Summary
We preset an integrated study of multi-stage hydraulic fracture stimulation of two horizontal wells in the Bakken Formation in the Williston Basin, North Dakota. We analyze and compare two sets of independently processed microseismic hypocenter locations recorded in six vertical observation wells during hydraulic fracturing of parallel wells X and Z, and find event locations of both datasets exhibit three consistent, but unusual patterns. First, rather than occurring in proximity to the stages being pressurized, many of the events occur along the length of well Y, a parallel well located between X and Z that had been in production for ~2.5 years at the time X and Z were stimulated. Second, relatively few fracturing stages are associated with an elongated cloud of events trending in the direction of SHmax as is commonly observed during hydraulic fracturing. Instead, the events in a number of stages appear to trend N75°E, ~30° from the direction of SHmax. Focal plane mechanisms confirm slip on faults with this orientation. Finally, the events are clustered at two distinct depths, one near the depth of the well being pressurized in the Middle Bakken Formation and the other ~800 ft above in the Mission Canyon Formation. We believe all three of these patterns result from the hydraulic stimulation being dominated by flow channeling along preexisting faults. We propose that steeply-dipping N75°E striking faults with a combination of normal and strike-slip movement were being stimulated during hydraulic fracturing and provide conduits for pore pressure to be transmitted to the overlaying formations.
