Effects Of Pore Fluid Pressure Research Articles

Effects of pore-fluid pressure on the motion of debris flows

ABSTRACT Pore-fluid pressure mediates the shear strength between soil particles and significantly controls the movement of debris flows. This study analyzes the effects by proposing a numerical model to solve the pore-fluid diffusion equation coupled to changes in surge motion of debris flows. The model was established by combining a Lagrangian method solving the depth-averaged one-dimensional equations for the debris-flow motion and a Fourier-series solution for the pore-fluid diffusion equation. Owing to the source term related to the change in surge depth, the Fourier-series solution shows that the excess pore-fluid pressure near the bed dissipates faster and exhibits a non-monotonic profile. The results of numerical calculations reveal that the diffusivity of excess pore-fluid pressure and the liquefaction condition have significant impacts on the depth profiles and velocities of debris-flow surges.

Slope stability analysis considering fully saturated poro-elasto-plasticity by an image-based scaled boundary finite element approach

Open pit mines are large geotechnical structures. Their stability is an important consideration in the mining industry. The deformations of geotechnical structures often involve the coupled interaction between the pore fluid pressure and the nonlinear deformation of soil, characterised by poro-elasto-plastic behaviour. This paper develops the scaled boundary finite element method (SBFEM) to address poro-elasto-plastic in slope stability problems. It builds upon a previously developed elasto-plastic formulation to consider the effect of pore fluid pressure and its interaction with the nonlinear deformation within the soil. The pore pressure field introduces an additional variable in the governing equations that is similarly discretised using SBFEM shape functions. The SBFEM is implemented together with a pixel-based quadtree mesh generation technique, enabling automatic meshing directly from digital images. This leads to efficient automation when modelling problems with iterative changes in the geometry such as in optimisation of construction processes during the rehabilitation of slopes. The formulation is validated first using a standard numerical benchmark. Application of the developed technique in construction applications in slopes where the stability and effect of pore water pressure is considered e.g., tailings dam construction and optimisation of backfilling process is demonstrated in three examples to demonstrate feasibility.

Shear instability in geothermal fields under hydromechanical feedback

A recent trend in improving the seismic safety and economic efficiency of enhanced geothermal systems in deep rock formation has led to an interest in the underlying mechanisms of shear stimulation. This contribution investigates the formation of shear localization patterns around an injection hole in deep geothermal reservoirs considering thermo-hydro-mechanical coupled processes. The constitutive formulation treats the target rock layer as an elasto-visco-plastic material with an Arrhenius dependence taking into account the effect of pore fluid pressure. Considering the thermal pressurization effect resulting from frictional heating, the role of system heterogeneity and the influence of injected fluid temperature are investigated. The findings of this study indicate that a deeper layer and a hotter injection would favor localized shear deformation around a deep borehole and hence an effective network of fluid pathways.

The State of Pore Fluid Pressure and 3‐D Megathrust Earthquake Dynamics

AbstractWe study the effects of pore fluid pressure (Pf) on the pre‐earthquake, near‐fault stress state, and 3‐D earthquake rupture dynamics through six scenarios utilizing a structural model based on the 2004 Mw 9.1 Sumatra‐Andaman earthquake. As pre‐earthquake Pf magnitude increases, effective normal stress and fault shear strength decrease. As a result, magnitude, slip, peak slip rate, stress drop, and rupture velocity of the scenario earthquakes decrease. Comparison of results with observations of the 2004 earthquake support that pre‐earthquake Pf averages near 97% of lithostatic pressure, leading to pre‐earthquake average shear and effective normal tractions of 4–5 and 22 MPa. The megathrust in these scenarios is weak, in terms of low mean shear traction at static failure and low dynamic friction coefficient during rupture. Apparent co‐seismic principal stress rotations and absolute post‐seismic stresses in these scenarios are consistent with the variety of observed aftershock focal mechanisms. In all scenarios, the mean apparent stress rotations are larger above than below the megathrust. Scenarios with larger Pf magnitudes exhibit lower mean apparent principal stress rotations. We further evaluate pre‐earthquake Pf depth distribution. If Pf follows a sublithostatic gradient, pre‐earthquake effective normal stress increases with depth. If Pf follows the lithostatic gradient exactly, then this normal stress is constant, shifting peak slip and peak slip rate updip. This renders constraints on near‐trench strength and constitutive behavior crucial for mitigating hazard. These scenarios provide opportunity for future calibration with site‐specific measurements to constrain dynamically plausible megathrust strength and Pf gradients.

Tensile Failure and Fracture Width of Partially Permeable Wellbores with Applications in Lost Circulation Material Design

SummaryEstimation of near-wellbore fracture widths remains central to designing the particle size distribution (PSD) and composition of lost circulation material (LCM) blends. Although elastic rock models are often used for this purpose, they fall short in capturing the substantial effect of pore fluid pressure on the fracture width. The problem is addressed in this paper by incorporating the poroelastic back stress in width estimation of axial fractures nearby an inclined wellbore. The poroelastic back stress is caused by a nonideal drilling fluid filter cake allowing for fluid pressure communication between the wellbore and pore space of the rock surrounding the wellbore. In this aspect, a proper definition of the filter-cake efficiency is made in terms of the wellbore pressure, far-field pore fluid pressure, and pore fluid pressure of the rock surrounding the wellbore. The value of this parameter is estimated from the standard drilling fluid filtration test results, as well as the formation rock permeability. The filter-cake efficiency is next used to develop the long-time, asymptotic analytical solution for the poroelastic stress of an inclined wellbore. By accounting for the obtained poroelastic back stress, an improved estimation of the wellbore tensile limit that depends on the filter-cake efficiency parameter is developed. For wellbore pressures beyond the wellbore tensile limit, the width of the near-wellbore fractures is estimated. The fracture width estimation is made through an analytical, dislocation-based fracture mechanics solution to the integral equation describing the nonlocal stress equilibrium along the fracture faces. The commonly practiced scheme for geometric design of LCM blends is enhanced by using the presented improvement in estimation of the near-wellbore fracture width. A case study is used to demonstrate the substantial effect of drilling fluid filtration properties and the resulting poroelastic back stress on the wellbore tensile limit, estimated fracture width, and consequently, composition of the recommended LCM blend.

Plastic modeling of porous rocks in drained and undrained conditions

This paper is devoted to numerical modeling of plastic behavior of porous rocks in both drained and undrained conditions. Based on the experimental data, we propose a new plastic model based on relevant micro-mechanical results, by considering plastic shearing and pore collapse processes. The effect of porosity on the macroscopic plastic strength is explicitly taken into account. A specific plastic hardening law of the solid matrix is identified. The concept of effective stress is used for accounting for the effect of pore fluid pressure on the plastic deformation process. The proposed model is verified against drained and undrained triaxial compression tests performed on two typical rocks.

Effect of Pore Fluid Pressure on the Normal Deformation of a Matched Granite Joint

The influence of pore fluid pressure on the normal deformation behaviors of joints is vital for understanding the interaction between hydraulic and mechanical processes of joints. The effect of pore fluid pressure on the normal deformation of a granite matched joint was investigated by laboratory experiments. Experimental results indicate pore fluid pressure significantly affects the normal deformation of jointed sample, and the relative normal deformation of jointed sample during fluid injection consists of the opening of the joint and the dilation of host rock. The action of pore fluid pressure on the joint follows the Terzaghi’s effective stress law. The normal deformation of the joint can be well quantitated by the generalized exponential model. The relative normal deformation of host rock during fluid injection would have a linear relationship with pore fluid pressure, and if affected by gas is more pronounced than water.

Computational simulation of iron ore-forming processes in the Caiyuanzi siderite ore district, Guizhou, China

By means of the FISH language in the FLAC code, the coupled pore-fluid flow, heat transfer and mineralization have been simulated first with a generic model, and then with an actual geological model in the Caiyuanzi siderite ore district, Guizhou. The effect of pore-fluid pressure on the equilibrium concentration of siderite has been considered. The main purposes of this paper are: (1) to illustrate some simulation results for the possibility of hydrothermal mineralization of siderite in a sedimentary environment; (2) to get a better understanding of the processes and mechanism of siderite orebody formation in the sedimentary environment; (3) to analyze the influence of the concentration of carbon dioxide on the siderite mineralization; and (4) to compare the simulation results with the actual situation, so as to demonstrate that this simulation method is effective and feasible for understanding the mineralization mechanism of iron ore in the Caiyuanzi siderite ore district.

蛇紋岩の弾性波速度に対する間隙水圧の影響

蛇紋岩の弾性波速度に対する間隙水圧の影響

Effects of pore fluid pressure and tectonic stress on diverse seismic activities around the Mt. Ontake volcano, central Japan

To understand the effects of pore fluid pressure and tectonic stress in generating diverse seismicity, we deployed 11 temporary seismic stations around an active volcano, Mt. Ontake, in central Japan during the summers of 2009–2011. We obtained well-constrained focal mechanism solutions from the seismic data recorded at the temporary stations and permanent stations located within 50 km around Mt. Ontake. We estimated the 3-D pore fluid pressure field, taking into consideration spatial changes in the tectonic stress pattern, by applying the method of focal mechanism tomography to the dataset. We found overpressurized fluid reservoirs with a peak of 100–150 MPa at depths between 5 and 12 km in the southeast and east flanks of Mt. Ontake. This region has experienced earthquake swarm activity since 1976. Many seismic events concentrated at the edge of the overpressurized fluid reservoirs, where pore fluid pressure gradients are steep. This indicates that a large amount of fluid flow would trigger seismic events by decreasing fault strength caused by an increase in pore fluid pressures. Many small events (M < 3) occurred on pre-existing faults that are unfavorably oriented or severely mis-oriented relative to the tectonic stress pattern, indicating that these events are driven by overpressurized fluids. However, the macroscopic rupture planes of microseismic clouds and larger events (M ≥ 3) appear to release tectonic stresses produced by steady plate subduction. The critical event magnitude (Mc = 3) at which seismic behavior changes would be characterized by a length scale of pore fluid pressure heterogeneity. This quantity might be an indicator of the maximum event magnitude in the region.

Effects of pore fluid pressure on slip behaviors: An experimental study

Slow slip behaviors are suggested to have a close correlation with the presence of excess pore fluid pressure. In this study, we conducted deformation experiments with and without excess pore pressure on intact porous sandstone samples to investigate effect of pore fluid pressure on rupture growth and slip instability. Experimental conditions are such that the samples failed either in brittle faulting or transitional regimes. The experimental results indicate that in the brittle faulting regime prone to seismic deformation, excess pore pressure causes reduction in brittle strength but no detectable difference in slip behavior compared to the cases without excess pore pressure. In the transitional regime, which is prone to stable deformation, excess pore pressure induces slip instability on otherwise stable sliding faults, but there are measurable differences compared to the unstable slip in the brittle faulting regime due to the interaction between unstable crack growth and dilatancy hardening.

Changes in seismic activity following the 2011 Tohoku-oki earthquake: Effects of pore fluid pressure

We examined the effects of pore fluid pressure on seismicity changes following the 2011 off the Pacific coast of Tohoku (Tohoku-oki) earthquake through the analysis of aftershocks based on the Coulomb failure criterion. Background tectonic stress fields in Northeast Japan are generally characterized by E–W compression. After the Tohoku-oki earthquake, as expected from decrease in the Coulomb failure function (=shear stress−0.4×normal compressive stress), seismicity in the upper crust of Northeast Japan decreased except some restricted regions, where we observed many aftershocks with unfavourable focal mechanisms to the background stress fields. By mapping the focal mechanisms of aftershocks on the 3-D Mohr diagram region by region, we confirmed that the aftershocks occurred on optimally oriented faults in some regions but on misoriented faults in other regions. The aftershocks on optimally oriented faults indicate the increase in regional ambient fluid pressure caused by the flow of over-pressurized fluid from a deep reservoir. On the other hand, the aftershocks on misoriented faults, which cannot be attributed to coseismic stress rotation, indicate the increase in fault-confined fluid pressure relative to the ambient fluid pressure.

Effect of pore fluid pressure on the frictional strength of antigorite serpentinite

Triaxial deformation experiments on antigorite serpentinite show a systematic correlation between shear stress and effective normal stress over a wide range of pore pressures (50<Pp<143MPa). Under dry conditions using Ar gas as the pore fluid, the steady-state friction coefficient (μ) is estimated to be 0.66±0.01. In contrast, under wet conditions using distilled water as the pore fluid, μ=0.51±0.01 (i.e., less than under dry conditions). This difference in μ may be caused by charged water on the serpentine surface, which reduces the frictional resistance of the sliding surface in wet environments. The linear correlation between shear stress and effective normal stress suggests that Coulomb's law applies to sliding surfaces during the injection and ejection of pore fluids. This results in a significant mechanical weakening due to elevated pore fluid pressures; consequently, the locations of asperities on plate boundaries may be controlled by the heterogeneous distribution of fluids on the subducting plate interface.

Effects of permeability and porosity evolution on simulated earthquakes

Numerical simulations are a fundamental tool to access the typical conditions attained during earthquake instabilities and to simulate the large number of dissipative processes taking places during faulting. In this study we consider a single-degree-of-freedom spring-slider system, a simplified fault model which can describe the whole seismic cycle and the dynamics of a fault with spatially homogeneous properties. We assume a rate- and state-dependent friction in which we incorporate the effects of pore fluid pressure, thermally-pressurized as a consequence of the frictional heat produced during sliding. We explore, in a single framework, the role of the time variations of the porosity, permeability or both, ultimately leading to changes in hydraulic diffusivity, which has been recognized as one of the key parameters in thermally-pressurized faults. Our synthetic ruptures show that the changes in the hydraulic diffusivity only due to porosity variations do not markedly affect the earthquake recurrence (cycle time), the traction evolution and the thermal history of the fault. On the contrary, when the evolutions of both the porosity and the permeability are accounted for, the cycle time is significantly reduced. This result has a clear implication in the context of the hazard assessment.

Effects of pore fluid pressure on the seismic response of a fractured carbonate reservoir

An effective medium model for the stress-dependent seismic properties of fractured reservoirs is developed here on the basis of a combination of a general theory of viscoelastic waves in rock-like composites with recently published formulae for deformation of communicating and interacting cavities (interconnected pores/cracks and fractures at finite concentration) under drained loading. The inclusion-based model operates with spheroidal cavities at two different length scales; namely, the microscopic scale of the pores and (grain-boundary) cracks, and the mesoscopic scale of the fractures (controlling the flow of fluid). The different cavity types can in principle have any orientation and aspect ratio, but the microscopic pores/cracks and mesoscopic fractures were here assumed to be randomly and vertically oriented, respectively. By using three different aspect ratios for the relatively round pores (representing the stiff part of the pore space) and a distribution of aspect ratios for the relatively flat cracks (representing the compliant part of the pore space), we obtained a good fit between theoretical predictions and ultrasonic laboratory measurements on an unfractured rock sample under dry conditions. By using a single aspect ratio for the mesoscopic fractures, we arrived at a higher-order microstructural model of fractured porous media which represents a generalization of the first-order model developed by Chapman et al. (2002,2003). The effect of cavity size was here modelled under the assumption that the characteristic time for wave-induced (squirt) flow at the scale of a particular cavity (pore/crack vs. fracture) is proportional with the relevant scale-size. In the modelling, we investigate the effect of a decreasing pore pressure with constant confining pressure (fixed depth), and hence, increasing effective pressure. The analysis shows that the attenuation-peak due to the mesoscopic fractures in the reservoir will move downward in frequency as the effective pressure increases. In the range of seismic frequencies, our modelling indicates that the P-wave velocities may change by more than 20% perpendicular to the fractures and close to 10% parallel to the fractures. In comparison, the vertical S-wave velocities change by only about 5% for both polarization directions (perpendicular and parallel to the fractures) when the effective pressure increases from 0 to 15 MPa. This change is mainly due to the overall change in porosity with pressure. The weak pressure dependence is a consequence of the fact that the S waves will only sense if the fractures are open or not, and since all the fractures have the same aspect ratio, they will close at the same effective pressure (which is outside the analysed interval).

Modeling seismic wave propagation during fluid injection in a fractured network: Effects of pore fluid pressure on time-lapse seismic signatures

Fluid flow in the earth's crust plays an important role in a number of geologic processes. In carbonate reservoirs, fluid flow is thought to be controlled by open macrofractures. The movement of fluids in the fractured media results in changes in the pore pressure and consequently causes changes in the effective stress, traction, and elastic properties. Many recent examples in time-lapse or 4D seismic surveys have demonstrated that seismic waves can be used to monitor changes in oil or gas reservoirs as a function of time (e.g., Landro, 2002; Angerer et al., 2002). During production from a reservoir, the movement of fluids is accompanied by substantial change in the pore pressure field. As fluids drain, pore pressure decreases, which increases the effective pressure on fractures, grain boundaries, and microcracks. Higher static load on these surfaces decreases their compliance nonlinearly and decreases fracture opening and/or pore throat size, thus increasing the stiffness of the rock (by increasing compressional and shear velocities) and decreasing permeability (Schoenberg, 2002). Conversely, pore pressure buildup due to injection leads to a decrease in effective pressure and an increase in rock compliance. Fractured rock is often modeled as a relatively rigid, defect-free, “background” medium with embedded sets of linear slip interfaces. A linear slip interface is a surface across which anomalously large strain occurs due to the passage of a wave. In linear slip deformation theory, the large strain is approximated by a displacement discontinuity across the surface that is linearly related to the dynamic traction acting on the interface (to the first order). The dynamic elastic properties of the rock are determined by adding the compliance tensor of the background to an excess fracture compliance tensor associated with the fractures (e.g., Liu et al., 2000). The linear parameters governing the infinitesimal slip on these planes have …

Hydrotectonic regime at soles of overthrust sheets: textural and fluid inclusion evidence from basal cataclasites of the Muráň nappe (Western Carpathians, Slovakia)

Newly formed, synkinematically grown minerals in basal cataclasites of the thin-skinned Muráň nappe contain primary fluid inclusions which represent samples of a hydraulic lubricant of the overthrusting nappe block. The inclusion fluid corresponds to a high-salinity brine (37–58 wt% NaCl eq.) of the system Na-K-Ca-Mg-Cl-SO 4-CO 2. Trapping temperatures between 213 and 471 °C are substantially higher than weak metamorphic imprint of the overlying thrust body. Strongly fluctuating inclusion fluid densities and trapping pressures between 0.2 and 5.4 kbar are interpreted to reflect locally supralithostatic overpressures induced by frictional heating beneath the moving nappe. The results of this study corroborate the hypothesis of Hubert and Rubey [Bull. Geol. Soc. Am. 70 (1959) 115–206], who envisaged a profound effect of pore fluid pressure at the base of moving thrust blocks.

A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna, India

Triggering of earthquakes by filling of artificial water reservoirs is known for over six decades. As of today, over 90 sites have been globally identified where earthquakes have been triggered by filling of water reservoirs. The question of earthquakes triggered by artificial reservoirs has been addressed and reviewed in a number of papers and books. In the present review, the book “Reservoir-Induced Earthquakes” by Gupta [Gupta, H.K., 1992. Reservoir-Induced Earthquakes. Elsevier, Amsterdam, 364 pp.], which contains all the necessary information on this topic till 1990, has been taken as the base. An effort has been made to add information on this important topic gathered over the last 10 years. Koyna, India continues to be the most significant site of artificial-water-reservoir-triggered earthquakes. During 1990s, two events exceeding M 5 and several smaller events occurred in the vicinity of Koyna, and recently impounded Warna Reservoir. Detailed studies have addressed the relocation of earthquakes, stress drop, nucleation, migration and other important aspects of these earthquakes. In a unique experiment, twenty-one 90- to 250-m deep borewells have been drilled in the seismically active Koyna–Warna region and the water levels are continuously monitored. Step-like coseismic changes of several centimeters have been observed in some wells associated with a few M≥4 events. Detailed tomographic studies conducted on one of the best recorded triggered earthquake sequence at Lake Oroville in California revealed that this sequence was associated with a southwest dipping structure characterized by low velocity, while in adjacent areas, seismic activity occurs in regions of higher velocity. Similar investigations in Aswan showed that shallow activity is associated with low P-wave velocity. Several new reservoir sites that have triggered earthquakes have been reported during 1990s. The most important being Srinagarind Dam in Thailand, which had an M 5.9 earthquake and the sequence had all the characteristics of triggered earthquake sequences. New theoretical work, particularly the effect of pore fluid pressure in anisotropic rocks and its implication in triggered seismicity is an important development. However, much more needs to be done to fully comprehend the role of artificial water reservoirs in triggering earthquakes.

A modeling study on the controlling factors of seismic source distribution and their strength index

A model, in which various seismic environmental factors are involved, has been developed in the paper on the bases of relevant data. The environmental factors include the crust structure, lithological conditions, fault plan attitude, the crust stress state, pore-fluid pressure and geothermal conditions, etc. The effect of each of the factors on the tectonic movement energy index has been analyzed. The model calculation results indicate that shear fracture energy of a fault generally increases with depth; it reached a peak value at a certain depth, then turns gradually attenuated. Of all the factors, the effects of pore-fluid pressure and geothermal conditions on the energy index are of prominence. High pore-fluid pressure and high temperature circumstance may result in decline of the peak value of the shear fracture energy curve, making the depths of the peak value and of the bell-waist value deepened. Such effects restrict strong seismic events, but suitable for micro-seismic activity and/or fault creep. The lower limit of focal depth under such an environment is relatively deep. Contrarily, low pore-fluid pressure and low geothermal temperature circumstance result in increase of the peak value of the shear fracture energy curve, and the curve becoming steep. The later circumstance is favorable to the formation of locked segment where possesses high strength and high-energy accumulation. If these two kind of segments are arranged interactively along a fault zone, such arrangement would boost the energy transition among the segments, forming mechanical circumstance of energy highly accumulated in one segment with adjacent segment(s) less-locked. Such contrast of the strength and energy would provide conditions for strong earthquakes with high stress drop. An analysis of the northwest segment of the Honghe fault has provided some evidence for the modeling results.

Debris-flow deposition: Effects of pore-fluid pressure and friction concentrated at flow margins

Measurements of pore-fluid pressure and total bed-normal stress at the base of several ~10 m 3 experimental debris flows provide new insight into the process of debris-flow deposition. Pore-fluid pressures nearly sufficient to cause liquefaction were developed and maintained during flow mobilization and acceleration, persisted in debris-flow interiors during flow deceleration and deposition, and dissipated significantly only during postdepositional sediment consolidation. In contrast, leading edges of debris flows exhibited little or no positive pore-fluid pressure. Deposition therefore resulted from grain-contact friction and bed friction concentrated at flow margins. This finding contradicts models that invoke widespread decay of excess pore-fluid pressure, uniform viscoplastic yield strength, or pervasive grain-collision stresses to explain debris-flow deposition. Furthermore, the finding demonstrates that deposit thickness cannot be used to infer the strength of flowing debris.