Previous Talks

Wednesday, November 13, 2013

Shearing on High-Angle Fracture Jogs during Hydraulic Fracturing

Jim Rutledge, Schlumberger

Abstract

Hydraulic-fracture microseismicity sometimes exhibits fairly uniform source mechanisms along the event cloud lengths. The common characteristics associated with the observation are: 1) simple fracture geometry revealed by linear or planar microseismic clouds, 2) one nodal plane aligned close to the hydraulic fracture event trend and maximum principal stress direction, and 3) primarily shear mechanisms and opposite sense of shearing on the aligned nodal planes. We present a number of examples of these observations from reservoirs of different rock types and varying stress regimes. The uniformity of shearing on planes with little or no expected in-situ shear stress and the simple geometry suggest that the signal generation is closely associated with the near-field stress and strain conditions of the hydraulic fracture.

Using two case studies we offer two possible models to explain the uniform mechanisms in terms of expected stress conditions created near fracture opening. In the first case, strike-slip failure is prevalently aligned with maximum horizontal stress (SHmax) direction in a series of hydraulic fracture injections in the Cotton Valley tight sands. The alignment of nodal planes with the principal stress direction and exhibiting both left- and right-lateral displacement, could be an indication for extension-shear fracturing. A preference for extension-shear fracturing requires low differential stress and high pore pressure, conditions that can be promoted immediately adjacent to a hydraulic fracture. Using calculations of stress changes associated with crack opening for a long, vertical hydraulic fracture results in a reduction of differential stress for strike-slip faulting (SHmaxShmin) in the immediate area of the crack, where Shmin denotes the minimum horizontal stress. The smaller Mohr circle representing the near-field stress condition can then first contact the failure envelope in the extension-shear domain at low angles to the principal stress direction. This mechanism requires the fracture pore pressure to effectively couple into the surrounding rock. In the second case, vertical dip-slip nodal planes are aligned with SHmax and hydraulic-fracture direction in the Barnett shale. Flips in first-motion polarities also indicate opposite sense of up/down shearing. It is not likely that the Barnett observations are due to extension-shear failure because: 1) coupling the fracture pore pressure into a shale is difficult, and 2) the stress changes close to a long hydraulic fracture, as outlined by the seismicity for the Barnett case, are similar for vertical stress (Sv) and Shmin so that differential stress will not be reduced to promote extensional-shear failure for dip-slip displacements. The alternate failure plane for a near-vertical dip-slip mechanism is a horizontal fracture plane. We interpret the uniform Barnett mechanisms as an indication that shearing prevalently occurs on bedding planes, driven by and accommodating hydraulic fracture opening.

The source model for the Barnett can be considered a high-angle fracture jog in which shearing is driven by adjacent fracture opening. This mechanism is also plausible for the Cotton Valley case, where the nodal planes near orthogonal to the event trend could be the slip plane. In this case fracture jogs would be generated by fracture opening being accommodated along intersecting vertical fractures. We show that the model of a high-angle fracture jog driven by crack opening is more likely to generate a shear-type source mechanism than low-angle extension-shear failure, consistent with the common observation of prevalently shear events during hydraulic fracturing. Further, the simple model of shearing on high-angle jogs driven by adjacent opening can be invoked in any stress regime. Recognition of the mechanisms allows one to distinguish events directly associated with crack opening, versus, for example, shearing on natural-fracture networks reflecting tectonic stress release through far-field pressure coupling
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Speaker Biography


Jim Rutledge received a BS in Geology from Pennsylvania State University and an MS in Geophysics from the University of Arizona. From 1984 to 2012 he worked as a contractor and staff seismologist at Los Alamos National Laboratory. From 2004 to 2012 he also worked as a consultant for Schlumberger Cambridge Research and MEQ Geo, Inc.  Since October 2012 he has been employed by Schlumberger’s Microseismic Services as a Principle Geophysicist.

His research interests include: induced seismicity, the mechanics and interpretation of hydraulic fracture microseismicity, wave propagation in layered media, sensor systems, and borehole seismology. Starting in 1989, James has led and participated in several studies that demonstrated the uses of microseismic monitoring in oil, gas and geothermal fields. Applications included hydraulic fracture imaging, natural fracture mapping during primary production, monitoring waterflood- and CO2-EOR operations, and detection of seismic deformation associated with reservoir compaction and subsidence.