Slow Fault Slip During Hydraulic-Fracturing Stimulation of Shale-Gas Reservoirs
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 155476, ’The Importance of Slow Slip on Faults During Hydraulic- Fracturing Stimulation of Shale-Gas Reservoirs,’ by Mark D. Zoback, SPE, Arjun Kohli, Indrajit Das, and Mark McClure, SPE, Stanford University, prepared for the 2012 Americas Unconventional Resources Conference, Pittsburgh, Pennsylvania, 5-7 June. The paper has not been peer reviewed.
Slow slip of pre-existing fractures and faults is an important deformation mechanism that contributes to the effectiveness of slickwater hydraulic fracturing for stimulating production in extremely-low-permeability shale-gas reservoirs. Experiments indicated that slippage of faults in shales that contain less than approximately 30% clay is expected to propagate unstably, thus generating conventional microseismic events. In contrast, formations containing more than approximately 30% clay are expected to slip slowly. Because slow fault slip does not generate high-frequency seismic waves, conventional microseismic monitoring does not routinely detect what appears to be a critical process during stimulation. Thus, microseismic events are expected to give only a generalized picture where pressurization is occurring in a shale-gas reservoir during stimulation, which helps explain why microseismic activity does not appear to correlate with relative productivity.
Introduction
Multistage hydraulic fracturing with slickwater in horizontal wells is an effective completion strategy for producing commercial quantities of natural gas from organic-rich shale-gas formations. Physical mechanisms responsible for reservoir stimulation are understood poorly. The prevalent hypothesis is that diffusion of water out of the hydraulic fracture stimulates shear failure of multiple small pre-existing fractures and faults in the shale. This shear slip creates a network of relatively permeable flow paths and, thus, enhances productivity from the extremely-low-permeability shale formations. Microseismic events recorded during hydraulic fracturing are evidence of this shear slip, and the clouds of microseismic events associated with multiple hydraulic-fracturing stages in a well generally are assumed to define the stimulated rock volume from which the gas is produced. However, simple mass-balance calculations show that the cumulative deformation associated with the microseismic events can account for only a small fraction of the production. In a single well, it has been shown that the number of microseismic events does not correlate with production from successive hydraulic-fracturing stages. Production from five wells in the Barnett shale was studied, and it did not correlate with the number of microseismic events generated by hydraulic fracturing in each well, even though the wells were stimulated in a similar manner.
Slow slip of numerous fault planes may occur in shale-gas reservoirs during stimulation and may be the dominant deformation mechanism during hydraulic stimulation. The shear deformation associated with the slowly slipping faults is expected to create a network of multiple permeable planes surrounding the induced hydraulic fractures.