Abstract

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.

Introduction

The Mississippian-Devonian Bakken formation is a restricted shallow-water mixed carbonate-clastic sequence deposited over most of the deep part of the Williston Basin (Sturm and Gomez, 2009). Utilization of horizontal drilling and multi-stage hydraulic fracturing led to the successful development of the Elm Coulee (Montana) and Parshall (North Dakota) fields and demonstrated the great potential of the Bakken formation. Despite the generally successful exploitation of the Bakken formation, questions remain regarding how to optimize hydraulic fracturing and the importance of pre-existing natural fractures as fluid pathways in the reservoir.

To help address these issues, we first report development of a geomechanical model that includes knowledge of the magnitude and orientation of principal stresses, the existence and orientation of natural fractures and faults and mechanical properties of the formations being produced. In addition, as analysis of microseismic event locations can be used to better understand fracture networks after hydraulic fracture stimulation (Fehler et al., 1987; Maxwell et al., 2002; Phillips et al., 1998; Rutledge et al., 1998; Williams-Stroud, 2008), and the stress state will influence both the hydraulic fracture propagation and the distribution of microseismic events. Therefore, linking observations of microseismic data with geological and geophysical information can help us better understand the geomechanical properties of the Bakken formation and to understand the role of pre-existing fractures and faults on the effectiveness of hydraulic fracturing stimulation in this shale oil reservoir. We hope this highly integrated approach will demonstrate the significance of geomechanics in guiding successful multi-stage fracturing performance in shale reservoirs. Relatively few published studies (Tezuka and Niitsuma, 2000; Verdon et al., 2011) have attempted to link microseismic events with the geomechanical properties of the reservoir and their impact on the hydraulic fracture stimulation.

URTeC 1580301

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