The Effects of Vertical Fractures on Areal Sweep Efficiency in Adverse Mobility Ratio Floods

Abstract

ABSTRACT The industry has recognized for many years that the creation of hydraulic fractures is an excellent means of improving well productivity and injectivity. These induced fractures, however, can have a significant impact on sweep efficiency. This paper presents the results of a simulation study using a finite element model to determine the effects of hydraulic fractures on the areal sweep efficiency of contact miscible displacements in five-spot and line-drive patterns. The influence of fracture orientation, length, and conductivity for mobility ratios ranging from one to ten is reported for patterns where either the producer or injector, or both the producer and injector are fracture stimulated. For favorably oriented fractures, the relative amount of improvement in areal sweep efficiency increases as the mobility ratio increases. Long highly conductive fractures at both the injector and producer are required to improve sweep for a five-spot pattern. Areal sweep in a line-drive pattern is more sensitive to fractures and can be improved by stimulating either or both wells. The greatest improvement occurs when both the injector and producer are fractured, while the smallest improvement occurs when only the producer is stimulated. Low conductivity fractures should be avoided because they can severely restrict the sweep improvement resulting from favorably oriented fractures. Care must be taken to ensure that fractures are oriented in a favorable direction, however, because a significant reduction in areal sweep efficiency occurs if the fractures are oriented in an unfavorable direction. Since fracture orientation is not always known or cannot always be controlled, this study also identifies which length fractures prevent detrimental effects to sweep for unit and adverse mobility ratio floods with unknown fracture orientation.