131,059 research outputs found
Influence of dilatancy on the frictional constitutive behavior of a saturated fault zone under a variety of drainage conditions
No full textWe use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence fault strength and friction constitutive behavior. Dilatant hardening can change the frictional response and the effective critical stiffness, Kcr, which determines the transition from stable to unstable sliding in velocity weakening fault zones. We study the frictional shear strength response to numerical velocity stepping experiments and show that when the duration of pore fluid decompression is long compared to the time necessary for frictional evolution (as dictated by rate and state friction) both the effective critical slip distance (DC) and the effective shear strength direct effect (A) are increased. We investigate the role of fault zone permeability (k), dilatancy coefficient (ε), and the magnitude of shearing velocity of the fault zone (vlp) and compare results using the Dieterich and Ruina state evolution laws. Over the range from k = 10-15 to 10-21 m2, DC increases from 25 m to ∼2 mm and A increases from 0.15 to ∼5 MPa. We vary ε from 10-5 to 10-3 and the size of the velocity perturbation from 3X to 1000X and find large increases in the values of D C and A, which may lead to inhibition of unstable sliding. Our results indicate that spatial variations, with either depth or lateral extent, in ε and k could result in significant changes in the drainage state in fault zones. Such variation may lead to spatial variation of the nucleation and propagation of earthquakes based upon the drainage capabilities of the fault zone. Copyright 2011 by the American Geophysical Union
Shear-induced dilatancy of fluid-saturated faults: Experiment and theory
No full textPore fluid pressure plays an important role in the frictional strength and stability of tectonic faults. We report on laboratory measurements of porosity changes associated with transient increases in shear velocity during frictional sliding within simulated fine-grained quartz fault gouge (d50 = 127 μm). Experiments were conducted in a novel true triaxial pressure vessel using the double-direct shear geometry. Shearing velocity step tests were used to measure a dilatancy coefficient (ε = Δφ/Δln(ν), where φ is porosity and ν is shear velocity) under a range of conditions: background shearing rate of 1 μm/s with steps to 3, 10, 30, and 100 μm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7 × 10-5 to 3.0 × 10 -4 and that it does not vary with effective normal stress. We use our measurements to model transient pore fluid depressurization in response to dilation resulting from step changes in shearing velocity. Dilatant hardening requires undrained response with the transition from drained to undrained loading indexed by the ratio of the rate of porosity change to the rate of drained fluid loss. Undrained loading is favored for high slip rates on low-permeability thick faults with low critical slip distances. Although experimental conditions indicate negligible depressurization due to relatively high system permeability, model results indicate that under feasible, but end-member conditions, shear-induced dilation of fault zones could reduce pore pressures or, correspondingly, increase effective normal stresses, by several tens of megapascals. Our results show that transient increases in shearing rate cause fault zone dilation. Such dilation would tend to arrest nucleation of unstable slip. Pore fluid depressurization would exacerbate this effect and could be a significant factor in generation of slow earthquakes, nonvolcanic tremors, and related phenomena. Copyright 2009 by the American Geophysical Union
Frictional strength and strain weakening in simulated fault gouge: Competition between geometrical weakening and chemical strengthening
No full textDespite the importance of hydromechanical effects in fault processes, not much is known about the interplay of chemical and mechanical processes, in part because the conditions are difficult to simulate in the laboratory. We report results from an experimental study of simulated fault gouge composed of rock salt sheared under conditions where pressure solution is known to operate. At sliding velocities above 10 m/s and high shear strains (>5), friction measurements show that layers of rock salt weaken significantly and ultimately slide unstably (i.e., stick-slip). Microstructural observations show the presence of a zone of comminuted grains along shear zone boundaries, forming boundary-parallel Y shears at high sliding velocities. Samples deformed at low sliding velocities do not show boundary-parallel shear but rather exhibit low porosity passive regions isolated by dilational zones in the Riedel shear orientation. We posit that the significant strain weakening observed at high sliding velocities is caused by severe grain size reduction as shear localization develops, i.e., by frictional wear, ultimately leading to the development of a throughgoing boundary parallel Y shear. Unstable slip is probably related to rupture on this Y shear surface with intermittent healing of the asperities by pressure solution. Furthermore, the data show that the weakening and subsequent unstable slip can be delayed (i.e., occur at higher strains) by lower sliding velocities, larger initial grain sizes, lower normal stresses, and the presence of fluids. This suggests a competition between mechanical wear and chemical processes. Our data highlight the importance of hydrothermal processes in tectonic faulting. Copyright 2010 by the American Geophysical Union
Fabric induced weakness of tectonic faults
No full textMature fault zones appear to be weaker than predicted by both theory and experiment. One explanation involves the presence of weak minerals, such as talc. However, talc is only a minor constituent of most fault zones and thus the question arises: what proportion of a weak mineral is needed to satisfy weak fault models? Existing studies of fault gouges indicate that >30% of the weak phase is necessary to weaken faults-a proportion not supported by observations. Here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt% talc, provided the talc forms a critically-aligned, through-going layer. Observations of foliated fault rocks in mature, large-offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults. Copyright © 2010 by the American Geophysical Union
Fault zone restrengthening and frictional healing: The role of pressure solution
No full textLaboratory and field observations note the significant role of strength recovery (healing) on faults during interseismic periods and implicate pressure solution as a plausible mechanism. Plausible rates for pressure solution to activate, and the magnitudes of ultimate strength gain, are examined through slide-hold-slide experiments using simulated quartz gouge. Experiments are conducted on fine-grained (110 μm) granular silica gouge, saturated with deionized water, confined under constant normal stress of 5 MPa and at modest temperatures of 20 and 65°C, and sheared at a maximum rate of 20 μm/s. Data at 20°C show a log linear relation between strength gain and the duration of holding periods, whereas the higher temperature observations indicate higher healing rates than the log linear dependencies; these are apparent for hold times greater than ∼1000 s. This behavior is attributed to the growth and welding of grain contact areas, mediated by pressure solution. The physical dependencies of this behavior are investigated through a mechanistic model incorporating the serial processes of grain contact dissolution, grain boundary diffusion, and precipitation at the rim of contacts. We use the model to predict strength gain for arbitrary conditions of mean stress, fluid pressure, and temperature. The strength gain predicted under the experimental conditions (σeff = 5 MPa and T = 65°C) underestimates experimental measurements for hold periods of less than ∼1000 s where other frictional mechanisms contribute to strength gain. Beyond this threshold, laboratory observations resemble the trend in the prediction by our mechanistic model, implicating that pressure solution is likely the dominant mechanism for strength gain. The model is applied to the long-term prediction of healing behavior in quartzite fault zones. Predictions show that both rates and magnitudes of gain in contact area increase with an increase in applied stresses and temperatures and that fault healing aided by pressure solution should reach completion within recurrence interval durations ranging from <1 to ∼104 years, depending on applied stresses, temperatures, and reaction rates. Copyright 2005 by the American Geophysical Union
The transition from steady frictional sliding to inertia-dominated instability with rate and state friction
No full textUnstable frictional slip motions are investigated with a rate and state friction law across the transitions from stable, quasi–static slip to dynamic, stick–slip motion, and finally to, inertia dominated quasi–harmonic vibration. We use a novel numerical method to capture the full dynamics and investigate the roles of inertial and quasistatic factors of the critical stiffness defining the transition to instability, Kc. Our simulations confirm theoretical estimates of Kc, which is dependent on mass and velocity. Furthermore, we show that unstable slip motion has two distinct dynamic regimes with characteristic limit cycles: (i) stick–slip motions in the quasi–static (slowly loaded) regime and (ii) quasi–harmonic oscillations in the dynamic (rapidly loaded) regime. Simulation results show that the regimes are divided by the dynamic frictional instability coefficient, η = MV2/σaDc and stiffness of the system K. The quasi–static regime is governed by the ratio K/Kc and both the period and magnitude of stick–slip cycles decrease with increasing loading rate. In the dynamic regime, slip occurs in harmonic limit cycles, the frequency of which increases with loading velocity to a limit set by the natural frequency of the system. Our results illuminate the origin of the broad spectrum of slip behaviors observed for systems ranging from manufacturing equipment to automobiles and tectonic faults, with particular focus on the role of elasto–frictional coupling in dictating the transition from slow slip to dynamic instability. We highlight distinct characteristics of friction–induced slip motions (stick–slip and friction–induced vibration) and show that the dynamic frictional instability coefficient (η) is a key parameter that both defines the potential for instability and determines the dynamic characteristics of instability
Laboratory evidence for particle mobilization as a mechanism for permeability enhancement via dynamic stressing
No full textIt is well-established that seismic waves can increase the permeability in natural systems, yet the mechanism remains poorly understood. We investigate the underlying mechanics by generating well-controlled, repeatable permeability enhancement in laboratory experiments. Pore pressure oscillations, simulating dynamic stresses, were applied to intact and fractured Berea sandstone samples under confining stresses of tens of MPa. Dynamic stressing produces an immediate permeability enhancement ranging from 1 to 60%, which scales with the amplitude of the dynamic strain (7 × 10 -7 to 7 × 10 -6) followed by a gradual permeability recovery. We investigated the mechanism by: (1) recording deformation of samples both before and after fracturing during the experiment, (2) varying the chemistry of the water and therefore particle mobility, (3) evaluating the dependence of permeability enhancement and recovery on dynamic stress amplitude, and (4) examining micro-scale pore textures of the rock samples before and after experiments. We find that dynamic stressing does not produce permanent deformation in our samples. Water chemistry has a pronounced effect on the sensitivity to dynamic stressing, with the magnitude of permeability enhancement and the rate of permeability recovery varying with ionic strength of the pore fluid. Permeability recovery rates generally correlate with the permeability enhancement sensitivity. Microstructural observations of our samples show clearing of clay particulates from fracture surfaces during the experiment. From these four lines of evidence, we conclude that a flow-dependent mechanism associated with mobilization of fines controls both the magnitude of the permeability enhancement and the recovery rate in our experiments. We also find that permeability sensitivity to dynamic stressing increases after fracturing, which is a process that generates abundant particulate matter in situ. Our results suggest that fluid permeability in many areas of the Earth's crust, particularly where pore fluids favor particle mobilization, should be sensitive to dynamic stressing. © 2014 The Authors
Permeability evolution in sorbing media. Analogies between organic-rich shale and coal
No full textShale gas reservoirs like coalbed methane (CBM) reservoirs are promising targets for geological sequestration of carbon dioxide (CO2). However, the evolution of permeability in shale reservoirs on injection of CO2 is poorly understood unlike CBM reservoirs. In this study, we report measurements of permeability evolution in shales infiltrated separately by nonsorbing (He) and sorbing (CO2) gases under varying gas pressures and confining stresses. Experiments are completed on Pennsylvanian shales containing both natural and artificial fractures under nonpropped and propped conditions. We use the models for permeability evolution in coal (Journal of Petroleum Science and Engineering, Under Revision) to codify the permeability evolution observed in the shale samples. It is observed that for a naturally fractured shale, the He permeability increases by approximately 15% as effective stress is reduced by increasing the gas pressure from 1 MPa to 6 MPa at constant confining stress of 10 MPa. Conversely, the CO2 permeability reduces by a factor of two under similar conditions. A second core is split with a fine saw to create a smooth artificial fracture and the permeabilities are measured for both nonpropped and propped fractures. The He permeability of a propped artificial fracture is approximately 2- to 3fold that of the nonpropped fracture. The He permeability increases with gas pressure under constant confining stress for both nonpropped and propped cases. However, the CO2 permeability of the propped fracture decreases by between one-half to one-third as the gas pressure increases from 1 to 4 MPa at constant confining stress. Interestingly, the CO2 permeability of nonpropped fracture increases with gas pressure at constant confining stress. The permeability evolution of nonpropped and propped artificial fractures in shale is found to be similar to those observed in coals but the extent of permeability reduction by swelling is much lower in shale due to its lower organic content. Optical profilometry is used to quantify the surface roughness. The changes in surface roughness indicate significant influence of proppant indentation on fracture surface in the shale sample. The trends of permeability evolution on injection of CO2 in coals and shales are found analogous; therefore, the permeability evolution models previously developed for coals are adopted to explain the permeability evolution in shales
Audrey Bely
No full textAudrey Bely was one of the most innovative prose writers in Russian in the twentieth century. This book traces the development of his technique as a novelist from the early experimental Symphonies (1902–8) to the last novel, Masks, published in 1932. In the first two chapters of the book, Dr Elsworth explores Bely's theoretical writings on the aesthetic theory of Symbolism, and his association, after 1912, with the doctrines of Rudolf Steiner. Bely regarded art as an active force for the transformation of the human personality and the resolution of the crisis that he diagnosed in the culture of his time. Both the subject matter and the stylistic peculiarities of his novels have their origin in this particular philosophy of culture, and it is in this context that the novels are examined in the second half of the book. This book will be essential reading for all those interested in Bely and the wider subject of Russian Symbolist doctrine and practice.</jats:p
Breakdown pressure and fracture surface morphology of hydraulic fracturing in shale with H2O, CO2 and N2
No full textSlick-water fracturing is the most routine form of well stimulation in shales; however N2, LPG and CO2 have all been used as “exotic” stimulants in various hydrocarbon reservoirs. We explore the use of these gases as stimulants on Green River shale to compare the form and behavior of fractures in shale driven by different gas compositions and states and indexed by breakdown pressure and the resulting morphology of the fracture networks. Fracturing is completed on cylindrical samples containing a single blind axial borehole under simple triaxial conditions with confining pressure ranging from 10 to 25 MPa and axial stress ranging from 0 to 35 MPa (σ1 > σ2 = σ3). Results show that: (1) under the same stress conditions, CO2 returns the highest breakdown pressure, followed by N2, and with H2O exhibiting the lowest breakdown pressure; (2) CO2 fracturing, compared to other fracturing fluids, creates nominally the most complex fracturing patterns as well as the roughest fracture surface and with the greatest apparent local damage followed by H2O and then N2; (3) under conditions of constant injection rate, the CO2 pressure build-up record exhibits condensation between ~5 and 7 MPa and transits from gas to liquid through a mixed-phase region rather than directly to liquid as for H2O and N2 which do not; (4) there is a positive correlation between minimum principal stress and breakdown pressure for failure both by transverse fracturing (σ3axial) and by longitudinal fracturing (σ3radial) for each fracturing fluid with CO2 having the highest correlation coefficient/slope and lowest for H2O. We explain these results in terms of a mechanistic understanding of breakdown, and through correlations with the specific properties of the stimulating fluids
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