Effect Of Pore Pressure Research Articles

Effects of pore pressure on coring-induced damage based on simulation by mesoscale stress-flow coupling numerical model

Effects of pore pressure on coring-induced damage based on simulation by mesoscale stress-flow coupling numerical model

Insights from the numerical analysis of axially loaded piles in liquefiable soils

Axially loaded piles in liquefiable soils can undergo severe settlement due to an earthquake event. During shaking, the settlement is caused by the decreased shaft and tip capacity from excess pore pressures (ue) generated around the pile. Post shaking, soil settlement from the reconsolidation of liquefied soil surrounding the pile results in the development of additional load (known as drag load), causing downdrag settlement of the pile. Estimating the axial load distribution and pile settlement is essential for designing and evaluating the performance of axially loaded piles in liquefiable soils. In practice, a simplified neutral plane solution method is used, where the liquefied soils are modeled as a consolidating layer without considering the effect of ue generation/dissipation. A TzQzLiq analysis models the load and settlement response of axially loaded piles in liquefiable soils by accounting for the effect of excess pore pressure (ue) generation/dissipation on the shaft and tip capacity. This paper presents the deficiencies of the simplified neutral plane method in predicting the drag load as well as the downdrag settlement by comparing it with the TzQzLiq analysis validated with hypergravity model tests. The results show that the drag load and the downdrag settlement predicted by the neutral plane method might be over- or under-estimated depending on the pile load, the rate of ue dissipation, and the soil settlement. For the cases studied, it was found that most of the pile settlement occurs during shaking due to the decrease in the pile's tip resistance from the development of ue in the soil surrounding it. While large drag loads develop during reconsolidation, the resulting downdrag settlement is small. While the neutral plane method generally predicted a downdrag settlement comparable to that of the TzQzLiq analysis, it overpredicted drag load and could not predict co-seismic settlement. Finally, the study advocates for the development and use of a displacement-based procedure (accounting for all the mechanisms occurring during and after an earthquake event) such as based on TzQzLiq analysis in accurately evaluating the performance of the pile (i.e., the pile settlement and the maximum load), thus providing an overall safe, efficient, and optimized design.

Effect of Excess Pore Pressure on Earthquake‐Induced Displacement of Partially Saturated Sandy Soil Slopes: Flexible Sliding Block Analysis

Effect of Excess Pore Pressure on Earthquake‐Induced Displacement of Partially Saturated Sandy Soil Slopes: Flexible Sliding Block Analysis

Understanding the effects of pore pressure–induced crack deformation on the acoustic anisotropy of rocks with aligned cracks

AbstractCracks are extensively existing in rocks and play a significant role in the acoustic anisotropy of cracked rocks. Rocks in nature are affected by pore pressure, whereas the crack deformation with pore pressure and the impacts of the crack deformation on the anisotropic acoustic properties remain little known. Combining the theoretical model with the laboratory measurements of the anisotropic velocities of artificial sandstone samples with and without aligned penny‐shaped cracks, we invert for the crack parameters that characterize the crack deformation as a function of pore pressure and theoretically simulate the impacts of pore pressure–induced variation in the crack parameters on the anisotropic velocities. The results show that with increasing pore pressure, the inverted crack porosity increases exponentially, whereas the inverted crack aspect ratio decreases exponentially and the two crack parameters are linearly correlated. Moreover, model calculation demonstrates that the anisotropic velocities exhibit distinct reductions with the variation in the crack parameters caused by increasing pore pressure. In particular, the reduction in the velocity of the shear wave travelling parallel to the crack plane with polarization perpendicular to the crack plane is the most pronounced. We also demonstrate that the effects of the pore pressure–induced increasing crack porosity on the anisotropic velocities are more pronounced than the impacts of the decreasing crack aspect ratio. The findings not only reveal the variation of the crack geometry with pore pressure and the effects of the crack deformation on the anisotropic velocities of the cracked rocks but also can provide theoretical support for improving the characterization of the cracks through seismic survey.

Multistep procedure for estimating non-linear soil response in low seismicity areas—a case study of Lucerne, Switzerland

SUMMARY The impact of non-linear soil behaviour on seismic hazard in low-to-moderate seismicity areas is often neglected; however, it may become relevant for long return periods. In this study, we used fully non-linear 1-D simulations to estimate the site-specific non-linear soil response in the low seismicity area, using the city of Lucerne in Switzerland as an example. The constitutive model considers the development of pore pressure excess and requires calibration of complex soil models, including the soil dilatancy parameters. In the absence of laboratory measurements, we mainly used the cone penetration test data to estimate the model variables and perform inversion for the dilatancy parameters. Our findings, using Swiss building code-compatible input ground motions, suggest a high probability of strong non-linear behaviour and the possibility of liquefaction at high ground motion levels in the case study area. While the non-linearity observations from strong-motion recordings are not available in Lucerne, the comparison with empirical data from other sites and other methods shows similarity with our predictions. Moreover, we show that the site response modelled is largely influenced by the strong pore pressure effects produced in thin sandy water-saturated layers. In addition, we demonstrate that the variability of the results due to the input motion and the soil parameters is significant, but within reasonable bounds.

Hydro‐Mechanical Characterization of a Fractured Aquifer Using Groundwater Level Tidal Analysis: Effect of Pore Pressure and Seismic Dynamic Shear Stresses on Permeability Variations

AbstractGroundwater level tidal analysis is a powerful technique to monitor aquifer's permeability and hence its change over time. Earthquakes are known to affect aquifer's properties, in their vicinity through static stress changes but also further away through dynamic stresses. Most often changes are in the form of permeability increases, but sometimes decreases; the changes can be either permanent or transient. These observations are relatively well documented but the physical processes behind these changes are not well understood. By combining solid‐earth and barometric tidal groundwater level responses in a borehole in a coherent poroelastic theoretical framework, and a bi‐layer hydrogeological model, we recover a 15 years‐long time series evolution of aquifer transmissivity and shear modulus. This study showcases the full potential of the tidal analysis method, coupling pore pressure diffusion and rock deformation, at the frontier of hydrogeology and rock physics. This unprecedented measurement of permeability and shear modulus evolution by tidal analysis reveals, during interseismic period, high sensitivity of this shallow aquifer to effective stress, and thus to pore pressure. Thanks to additional finite element simulation of seismic wave propagation, we explore the different mechanisms affecting permeability and shear modulus in the studied fractured andesite aquifer. This study confirms the predominant role of seismic dynamic stresses, and more precisely of dynamic shear stresses, in the change of permeability following an earthquake.

Nonlinear hydroelastic vibration of foamed concrete beams via peridynamic differential operator

Evaluating the hydroelastic responses of underwater cementitious structural elements is critical for ensuring the sustainability and durability of energy-saving marine infrastructures. Existing work on the hydroelastic analysis of porous structures has been mostly developed using the general elastic constitutive relation; however, it fails to capture the influence of saturation. To fill this knowledge gap, we for the first time propose a novel fluid-porous structure interactive model that incorporates the combined effects of hydrodynamic pressure and saturation-induced pore pressure. One more pioneering effort is to solve this nonlinear hydroelastic problem by introducing peridynamic differential operator (PDDO). It is worth noting that the introduction of PDDO removes the inherent drawback employing the local-theory based techniques, namely being prone to singularities arising from the presence of discontinuity. The accuracy and reliability of the proposed numerical framework are validated by comparing the results with the degraded model in the reported literature. Moreover, our results highlight that the angular frequencies are underestimated when ignoring the effect of saturation in foamed concrete beams. The presented method provides a profound understanding of the underwater structural dynamic monitoring that benefits the design of marine infrastructures.

Coupled DEM/CFD analysis of impact of free water on the static and dynamic response of concrete in tension regime

In this paper, the quasi-static and dynamic behavior of partially fluid-saturated concrete under two-dimensional (2D) uniaxial tension at the mesoscale is investigated numerically. It was calculated what effect the free pore fluid content (gas and water) had on the fracture process and strength of concrete in a tension regime. An improved pore-scale hydro-mechanical model based on DEM/CFD was used to simulate the behavior of fully and partially fluid-saturated concrete in quasi-static and dynamic conditions. The foundation of the fluid flow idea was a network of channels in a continuous area between discrete elements. In very porous, partially saturated concrete, a two-phase laminar fluid flow was proposed. To track the liquid/gas content, the position and volumes of the pores and cracks were considered. Numerical simulations of bonded granular specimens of a simplified spherical mesostructure that resembled concrete were carried out under dry and wet circumstances for two different strain rates. Investigations were conducted on the effects of fluid pore pressures, fluid saturations, and fluid viscosities on the strength and fracture process of concrete. The quasi-static tensile strength dropped nonlinearly with increasing fluid saturation and fluid viscosity during fluid migration through pores and cracks which contributed to a promoted fracture process. However, during rapid dynamic tensile deformation, a fracture process attenuated due to a fluid migration constraint from insufficient time to exit the pores. It caused the dynamic tensile strength to rise nonlinearly as fluid saturation and fluid viscosity increased.

Kinetics mechanism of pore pressure effect on CO2-water-rock interactions: An experimental study

Carbon capture, utilization and storage (CCUS) is an effective technology to reduce carbon dioxide (CO2) content in atmosphere, while due to the insufficient mastery of deterioration laws of reservoir rocks caused by CO2, the extensive application of carbon storage projects is hindered by the leakage risk. Therefore, it was aimed to study the kinetics mechanism of pore water pressure on CO2-water–rock interactions. First, the pressure effect on water–rock interactions was studied to obtain the relationship between ratio of solid–liquid contact area Ψ and pore pressure, and the controlling mechanism of Ψ on reaction rate ri was verified. On this basis, the pressure effect on CO2-water–rock interactions was further studied. Considering the effects on Ψ and solution acidity, the original CO2-applicable kinetics equation was modified to reduce the error of calculating reaction rate by 33.60% on average. Meanwhile, a forecast method was proposed for the reaction rate under different pressure conditions, and the average calculation error was 41.00% lower than that of original kinetics equation. After verifying the accuracy of these two theoretical methods to calculate reaction rate, the reliability to calculate porosity evolution process was further verified, and the calculation errors could be reduced by 29.52% and 31.81%, respectively.

Damage Behavior of Porous Asphalt Concrete under Load–Pore Pressure Effects

Damage Behavior of Porous Asphalt Concrete under Load–Pore Pressure Effects

Data analytics and modelling in context to determination of moisture susceptibility of reclaimed asphalt foamed bituminous mix

The presence of water badly affects the moisture susceptibility of the reclaimed asphalt Foamed Bituminous Mix (FBM). The present study is mainly emphasized to assess the moisture susceptibility of reclaimed asphalt FBM, Where RAP is being incorporated as a replacement of fresh aggregates. Moisture susceptibility of the mix is evaluated in terms of tensile strength ratio (TSR) and resilient modulus ratio, subjected to different conditioning procedures namely AASHTO T283, modified IDOT, TG-2 guidelines, and MIST. Further data analytics and regression modeling are also carried out to determine the moisture susceptibility of the mix and to check the statistics among the variables. The findings show that the incorporation of RAP in the FBM improves moisture resistance. Further, FBM containing 100% RAP shows the least moisture susceptibility in terms of TSR and Mr ratio irrespective of any conditioning type. Moreover, MIST conditioning may be preferred to assess the moisture sensitivity as it simulates the field pore pressure effects. Further, mathematical analysis is carried out to predict the moisture susceptibility of mix. Adjusted R square coefficient indicates a better fit of the prediction model developed. Overall, the study may be helpful to highway professionals in analyzing the conditioning procedures and determining the moisture sensitivity of the reclaimed asphalt Foamed Bituminous Mix.

Molecular Simulation of Competitive Adsorption of Hydrogen and Methane: Analysis of Hydrogen Storage Feasibility in Depleted Shale Gas Reservoirs

Summary As a clean energy carrier, hydrogen (H2) is considered an indispensable part of the energy transition roadmap. To meet increasing energy demand, extremely large storage capacities are required. Previous studies have focused on underground H2 storage in conventional depleted gas reservoirs, salt caverns, and saline aquifers. The increasing number of depleted shale gas reservoirs may be good candidates for H2 storage. In this work, we analyze the potential of H2 storage in depleted gas reservoirs using Monte Carlo (MC) simulations. The competitive adsorption of a methane-hydrogen (C1-H2) system under nanoscale conditions is investigated, including the effects of pore size, temperature, pressure, boundary material, and fluid composition. Our results show that C1 is preferentially adsorbed in a C1-H2 system. C1 forms the adsorption layer near the boundary surface, while H2 molecules are freely distributed in the pore. The fluid distribution indicates that H2 can be easily produced during H2 recovery processes, which contributes to H2 storage in depleted shale gas reservoirs. In addition, the effect of water on C1-H2 competitive adsorption is analyzed. The strong interactions between water and boundary atoms force C1 molecules away from the adsorbed region. This work provides a foundation for hydrogen storage in depleted shale gas reservoirs at a molecular level.

Experimental Study on the Evolution of Fracture Aperture of Single‐Fracture Granite during Liquid Nitrogen Cold Shock Cycling

Fractures in hot dry rock (HDR) reservoirs are the locations where heating fluid flows exchange heat with the HDR matrix. Cold shock with liquid nitrogen is one method for stimulating cracks. This study investigates the evolution law of fracture aperture under cold shock with liquid nitrogen. The real‐time high‐temperature triaxial servo control rock testing machine was used to conduct permeability experiments to examine the fracture aperture of single‐fracture granite during liquid nitrogen shock cycles at various temperatures. The effects of pore pressure, temperature, and shocking cycles on the fracture aperture are analyzed, and the difference in fracture aperture variation under liquid nitrogen cooling and natural cooling modes is compared. The results showed that (1) during liquid nitrogen cooling, the fracture aperture expands as pore pressure rises; the effect of pore pressure on the fracture aperture becomes more robust as the number of liquid nitrogen shocking cycles and initial temperature increases; (2) under 1‐2 soaking cycles, fracture aperture decreases as the temperature rises. Under two or more soaking cycles, the fracture aperture first increases and then decreases with increasing temperature; (3) when the initial temperature of fractured granite is 100°C, the fracture aperture is not significantly changed by repeated cold soaking cycles. However, with a higher initial temperature, the fracture aperture develops with more liquid nitrogen cold soaking cycles. The liquid nitrogen cooling method is more conducive to increasing the fracture aperture than natural cooling. The experimental results can provide primary experimental data for future research into controlling the evolution of granite cracks.

Effect of Hydrate Saturation and Pore Pressure on the Safe Exploitation of Natural Gas Hydrate Resources

A comprehensive study of the mechanical behaviors of hydrate-bearing sediments (HBSs) is the key to safely exploiting hydrate resources. The mechanical behaviors of HBSs are related to many variables, among which hydrate saturation (Sh) and pore pressure (PP) are vital factors. In addition, Sh and PP are related to the location of hydrates in the subsea layer, so it is of positive significance to investigate their comprehensive influence on the mechanical behavior of HBSs. In this work, a series of triaxial tests were conducted on the HBSs synthesized in the laboratory to explore the influence of Sh and PP on the mechanical properties of the HBSs. The results show that the strength of HBSs increases with increasing PP and Sh. With increased PP and Sh, the stress-strain behaviors will shift from strain-hardening to strain-softening. Moreover, under different Sh conditions, a critical PP point of strain-hardening and softening transition exists in numerical perspective. The critical PP point will develop towards low pressure with increased Sh.

Pore-pressure and stress-coupled creep behavior in deep coal: Insights from real-time NMR analysis

Understanding the variations in microscopic pore-fracture structures (MPFS) during coal creep under pore pressure and stress coupling is crucial for coal mining and effective gas treatment. In this manuscript, a triaxial creep test on deep coal at various pore pressures using a test system that combines in-situ mechanical loading with real-time nuclear magnetic resonance (NMR) detection was conducted. Full-scale quantitative characterization, online real-time detection, and visualization of MPFS during coal creep influenced by pore pressure and stress coupling were performed using NMR and NMR imaging (NMRI) techniques. The results revealed that seepage pores and microfractures (SPM) undergo the most significant changes during coal creep, with creep failure gradually expanding from dense primary pore fractures. Pore pressure presence promotes MPFS development primarily by inhibiting SPM compression and encouraging adsorption pores (AP) to evolve into SPM. Coal enters the accelerated creep stage earlier at lower stress levels, resulting in more pronounced creep deformation. The connection between the micro and macro values was established, demonstrating that increased porosity at different pore pressures leads to a negative exponential decay of the viscosity coefficient. The Newton dashpot in the ideal viscoplastic body and the Burgers model was improved using NMR experimental results, and a creep model that considers pore pressure and stress coupling using variable-order fractional operators was developed. The model’s reasonableness was confirmed using creep experimental data. The damage-state adjustment factors ω and β were identified through a parameter sensitivity analysis to characterize the effect of pore pressure and stress coupling on the creep damage characteristics (size and degree of difficulty) of coal.

Effective yield strength of a saturated porous medium with a spheroidal meso-pore and spherical micro-pores

A macroscopic yield criterion has been derived in the present work for a double saturated porous medium with a spheroidal pore at the mesocale and spherical pores at the microscale. These two types of pores are well separated at two different scales. The meso spheroidal pore saturated by a pore pressure which is different from the one in the micro spherical pores. A Drucker-Prager type criterion is adopted for the solid phase at the microscopic scale to describe its asymmetric behavior between tension and compression. The methodology to formulate this criterion is based on the limit analysis approach of a spheroidal volume containing a confocal spheroidal pore subjected to a uniform strain rate boundary conditions. The matrix at the mesoscopic scale obeys to a general elliptic yield criterion. Based on a two-step homogenization step, the influence of meso-pore shape (spherical, prolate or oblate), micro-porosity, meso-porosity and the effect of pore pressures at different scales are taken into account explicitly by this macroscopic yield criterion.

State-Spatial Viewpoint of Stress in Thermally Postbuckled and Nonlinearly Bent FG Saturated Poroelastic Circular Plates

A dimensionless discrete state-space mathematical model is proposed for stress analysis of an axially-symmetric saturated poroelastic circular plate being loaded by temperature rise of one of the flat faces. Computation of stress by means of this model is free from a need for differentiation. The curves of thermal postbuckling, thermal nonlinear bending, radial stress, circumferential stress and resultant radial force have been achieved by means of six state-space variables and differential quadrature procedure. The effects of loading temperature, boundary conditions, porosity, pore distribution and fluid pore pressure on the thermal post buckling and nonlinear bending phenomena have been investigated. The critical postbuckling thermal load has been determined by means of a numerical algorithm that does not converge to trivial solution because it starts from a relatively great numerical thermal load. Contrary to the governing equation in usual form, the governing equations in state space together with the state equations equalize the number of the governing assembled difference equations and that of the unknowns, and eventually do not require to apply the so-called auxiliary extra boundary conditions at the nodes adjacent to the boundary nodes. As another advantage, in state-spatial viewpoint, there is not any necessity for computing the coefficients of derivatives with order higher than unity.

Microporomechanics of quasi-brittle rocks: Theoretical formulations and analytical simulations

Hydromechanical coupling is one of the essential theoretical and practical issues in rock mechanics and rock engineering. In geological context, it is critically important to incorporate the effect of pore pressure into the constitutive equations with full account of cracking-induced material anisotropies and friction-related plastic deformation. Focus here is put on poromechanical formulations for quasi-brittle rocks weakened by microcracks and saturated with pore fluid pressure. The linear homogenization method applied to derive the effective properties and system free energy is combined with the irreversible thermodynamics and the problem decomposition. Constitutive derivations include the determination of the system free energy, state equations associated with and evolution laws of the internal variables, closed-form failure criterion, etc. Inherent coupling between anisotropic unilateral damage, friction-induced plastic deformation and the fluid pressure constitutes one of significant novelties of the work. Continuities required for the total free energy, macroscopic stress and the global porosity are guaranteed at any opening-closure transition of cracks. Through strength and deformation coupling analyses, the effects of fluid pressure on material strength and deformation are elucidated and also validated by experiments we performed upon a sandstone.

Shear strength degradation of gas hydrate-bearing sediment due to partial hydrate dissociation

Extraction of methane hydrate from subseafloor reservoir may potentially trigger seabed slides and induce subsidence. To address the problems, it is crucial to properly characterize the phase equilibrium condition of pore hydrate and the shear strength of the soil. As one of the key constitutive components, the phase equilibrium condition enforces a constraint over pore gas pressure, temperature and unhydrated water content. Such a constraint, however, has been traditionally ignored in analyzing the mechanical behavior of hydrate-bearing soil. In this paper, a series of stepwise hydrate dissociation tests was performed, and the phase equilibrium condition of pore hydrate was determined, providing an effective way to evaluate the unhydrated water content during hydrate dissociation. Meanwhile, a series of direct shear tests was also conducted to explore the shear strength characteristics of the soil. It is shown that the shear strength of the hydrate-bearing soil can be significantly influenced by pore gas pressure, unhydrated water content, hydrate saturation and several other factors. In particular, the measured shear strength depends upon the initial water content of the sample, pointing to a potential problem that the shear strength could be wrongly determined if not properly interpreted. A shear strength criterion, which enforces the equilibrium condition of pore hydrate, is developed for hydrate-bearing soil, establishing a link between the equilibrium condition and the shear strength. The proposed equation describes well the shear strength characteristics of hydrate-bearing soils, remarkably unifying the effects of pore pressure, temperature, water content and hydrate saturation.

Molecular dynamics simulations of methane adsorption and displacement from graphenylene shale reservoir nanochannels

Methane is the main component of shale gas and is adsorbed in shale pores. Methane adsorption not only affects the estimation of shale gas reserves but also reduces extraction efficiency. Therefore, investigating the behavior of methane adsorption in shale reservoirs is important for evaluating shale gas resources, as well as understanding its desorption and displacement from the nanochannels of shale gas reservoirs. In this research, molecular dynamics simulations were used to investigate the adsorption behavior of methane gas in organic shale pores made of graphenylene, followed by its displacement by CO2 and N2 injection gases. The effects of pore size, pressure, and temperature on adsorption were examined. It was observed that increasing the pore size at a constant pressure led to a decrease in the density of adsorbed methane molecules near the pore surface, while a stable free phase with constant density formed in the central region of the nanopore. Moreover, adsorption increased with increasing pressure, and at pressures ranging from 0 to 3 MPa, 15 and 20 Å pores exhibited lower methane adsorption compared to other pores. The amount of adsorption decreased with increasing temperature, and the observed adsorption isotherm followed the Langmuir adsorption isotherm. The mechanism of methane displacement by the two injected gases differed. Carbon dioxide filled both vacant adsorption sites and directly replaced the adsorbed methane. On the other hand, nitrogen only adsorbed onto the vacant sites and, by reducing the partial pressure of methane, facilitated the displacement of methane.