Dilatancy stabilises shear failure in rock

Mapping Intimacies ◽

10.5194/egusphere-egu21-5626 ◽

2021 ◽

Author(s):

Franciscus Aben ◽

Nicolas Brantut

Keyword(s):

Pore Pressure ◽

Fault Zone ◽

Shear Failure ◽

Rock Fracture ◽

Fluid Pressure ◽

Fault Slip ◽

Pore Fluid ◽

Lithostatic Pressure ◽

Strengthening Effect ◽

Pore Fluids

Failure and fault slip in crystalline rocks is associated with dilation. When pore fluids are present and drainage is insufficient, dilation leads to pore pressure drops, which in turn lead to strengthening of the material. We conducted laboratory rock fracture experiments with direct in-situ fluid pressure measurements which demonstrate that dynamic rupture propagation and fault slip can be stabilised (i.e., become quasi-static) by such a dilatancy strengthening effect. We also observe that, for the same effective pressures but lower pore fluid pressures, the stabilisation process may be arrested when the pore fluid pressure approaches zero and vaporises, resulting in dynamic shear failure. In case of a stable rupture, we witness continued after slip after the main failure event that is the result of pore pressure recharge of the fault zone. All our observations are quantitatively explained by a simple spring-slider model combining slip-weakening behaviour, slip-induced dilation, and pore fluid diffusion. Using our data in an inverse problem, we estimate the key parameters controlling rupture stabilisation, fault dilation rate and fault zone storage. These estimates are used to make predictions for the pore pressure drop associated with faulting, and where in the crust we may expect dilatancy stabilisation or vaporisation during earthquakes. For intact rock and well consolidated faults, we expect strong dilatancy strengthening between 4 and 6 km depth regardless of ambient pore pressure, and at greater depths when the ambient pore pressure approaches lithostatic pressure. In the uppermost part of the crust (<4 km), we predict vaporisation of pore fluids that eliminates dilatancy strengthening. The depth estimates where dilatant stabilisation is most likely coincide with geothermal energy reservoirs in crystalline rock (typically between 2 and 5 km depth) and in regions &#160;where slow slip events are observed (pore pressure that approaches lithostatic pressure).&#160;

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On the loading conditions for pore fluid stabilization of failure in crustal rock

10.5194/egusphere-egu2020-18129 ◽

2020 ◽

Author(s):

Franciscus Aben ◽

Nicolas Brantut

Keyword(s):

Pressure Drop ◽

Shear Failure ◽

Fluid Pressure ◽

Confining Pressure ◽

Pore Fluid ◽

Failure Zone ◽

Effective Pressure ◽

Loading Conditions ◽

Dynamic Failure ◽

Pore Fluid Pressure

During shear failure in rock, fracture damage created within the failure zone causes localized dilation, which, under partially drained conditions, results in a localized pore fluid pressure drop. The effective normal stress within the failure zone therefore increases, and with it the fracture and frictional strengths. This effect is known as dilatancy hardening. Dilatancy hardening may suppress rupture propagation and slip rates sufficiently to stabilize the rupture and postpone or prevent dynamic failure. Here, we study the loading conditions at which the rate of dilatancy hardening is sufficiently high to stabilize failure. We do so by measuring the local pore fluid pressure during failure and the rate of dilatancy with slip at a range of confining and pore fluid pressures.We performed shear failure experiments on thermally treated intact Westerly granite under triaxial loading conditions. The samples were saturated with water and contained notches to force the location of the shear failure zone. For each experiment, we imposed a different combination of confining pressure and pore fluid pressure, so that the overall effective pressure was either 40 MPa or 80 MPa prior to axial deformation at 10-6 s-1 strain rate. Dynamic shear failure was recognized by a sudden audible stress drop, whereas the stress drop during stabilized shear failure took longer and was inaudible. Local pore fluid pressure was measured with in-house developed pressure transducers placed on the trajectory of the prospective failure.At effective pressures of 40 MPa and 80 MPa, we observe stabilized failure for a ratio &#955; (imposed pore fluid pressure over confining pressure) > 0.5. For &#955; < 0.5, we observe dynamic failure. Of two experiments performed at &#955; = 0.5 and 80 MPa effective pressure, one showed stabilized failure and one failed dynamically. For &#955; > 0.5, we observe pore fluid pressure drops in the failure zone of 30-45 MPa for 40 MPa effective pressure, and 60 MPa for 80 MPa confining pressure. The local pore fluid pressure during dynamic failure (&#955; < 0.5) is 0 MPa, strongly suggesting local fluid vaporization. Of the two experiments at &#955; = 0.5, the dilation rate with slip is higher for the dynamic failure relative to the stabilized failure.We show that with increasing effective pressure, dilatancy hardening increases as the local pore fluid pressure drop during failure becomes larger. For &#955; < 0.5, dilatancy hardening is insufficient to stabilize failure because the local pore fluid pressure drop is larger than the absolute imposed pore fluid pressure. Near &#955; = 0.5, small variations in dilatancy control rupture stability. For &#955; > 0.5, dilatancy hardening is sufficient to suppress dynamic failure.

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Integrated Reservoir Geomechanics Approach for Reservoir Management

10.2523/iptc-21216-ms ◽

2021 ◽

Author(s):

Vai Yee Hon ◽

M Faizzudin Mat Piah ◽

Noor 'Aliaa M Fauzi ◽

Peter Schutjens ◽

Binayak Agarwal ◽

...

Keyword(s):

Pore Pressure ◽

Water Injection ◽

Fluid Pressure ◽

Integrated Approach ◽

Fault Slip ◽

Fault Reactivation ◽

Analytical Models ◽

Reservoir Compaction ◽

Stress Changes ◽

Reservoir Geomechanics

Abstract An integrated 3D dynamic reservoir geomechanics model can provide a diverse 3D view of depletion-injection-induced field stress changes and the resulting deformation of both reservoir and overburden formations at various field locations. It enables the assessment of reservoir compaction, platform site subsidence, fault reactivation and caprock integrity associated with multiple production and injection reservoirs of the field. We demonstrated this integrated approach for a study field located in the South China Sea, Malaysia, which is planned for water injection for pressure support and EOR scheme thereafter. Reservoir fluid containment during water injection is an important concern because of the intensive geologic faulting and fracturing in the collapsed anticlinal structure, with some faults extending from the reservoirs to shallow depths at or close to the seafloor. Over 30 simulations were done, and most input parameters were systematically varied to gain insight in their effect on result that was of most interest to us: The tendency of fault slip as a function of our operation-induced variations in pore pressure in the reservoir rocks bounding the fault, both during depletion and injection. The results showed that depletion actually reduces the risk of fault slip and of the overburden, while injection-induced increase in pore fluid pressure will lead to a significant increase in the risk of fault slip. Overall, while depletion appears to stabilize the fault and injection appears to destabilize the fault, no fault slip is predicted to occur, not even after a 900psi increase in pore pressure above the pore pressure levels at maximum depletion. We present the model results to demonstrate why depletion and injection have such different effects on fault slip tendency. The interpretation of these geomechanical model results have potential applications beyond the study field, especially for fields with a similar geology and development plan. This is a novel application of 3D dynamic reservoir geomechanics model that cannot be obtained from 1D analytical models alone.

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Experimental Study on the Mechanical Properties and Seepage Characteristics of Red Sandstone with a Single Persistent Joint under Triaxial Compression

Geofluids ◽

10.1155/2021/3884605 ◽

2021 ◽

Vol 2021 ◽

pp. 1-13

Author(s):

Wei Wang ◽

Shifan Liu ◽

Chong Shi ◽

Shanxi Zheng ◽

Qizhi Zhu

Keyword(s):

Mechanical Properties ◽

Pore Pressure ◽

Failure Modes ◽

Shear Failure ◽

Triaxial Compression ◽

Confining Pressure ◽

Strengthening Effect ◽

Red Sandstone ◽

Inclination Angles ◽

Seepage Characteristics

In this research, the conventional triaxial compression experiments for intact red sandstone specimens and the specimens with a single persistent joint at different inclination angles, i.e., 0°, 30°, 45°, and 90°, were conducted at first. Based on the results of the conventional tests, the effects of the confining pressure and the joint inclination angle on the mechanical properties including deformation behavior and strength parameters were summarized and analyzed, respectively. We find that the strength and deformation of jointed red sandstone are enlarged due to the increment of confining pressure, and the mechanical parameters of specimens show a U-shaped development with the rise of the joint angle. Besides, to investigate the effects of the pore pressure on seepage characteristics of rocks with joint angles at 0°, 45°, and 90°, a series of triaxial compression drainage tests on the jointed red sandstone were performed. The results show that the pore pressure has a weakening effect on the strength of jointed specimens, which can reduce the strengthening effect induced by confining pressure. Meanwhile, the tested specimens mostly present shear failure modes. As a result, the mechanical responses, seepage characteristics, and cracking modes in red sandstone containing a single persistent joint under triaxial compression are revealed.

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Temporal changes in pore fluid pressure during slow earthquake cycle estimated from foliation-parallel extension cracking

10.5194/egusphere-egu21-7760 ◽

2021 ◽

Author(s):

Makoto Otsubo ◽

Kohtaro Ujiie ◽

Hanae Saishu ◽

Ayumu Miyakawa ◽

Asuka Yamaguchi

Keyword(s):

Quartz Vein ◽

Pressure Ratio ◽

Fluid Pressure ◽

Pore Fluid ◽

Lithostatic Pressure ◽

Mode I ◽

Pore Fluid Pressure ◽

Slow Earthquake ◽

Vein Formation ◽

Parallel Extension

Pore fluid pressure (Pf) is of great importance to understand slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine m&#233;lange eastern Kyushu, SW Japan. The m&#233;lange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: a few tens of microns) may represent episodic tremor and slow slip (Ujiie et al., 2018). The foliation-parallel extension cracks can function as the fluid pathway in the m&#233;lange. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel extension vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal &#963;1-axis trending NNW&#8211;SSE and a sub-vertical &#963;3-axis, and the driving pore fluid pressure ratio P* (P* = (Pf &#8211; &#963;3) / (&#963;1 &#8211; &#963;3)) is ~0.1. When the pore fluid pressure exceeds &#963;3, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds &#963;3 is the pore fluid overpressure &#916;Pf (&#916;Pf = Pf &#8211; &#963;3). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). Pf and &#916;Pf in the case of the Makimine m&#233;lange are ~280 MPa and 80&#8211;160 kPa (assuming depth = 10 km, density = 2800 kg/m3, tensile strength = 1 MPa and Young&#8217;s modulus = 7.5&#8211;15 GPa). When the pore fluid overpressure is released, the cracks are closed and the reduction of pore fluid pressure is stopped (Otsubo et al., 2020). After the pore fluid overpressure is reduced, the normalized pore pressure ratio &#955;* (&#955;* = (Pf &#8211; Ph) / (Pl &#8211; Ph), Pl: lithostatic pressure; Ph: hydrostatic pressure) is ~1.01 (Pf > Pl). The results indicate that the pore fluid pressure constantly maintains the lithostatic pressure during the extension cracking along the foliation.References: Gudmundsson (1999) Geophys. Res. Lett., 26, 115&#8211;118; Jolly and Sanderson (1997) Jour. Struct. Geol., 19, 887&#8211;892; Otsubo et al. (2020) Sci. Rep., 10:12281; Palazzin et al. (2016) Tectonophysics, 687, 28&#8211;43; Sato et al. (2013) Tectonophysics, 588, 69&#8211;81; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371&#8211;5379, https://doi.org/10.1029/2018GL078374.

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Excess pore pressure in a poroelastic seabed saturated with a compressible fluid

Canadian Geotechnical Journal ◽

10.1139/t94-113 ◽

1994 ◽

Vol 31 (6) ◽

pp. 989-1003 ◽

Cited By ~ 27

Author(s):

Z.Q. Yue ◽

A.P.S. Selvadurai ◽

K.T. Law

Keyword(s):

Pore Pressure ◽

Fluid Pressure ◽

Offshore Structures ◽

Pore Fluid ◽

Excess Pore Pressure ◽

Soil Consolidation ◽

Normal Surface ◽

Compressible Pore Fluid ◽

Poroelastic Seabed ◽

Excess Pore

This paper presents an analytical investigation on the excess pore-fluid pressure in a finite seabed layer by taking into account the influence of a compressible pore fluid. The seabed layer is modeled as a poroelastic layer saturated with a compressible pore fluid and resting on a rough, rigid impermeable base. The surface of the poroelastic seabed layer is either completely pervious or completely impervious, and subjected to a normal surface traction induced by offshore structures. The paper presents analytical and numerical results to illustrate the time-dependent behaviour of excess pore pressure in the poroelastic seabed. The results demonstrate that the presence of a compressible pore fluid reduces the generation of excess pore pressure in the poroelastic seabed layer. Key words : excess pore pressure, poroelastic seabed layer, soil consolidation, compressible pore fluid, integral transforms.

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Mechanisms for Pore Fluid Stabilization of Fault Propagation and Slip

10.5194/egusphere-egu2020-11918 ◽

2020 ◽

Author(s):

Wen-lu Zhu ◽

Tiange Xing ◽

Takamasa Kanaya ◽

Zachary Zega ◽

Melodie French

Keyword(s):

Fluid Pressure ◽

Fault Slip ◽

Pore Fluid ◽

Initial Porosity ◽

Effective Pressure ◽

Strain Accumulation ◽

Fault Propagation ◽

Pore Fluid Pressure ◽

Wide Range ◽

Fault Growth

Sudden motions of fault (i.e., fault propagation and slip) cause earthquakes. Understanding the mechanics of earthquakes requires quantitative knowledge of fault propagation and slip instability, which has long been a focus of experimental rock mechanics. In a classic framework based on the elastic rebound theory, the earthquake cycle includes the interseismic period of strain accumulation and the coseismic period of sudden strain release along a tectonic fault.Geophysical observations reveal diverse behaviorsof fault motions resulted from strain accumulation and release, from aseismic creep to slow slip events (SSEs) to regular earthquakes. Discovery of SSEs during the interseismic period provides a new means to assess the mechanical states of a seismogenic fault between earthquakes. Most seismic studies link SSEs to high pore fluid pressure. Yet, the mechanical link between slow fault slip and high pore fluid pressure is not well understood. We conduct experimental investigation to elucidate the mechanisms responsible for pore fluid stabilization of fault propagation and slip.Our experimental results show that slip events along gouge bearing faults can transform from fast to slow with increasing pore fluid pressures while keeping the effective pressure (i.e., confining pressure minus pore fluid pressure) constant. In these experiments, a layer of fine-grained quartz gouge was placed between the saw-cut surfaces in porous sandstone samples. The saw-cut samples were subject to conventional triaxial loading under a constant effective pressure using various combinations of confining and pore fluid pressures. Different slip events, from dynamic, audible stick-slip to slow, silent&#160; slip, with a range of slip rates and stress drops were produced along the gauge-filled saw-cut surface. These results suggest that on the same fault, varying pore fluid pressure alone could result in a range of fault slip behaviors from dynamic to creep.Experimental data further demonstrate that under the same effective pressure, high pore fluid pressure conditions stabilize fault propagation in a wide range of intact rocks including granite, serpentine, and sandstones. In &#160;compact rocks (initial porosity <5%) the stabilization effect can be explained by dilatant hardening. When dilatancy occurs faster than fluid diffusion along a propagating fracture, the resultant increase in effective normal stress impedes further fracture growth. In porous sandstones (initial porosity >10%), however, dilatancy hardening alone could not adequately explain the stable&#160; post-peak fault growth observed at slow loading rates where drained conditions are achieved. Based on the quantitative microstructural analysis of the deformed samples, we propose that the stable fault growth in highly permeable sandstones manifests stable cracking due to stress corrosion. These results elucidate the important controls of pore fluid on rock strength and fault slip beyond the effective stress law. The results provide a mechanic link between the spatially correlated SSEs and high pore fluid pressure conditions.

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Change of Pore Fluid Pressure Versus Frictional Coefficient During Fault Slip

Earthquake Research and Analysis - Seismology, Seismotectonic and Earthquake Geology ◽

10.5772/29118 ◽

2012 ◽

Cited By ~ 1

Author(s):

Yuta Mitsui

Keyword(s):

Fluid Pressure ◽

Frictional Coefficient ◽

Fault Slip ◽

Pore Fluid ◽

Pore Fluid Pressure

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Is complex fault zone behaviour a reflection of rheological heterogeneity?

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2019.0421 ◽

2021 ◽

Vol 379 (2193) ◽

pp. 20190421 ◽

Cited By ~ 1

Author(s):

Å. Fagereng ◽

A. Beall

Keyword(s):

Fault Zone ◽

Numerical Models ◽

Finite Thickness ◽

Fluid Pressure ◽

Plate Boundary ◽

Shear Zones ◽

Fault Slip ◽

Two Phase ◽

Fault Properties ◽

Stress Amplification

Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

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Analysis of the influence of joint direction on production optimization in enhanced geothermal systems

Journal of Petroleum Exploration and Production Technology ◽

10.1007/s13202-021-01254-7 ◽

2021 ◽

Author(s):

Dorcas S. Eyinla

Keyword(s):

Pore Pressure ◽

Production Rate ◽

Shear Failure ◽

Fluid Pressure ◽

Production Optimization ◽

Stress Change ◽

Geothermal Reservoir ◽

Injection Well ◽

Joint Models ◽

Joint Orientation

AbstractHeat extraction from geothermal reservoir by circulating cold water into a hot rock requires an amount of fluid pressure, which is capable of inducing fault opening. Although stress change promotes the potential of fault failure and reactivation, the rate at which fluid pressurization within the fault zone generates variations in pore pressure as fault geometry changes during geothermal energy production have not been thoroughly addressed to include the effects of joint orientation. This study examines how different fault/joint models result in different tendency of injection-induced shear failure, and how this could influence the production rate. Here, a numerical simulation method is adopted to investigate the thermo-hydro-mechanical (THM) response of the various fault/joint models during production in a geothermal reservoir. The results indicate that pore pressure evolution has a direct relationship with the evolution of production rate for the three joint models examined, and the stress sensitivity of the individual fault/joint model also produced an effect on the production rate. Changing the position of the injection well revealed that the magnitude of shear failure on the fault plane could be controlled by the hydraulic diffusivity of fluid pressure, and the production rate is also influenced by the magnitude of stress change at the injection and production wells. Overall, the location of the injection well along with the fault damage zone significantly influenced the resulting production rate, but a more dominating factor is the joint orientation with respect to the maximum principal stress direction. Thus, the rate of thermal drawdown is affected by pore pressure elevation and stress change while the fault permeability and the production rate are enhanced when the joint’s frictional resistance is low.

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