The Importance of Slow Slip on Faults During Hydraulic Fracturing Stimulation of Shale Gas Reservoirs

We utilize several lines of evidence to argue that slow slip on pre-existing fractures and faults is an important deformation mechanism contributing to the effectiveness of slick-water hydraulic fracturing for stimulating production in extremely low permeability shale gas reservoirs. First, we carried out rate and state friction experiments in the laboratory using shale samples from three different formations with a large range of clay content. These experiements indicated that slip on faults in shales comprised of less than about 30% clay is expected to propagate unstably, thus generating conventional microseismic events. In contrast, in formations containing more than about 30% clay are expected to slip slowly. Second, we illustrate through modeling that slip induced by high fluid pressure on faults that are poorly oriented for slip in the current stress field is expected to be slow, principally because slip cannot occur faster than fluid pressure propagates along the fault plane. 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 of where pressurization is occurring in a shale gas reservoir during stimulation which helps explain why microseismicity does not appear to correlate with relative productivity. We review observations of long-period-longduration seismic events that appear to be generated by slow slip on mis-oriented fault planes during stimulation of the Barnett shale. Prediction of how pre-existing faults and fractures shear in response to hydraulic stimulation can help optimize field operations and improve recovery. Introduction Multi-stage hydraulic fracturing with slick-water in horizontal wells is an effective completion strategy for producing commercial quantities of natural gas from organic-rich shale gas formations. That said, the physical mechanisms responsible for reservoir stimulation are poorly understood. The prevalent paradigm is that diffusion of water out of the hydraulic fracture stimulates shear failure on 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 are generally assumed to define the stimulated rock volume (SRV) from which the gas is being produced (Warpinki et al., 2012). While this paradigm is generally useful, a simple mass balance calculation illustrates 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 (Moos et al., 2011). Production from five wells in the Barnett shale studied by Vermylen and Zoback (2011) does not correlate the number of microseismic events generated by hydraulic fracturing in each well even though the wells were stimulated in a similar manner. In this paper we argue that slow slip on numerous fault planes is occurring in shale gas reservoirs during stimulation. In fact, we believe this is likely to 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. In the sections below, we first review evidence of