An Integrated Geomechanical and Microseismic Study of Multi-Well Hydraulic Fracture Stimulation in the Bakken Formation

We present an integrated study of multi-stage hydraulic fracture stimulation of two parallel horizontal wells in the Bakken formation in the Williston Basin, North Dakota. There are two distinct parts of this study: development of a geomechanical model for the study area, and analysis of the microseismic and hydraulic fracturing data. Despite the limited amount of well log data available, we estimate the current stress state to be characterized by a NF/SS regime, with Pp= 0.66 psi/ft, Sv=1.05 psi/ft, Shmin=0.79-0.85 psi/ft. Published data of drilling-induced tensile fractures observed in the FMI image logs just south of the study area indicate that SHmax is oriented ~N40-50°E. The microseismic events were recorded in six observation wells during hydraulic fracturing of parallel wells X and Z, and three unusual patterns were observed: First, rather than occurring in proximity to the stages being pressurized in X or Z, many of the events occur along the length of well Y (a parallel well located between X and Z that has been in production for about 2.5 years). Second, relatively few fracturing stages are associated with the expected elongate cloud of events trending in the direction of SHmax. Instead, the microseismic events from many stages appear to trend ~30° from the direction of SHmax. Finally, the microseismic events are clustered at two distinct depths, one close to the depth of the well being pressurized and the other about 800 ft above in another formation. We believe all three of these patterns result from the hydraulic stimulation being dominated by flow channeling along pre-existing fractures and faults.