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

Abstract

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.

Creating the link between micro-seismic observations and hydro-mechanical changes in the reservoir

“…3 of Ikari [41]; many other experiments exist [42][43][44][45]. Shear--induced permeability decrease is consistently observed in clayey materials, indicating that though one may accomplish plastically, aseismically, slow--slip regions in such materials [40], the accompanying permeability changes would be deleterious to production goals. Specific hydro--mechanical properties of shales, specifically, can be found found in a number of references, e.g., [46][47][48].…”

“…Generally, lab experiments and behavior extrapolated from field observations of seismic events agree in terms of qualitatively similar functional forms of strain--friction response (e.g., 2 inflection points, a local maximum at strains below 10%, etc.). Functional relationships of the type proposed by Dieterich [33], [34] and others [35][36][37][38][39] provide a reasonable form for capturing this behavior; recently, Zoback [40] has suggested using such models for capturing slow slip events in stimulated reservoirs. An example of laboratory data recording the behavior of sheared clayey materials is shown at left from Fig.…”

“…Experiments performed by Nemoto [52] show how a wet joint can transition between the modes depending on the roughness of the joint, as shown in the figure to the right. Thus, gouge formation [40] is not the only mechanism for inducing slow slip events. Determining the roughness, though, in a practical application is non--trivial, since diagnostics are limited to small patches and often acquired through quasi--one--dimensional cross--sections via well logs.…”

International Collaboration Activities in Different Geologic Disposal Environments

“…Results obtained by the experiment are crucial in defining mechanisms of natural and induced earthquakes, their precursors and risk assessment, but also the loss of integrity of natural low permeability barriers. Recent studies suggest that slow slip on faults may be a dominant deformation mechanism of shales during hydraulic stimulation or other large subsurface injection activities (Zoback et al, 2012). The same mechanism may be of importance in many other contexts where a stress perturbation is high enough to reactivate the faults, for example drilling a network of underground galleries for radioactive waste emplacement could activate slip on pre-existing faults and eventually enhance the formation permeability.…”

Coupled Thermo-Hydro-Mechanical-Chemical Modeling of Water Leak-Off Process during Hydraulic Fracturing in Shale Gas Reservoirs

“…Many researchers have conducted studies on the capillarity in shales [3,[15][16][17]. Shear reactivation of natural fractures in shale gas reservoirs is likely to be the main deformation mechanism during hydraulic fracturing operations [18]. Weng et al [19] believe that natural fracture dilates due to low effective normal stress and the aperture creates hydraulic conductivity during the pumping of hydraulic fracturing.…”