Journal of Petroleum Technology — October 2012
Change Language:
Slow Fault Slip During Hydraulic-Fracturing Stimulation Of Shale-Gas Reservoirs

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 extremelylow- 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 highfrequency 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 shalegas 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 massbalance 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.

Evidence of Slow Slip

Fig.1 shows sudden bursts of energy with a duration of 10–100 seconds in some spectrograms recorded during the microseismic recordings. This energy is most conspicuous between 10 and 80 Hz, as shown in Fig.1b, in which three of these events are plotted with an expanded time scale. In this frequency band, generally not considered in microseismic analysis, the recorded waveforms (Fig.1c) are similar to those of a tectonic tremor observed in subduction zones and strike/slip margins (Fig.1d). A tectonic tremor typically is observed between 1 and 15 Hz, a frequency band that is not recorded well by the 15-Hz geophones used for microseismic monitoring. Crustal-deformation measurements during periods of a tectonic tremor (and migration of the tremor) indicate that the deformation results from slow slip.

From seismograms recorded with a borehole array in Well C, the directions from which the long-period/longduration (LP/LD) seismic events originated were determined. It has been proposed that the events come from several pre-existing faults, parallel to those seen in an imaging log in Well C, that crosscut the hydraulic-fracture planes created during the fracture stages in Wells A and B. The exact locations of the hydraulic-fracture planes in any given stage are unknown, but they are expected to be perpendicular to the minimum horizontal stress (and the well paths). The study also showed that the high pore pressure during pumping near the hydraulic-fracture planes at the exact stages that recorded the largest number of LP/LD events is able to activate slip on crosscutting fault planes that are misoriented for slip in the current stress field. In other words, slip would not occur on these faults had pore pressure not been elevated during hydraulic fracturing.

Laboratory Friction Experiments

Understanding the conditions responsible for induced seismic and slowslip events during stimulation stages requires development of frictionalstability criteria for shale-gas-reservoir rocks. Previous fault-mechanics studies examining frictional stability focused on clay-rich gouges, which are hypothesized to control the transition from seismic to slow or stable slip in the Earth through changes in mechanical and hydraulic properties with depth. Although previous studies of the frictional stability of clay-rich rocks were carried out under specific conditions, no systematic investigations under the conditions (and rock types) relevant to reservoir stimulation of organic-rich shale-gas formations have been presented previously.

The Dietrich-Ruina rate and stateconstitutive laws show strong agreement with the results of laboratory rock- friction studies on simulated fault gouge. Terms a and b are material parameters representing the frictional evolution over a velocity step. In the formulation, detailed in the complete paper, the friction parameter (a.b) is diagnostic of frictional evolution over a stepwise change in sliding velocity, with (a.b)>0 describing a velocitystrengthening or stable response and (a.b)<0 describing a velocityweakening or potentially unstable response. In addition to the effect of frictional evolution, the slip behavior will depend on the effects of fluidpressure changes in the slipping zone.

Laboratory friction experiments were performed on gouge samples prepared from cores from the Barnett, Haynesville, and Eagle Ford shale reservoirs to measure frictional strength and the rate and state-constitutive parameters. Sliding velocities ranging from 0. 1 to 10 ìm/s were implemented in approximately even displacement increments up to 5 mm, which represents the maximum allowed sliding displacement in this experimental geometry resulting from the use of a Viton sample jacket around the sample.

Fig.2 shows that the coefficient of friction decreases monotonically from approximately 0.75 to 0.35 as the weight percent of clay plus kerogen increases to approximately 60%. In addition, for samples with less than approximately 30% clay plus kerogen, (a.b)<0 indicating unstable (seismic) sliding, whereas for samples with more than approximately 30% clay plus kerogen, (a.b)>0, indicating stable sliding. This threshold appears to be consistent for all three shale formations.

Slow Slip on Misoriented Faults

In fractured-rock masses, there are preexisting fractures and faults at a variety of orientations. Some of these faults are likely to be well-oriented for slip in the ambient-stress field, and these, sometimes, are termed critically stressed faults. While faults that are misoriented for slip in the ambient-stress field normally would not be expected to be capable of slipping, the strong elevation of fluid pressure during hydraulic fracturing is capable of triggering slip on such faults. The authors were able to demonstrate that slip induced on misoriented faults is expected to be slow slip, undetected in microseismic surveys. Also, slow slip of faults is likely to be a fundamental component of hydraulic stimulation.

Elevated pore pressure initiates slip of misoriented planes, as known from the basic principles of fault mechanics. What is not well known is that, while slip on a critically stressed fault could propagate rapidly as a microearthquake when triggered [assuming (a-b)<0], induced slip of misoriented planes will propagate slowly and go undetected during normal microseismic surveys. Simply put, the reason is that slip of a portion of a misoriented fault will occur only where the pore pressure is anomalously high. Thus, slip will propagate along a misoriented fault only as rapidly as increased pore pressure propagates along it.

The modeling used a procedure that incorporates fluid flow along the fault planes, rate and state friction, and an associated change in permeability of the fault plane once slip is induced. Faults of various orientations were considered in the model. A few of the preexisting- fracture and -fault planes are critically stressed under normal reservoir conditions, but most of the fracture and fault planes will not slip unless fluid pressure is increased.

Fig.3 summarizes the results of modeling on faults of many orientations. Except for fault orientation, the parameters used in all of the calculations were the same. The color of the symbol indicates the speed of rupture propagation. The red symbols indicate the triggering of seismic slip of faults that are well-oriented for slip (critically stressed under initial reservoir conditions). In all cases, (a-b)<0 indicated that slip could be unstable. However, it was observed that the triggered slip was slow for faults misoriented to the current stress field (as shown in purple). For the friction parameters and porepressure perturbation used in the calculations, slip did not occur on the mostseverely misoriented faults.

Conclusions

The authors suggest that pervasive slow slip of faults may be critical during hydraulic fracturing if the treatment is to be effective in stimulating production from extremely-low-permeability shale-gas reservoirs. If this is the case, it is important to approach shale-gas development from a predictive perspective rather than carrying out the hydraulic fracturing with predetermined regularized spacing, volumes, and rates. Mapping the distribution of fractures and faults in a reservoir, developing an understanding of the magnitude and orientation of the three principal stresses, and predicting the pressure at which slip will be induced on misoriented faults can be accomplished with well-established technologies. By use of such knowledge, it is possible to design reservoir stimulations in a manner that is optimally efficient and most productive.

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.
VIEW ALL ARTICLES
Message
SEND