Experimental study of impact of shearing velocity and effective normal stress on post-shearing permeability evolution of silica fault gouges

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with near 100 % organic content, e.g. coal, and the controlling microscale mechanisms, remain unclear. Here, we report seven velocity stepping (VS) and one slide-hold-slide (SHS) friction experiments performed on simulated fault gouges prepared from bituminous coal, collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 °C, employing sliding velocities of 0.1–100 μm s−1, using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip weakening behaviour at shear displacements beyond ~ 1–2 mm, from a peak friction coefficient approaching ~ 0.5 to (near) steady state values of ~ 0.3, regardless of effective normal stress or whether vacuum dry flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip weakening behaviour can be attributed to the development of R-, B- and Y- shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ~ 0.006. We also determined the rate-dependence of steady state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities > 1 μm s−1 in the coal sample under vacuum dry conditions, but at > 10 μm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. This may be controlled by competition between dilatant granular flow and compaction enhanced by presence of water. Together with our previous work on frictional properties of coal-shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear-out coal seams may promote unstable, seismogenic slip behaviour, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Download Full-text

Permeability Evolution of an Intact Marble Core during Shearing under High Fluid Pressure

Geofluids ◽

10.1155/2021/8870890 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Yuan Wang ◽

Yu Jiao ◽

Shaobin Hu

Keyword(s):

Normal Stress ◽

Laser Scanning ◽

Shear Failure ◽

Fluid Pressure ◽

Shear Displacement ◽

Permeability Evolution ◽

Fracture Permeability ◽

Effective Normal Stress ◽

High Fluid ◽

High Fluid Pressure

The progressive shear failure of a rock mass under hydromechanical coupling is a key aspect of the long-term stability of deeply buried, high fluid pressure diversion tunnels. In this study, we use experimental and numerical analysis to quantify the permeability variations that occur in an intact marble sample as it evolves from shear failure to shear slip under different confining pressures and fluid pressures. The experimental results reveal that at low effective normal stress, the fracture permeability is positively correlated with the shear displacement. The permeability is lower at higher effective normal stress and exhibits an episodic change with increasing shear displacement. After establishing a numerical model based on the point cloud data generated by the three-dimensional (3D) laser scanning of the fracture surfaces, we found that there are some contact areas that block the percolation channels under high effective stress conditions. This type of contact area plays a key role in determining the evolution of the fracture permeability in a given rock sample.

Download Full-text

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Solid Earth ◽

10.5194/se-11-1399-2020 ◽

2020 ◽

Vol 11 (4) ◽

pp. 1399-1422

Author(s):

Caiyuan Fan ◽

Jinfeng Liu ◽

Luuk B. Hunfeld ◽

Christopher J. Spiers

Keyword(s):

Steady State ◽

Normal Stress ◽

Hold Time ◽

Shear Bands ◽

Fluid Pressure ◽

Organic Content ◽

Local Conditions ◽

Frictional Properties ◽

Effective Normal Stress ◽

Fault Gouges

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Download Full-text

Dilation and compaction accompanying changes in slip velocity in clay-bearing fault gouges

10.5194/egusphere-egu21-8679 ◽

2021 ◽

Author(s):

Isabel Ashman ◽

Daniel Faulkner

Keyword(s):

Normal Stress ◽

Pore Pressure ◽

Volume Change ◽

Slip Velocity ◽

Fault Slip ◽

Slow Slip ◽

Volume Changes ◽

Effective Normal Stress ◽

Earthquake Nucleation ◽

Fault Gouges

Many natural fault cores comprise volumes of extremely fine, low permeability, clay-bearing fault rocks. Should these fault rocks undergo transient volume changes in response to changes in fault slip velocity, the subsequent pore pressure transients would produce significant fault weakening or strengthening, strongly affecting earthquake nucleation and possibly leading to episodic slow slip events. Dilatancy at slow slip velocity has previously been measured in quartz-rich gouges but little is known about gouge containing clay. In this work, the mechanical behaviour of synthetic quartz-kaolinite fault gouges and their volume response to velocity step changes were investigated in a suite of triaxial deformation experiments at effective normal stresses of 60MPa, 25MPa and 10MPa. Kaolinite content was varied from 0 to 100wt% and slip velocity was varied between 0.3 and 3 microns/s.Upon a 10-fold velocity increase or decrease, gouges of all kaolinite-quartz contents displayed measurable volume change transients. The results show the volume change transients are independent of effective normal stress but are sensitive to gouge kaolinite content. Peak dilation values did not occur in the pure quartz gouges, but rather in gouges containing 10wt% to 20wt% kaolinite. Above a kaolinite content of 10wt% to 20wt%, both dilation and compaction decreased with increasing gouge kaolinite content. At 25MPa effective normal stress, the normalised volume changes decreased from 0.1% to 0.06% at 10wt% to 100wt% kaolinite.&#160; The gouge mechanical behaviour shows that increasing the gouge kaolinite content decreases the gouge frictional strength and promotes more stable sliding, rather than earthquake slip. Increasing the effective normal stress slightly decreases the frictional strength, enhances the chance of earthquake nucleation, and has no discernible effect on the magnitude of the pore volume changes during slip velocity changes.Low permeabilities of clay-rich fault gouges, coupled with the observed volume change transients, could produce pore pressure fluctuations up to 10MPa in response to fault slip. This assumes no fluid escape from an isolated fault core. Where the permeability is finite, any pore pressure changes will be mediated by fluid influx into the gouge. Volume change transients could therefore be a significant factor in determining whether fault slip leads to earthquake nucleation or a dampened response, possibly resulting in episodic slow slip in low permeability fault rock volumes.

Download Full-text

Residual Strength: Does BS 5930 Help or Hinder?

Geological Society London Engineering Geology Special Publications ◽

10.1144/gsl.1986.002.01.50 ◽

1986 ◽

Vol 2 (1) ◽

pp. 279-282 ◽

Cited By ~ 9

Author(s):

A. B. Hawkins ◽

K. D. Privett

Keyword(s):

Stability Analysis ◽

Normal Stress ◽

Residual Strength ◽

Clay Content ◽

Failure Envelope ◽

Strength Parameters ◽

Effective Normal Stress ◽

Ring Shear ◽

Stress Dependent ◽

Shear Box

AbstractBS 5930 offers little assistance to engineers wishing to use residual strength parameters in slope stability analysis. It wrongly suggests the ring shear gives lower parameters than the shear box.BS 5930 does not mention the fact that the residual strength is stress dependent, hence the failure envelope is curved and the parameters must be assessed using an appropriate effective normal stress. For this reason the correlation charts relating ϕ′R to plasticity index or clay content need replacing with a series of charts in which these properties are plotted against ϕ′R values obtained at a number of effective normal stress loadings. Even then such correlations should be treated with caution.

Download Full-text

Permeability evolution of fracture during slipping under normal stress condition

Deep Rock Mechanics: From Research to Engineering ◽

10.1201/9781351042666-23 ◽

2018 ◽

pp. 235-242

Author(s):

Q. Zhang ◽

X.C. Li

Keyword(s):

Normal Stress ◽

Stress Condition ◽

Permeability Evolution

Download Full-text

Two types of reservoir-induced seismicity

Bulletin of the Seismological Society of America ◽

10.1785/bssa0780062025 ◽

1988 ◽

Vol 78 (6) ◽

pp. 2025-2040

Author(s):

D.W. Simpson ◽

W.S. Leith ◽

C.H. Scholz

Keyword(s):

Normal Stress ◽

Pore Pressure ◽

Induced Seismicity ◽

Elastic Stress ◽

Pore Space ◽

Temporal Distribution ◽

The Other ◽

Elastic Response ◽

Reservoir Induced Seismicity ◽

Effective Normal Stress

Abstract The temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response. At some reservoirs, seismicity begins almost immediately following the first filling of the reservoir. At others, pronounced increases in seismicity are not observed until a number of seasonal filling cycles have passed. These differences in response may correspond to two fundamental mechanisms by which a reservoir can modify the strength of the crust—one related to rapid increases in elastic stress due to the load of the reservoir and the other to the more gradual diffusion of water from the reservoir to hypocentral depths. Decreased strength can arise from changes in either elastic stress (decreased normal stress or increased shear stress) or from decreased effective normal stress due to increased pore pressure. Pore pressure at hypocentral depths can rise rapidly, from a coupled elastic response due to compaction of pore space, or more slowly, with the diffusion of water from the surface.

Download Full-text