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. Introduction Historically, the industry has hydraulically fracture stimulated many wells in order to increase oil production rates and accelerate project timing. Many articles have been published which document the relationship between vertical fractures and areal sweep for five-spot and line-drive systems under unit mobility ratio conditions. These results have been used to provide insight into how hydraulic fractures affect sweep in pattern waterfloods. Some of these hydraulically fractured wells are located in fields that are now undergoing or being evaluated for enhanced oil recovery processes such as carbon dioxide or enriched gas injection. These enhanced gas drive (EGD) floods are adverse mobility ratio displacements which create unique sweep efficiency concerns not typically encountered in waterflood operations. In unfractured homogeneous patterns, the difference in sweep between a waterflood and adverse mobility ratio displacement occurs because the injection volume at which flow deviates from radial behavior and begins to cusp toward the producer decreases as the mobility ratio increases. Cusping also becomes more pronounced as the mobility ratio increases which causes earlier breakthrough and decreased sweep efficiency at a given injection volume. Two articles have been published which address the effects of vertical fractures on the areal sweep efficiency of adverse mobility ratio displacements. Both articles present the results of X-ray shadowgraph experiments conducted on a homogeneous five-spot pattern. The first paper was published by Dyes, Kemp, and Caudle in 1958 and documents the results of tests conducted at a mobility ratio of three. P. 611^