reservoir were unfractured. Introduction Since the initiation of commercial reservoir fracturing techniques it has been desirable to determine the effect of fractures on the areas swept after breakthrough. Most water flooding or gas cycling projects are continued for substantial periods after the breakthrough of the injected fluid. Although the sweep efficiency serves as one criterion for rating various flooding patterns, the area swept after breakthrough for various water-oil ratios or percentage wet gas, if cycling, is of perhaps more importance than the sweep efficiency alone. Sweep efficiency data on the vertically fractured five-spot have been presented. Previous work on the line-drive pattern has shown the effect of vertical fractures on the area swept after breakthrough for the case in which the distance between injection and producing wells divided by the distance between adjacent input wells was equivalent to 1.5 (see Ref. 2). The data indicated that for the line-drive pattern it may be desirable to flood or cycle substantially perpendicular to the fractures in order to achieve the greatest recovery for the smallest volume of fluid injected. For this study the center well of a five-spot is assumed as the fractured well.