Lithologically-Controlled Variations of the Least Principal Stress with Depth and Its Affect on Multi-Stage Hydraulic Fracturing and Earthquake Propagation
Mark D. Zoback
Stanford University, USA
Dr. Mark D. Zoback is the Benjamin M. Page Professor of Geophysics, Emeritus at Stanford University, where he was also the Director of the Stanford Natural Gas Initiative and Co-Director of the Stanford Center for Induced and Triggered Seismicity and the Stanford Center for Carbon Storage and Senior Fellow in the Precourt Institute for Energy. Dr. Zoback conducts research on in situ stress, fault mechanics, and reservoir geomechanics with an emphasis on shale gas, tight gas and tight oil production as well as CO2 sequestration. Dr. Zoback served on the Secretary of Energy Subcommittee on shale gas development and the National Academy of Engineering Committee that investigated the Deepwater Horizon accident. He is the author of two textbooks and the author/co-author of about 400 technical papers. His most recent book, Unconventional Reservoir Geomechanics, was written with Arjun Kohli, and published in 2019 by Cambridge University Press. His online course, Reservoir Geomechanics, has been completed by over 10,000 people around the world. Dr. Zoback has received a number of awards and honors including election to the U.S. National Academy of Engineering in 2011 and the Robert R. Berg Outstanding Research Award of the AAPG in 2015. He was the 2020 chair of the Society of Petroleum Engineers Technical Committee on Carbon Capture, Utilization and Storage and 2021 Honorary Lecturer for the Society of Exploration Geophysicists.
Introduction of the Lecture
I will present observational data and modeling results which show layer-to-layer stress variations of the least principal stress as large as ~10 MPa (~1500 psi) in areas where horizontal drilling and multi-stage hydraulic fracturing is occurring at very large scale. These stress variations are lithologically controlled and related to viscoplastic stress relaxation, a process we have studied under laboratory conditions for the past decade. Monotonic variations of the least principal stress with depth straightforwardly imply either upward or downward hydraulic fracture growth. More interestingly, we present several case studies of complex patterns of vertical and horizontal hydraulic fracture growth governed by the detailed variation of the magnitude of the least horizontal stress with depth and the exact landing zone of the lateral. In gun barrel view, this complex geometry is suggestive of a fingerprint that depends on the vertical position of a frac stage with respect to the variations of the least principal stress in the layers both above and below the stage depth. Another aspect of relatively high stress layers associated with viscoplastic stress relaxation acting as “frac barriers” is that they may also act as barriers to rupture propagation associated with induced seismicity. In other words, it is easy to cases in which frac barriers also act as fault barriers.