Initial Pore Research Articles

Investigating the Interaction Effects between Reservoir Deformation and Hydrate Dissociation in Hydrate-Bearing Sediment by Depressurization Method

Natural gas hydrate (NGH) has been widely focused on having great potential for alternative energy. Numerous studies on gas production from hydrate-bearing sediments have been conducted in both laboratory and field. Since the strength of hydrate-bearing sediments depends on the saturation of NGH, the decomposition of NGH may cause the failure of sediments, then leading to reservoir deformation and other geological hazards. Plenty of research has shown that the reservoir deformation caused by hydrate decomposition is considerable. In order to investigate this, the influence of sediment deformation on the production of NGH, a fully coupled thermo-hydro-chemo-mechanical (THMC) model is established in this study. The interaction effects between reservoir deformation and hydrate dissociation are discussed by comparing the simulation results of the mechanical coupling and uncoupled models on the laboratory scale. Results show that obvious differences in behaviors between gas and water production are observed among these two models. Compared to the mechanical uncoupled model, the mechanical coupling model shows a significant compaction process when given a load equal to the initial pore pressure, which leads to a remarkable decrease of effective porosity and reservoir permeability, then delays the pore pressure drop rate and reduces the maximum gas production rate. It takes a longer time for gas production in the mechanical coupling model. Since the reservoir temperature is impacted by the comprehensive effects of the heat transfer from the boundary and the heat consumption of hydrate decomposition, the reduced maximum gas production rate and extended gas production process for the mechanical coupling model lead to the minimum reservoir temperature in the mechanical coupling model larger than that of the mechanical uncoupled model. The reduction of the effective porosity for the mechanical coupling model causes a larger cumulative water production. The results of this paper indicate that the reservoir deformation in the gas production process should be taken into account by laboratory and numerical methods to accurately predict the behaviors of gas production on the field scale.

Fracture Prediction Based on Evaluation of Initial Porosity Induced By Direct Energy Deposition

Additive manufacturing (AM) of metals proved to be beneficial in many industrial and non-industrial areas due to its low material waste and fast stacking speed to fabricate high performance products. The present contribution addresses several known challenges including mechanical behaviour and porosity analysis on directed energy deposition (DED) manufactured stainless steel 316L components. The experimental methodology consisting of metal deposition procedure, hardness testing and fractographic observations on manufactured mini-tensile test samples is described. A ductile fracture material model based on the Rousselier damage criterion is utilized within a FE framework for evaluation of material global response and determination of initial porosity value representing the structure’s nucleating void population. Alternatively, the initial pore sizes are characterized using the generalized mixture rule (GMR) analysis and the validity of the approach is examined against the experimental results.

Influence of unconnected pores on effective stress in porous geomaterials: Theory and case study in unconventional oil and gas reservoirs

Unconventional oil and gas reservoirs are mostly characterized by unconnected pores and abnormal pore pressures. These conventional effective stress theories, such as Biot's theory, cannot display the quantitative relationship between unconnected pores' micro characteristics and the macro stress applied to the solid structure. These parameters related to the unconnected pores should be quantitatively incorporated in the effective stress formula. In this paper, two modified models were proposed to evaluate the solid framework's real effective stress and deformation. The effects of the unconnected pore pressure, unconnected porosity, and unconnected pore fluid's compressibility on effective stress and volumetric strain were theoretically investigated. It was found that the increase of unconnected pore pressure can decrease the effective stress and volumetric strain. For an abnormal high pore pressure formation, the growth of unconnected porosity can reduce the effective stress but increase the volumetric strain. The decrease of the unconnected pore fluid's compressibility can increase and decrease the effective stress and volumetric strain for abnormal low and high pore pressure reservoirs, respectively. Two case studies on the abnormal pore pressures induced by under-compaction and hydrocarbon generation were conducted. The results showed that the pore pressure exhibits significant increases when the unconnected pores and unconnected pore fluids are proactively compressed in the processes of under-compaction and hydrocarbon generation, respectively. The abnormal high pore pressures in these tight sandstone reservoirs in the Sichuan Basin, Tarim Basin, and Bohai Bay Basin can be theoretically generated when the pore space is compressed to 95%–84% of the initial pore space. The model can evaluate the effective stress and deformation for reservoirs with considerable unconnected porosities. In turn, it can also interpret the abnormal pressure generation induced by the changes of unconnected porosity and pore fluid's compressibility.

Research results of triaxial geomechanical tests of a coal sample in the light of tomographic tests

The study aimed to check the possibility of using computed tomography to observe fractures in coal samples before and after a triaxial compression test (TRX). The paper presents an innovative use of two specialized laboratory techniques: computed tomography and geomechanical triaxial compression test. Geomechanical tests were performed on a TRX-070, GCTS (USA) measuring triaxial system. A full stress-strain characteristic was recorded. Also, during the triaxial compression tests, the P and S wave velocities were measured and dynamic elastic moduli were determined. The results were compared. The sample was analyzed in two stages: CT measurement performed on raw coal sample (stage 1) and after the TRX test which destroyed the sample structure (stage 2). First, the sample was scanned reflecting the initial pores and microcracks system. The final stage represents the sample after the geomechanical test. A detailed pore structure analysis was performed for each of the stages. As a result of the proposed above-mentioned method, images of the pore structure were obtained after each step. The sample was analyzed for crack propagation and changes in the pore structure. The qualitative and quantitative interpretation was based on the determination of the number of voids, crack diameters, collapse angles, and comparison of the pore structure between the initial state of the sample and after the triaxial test. In the first stage (before the test), the largest object (identified as a microcrack) had a volume of approximately 16 mm3, after the sample was stress-tested (after the sample was destroyed), the largest identified object was about 15 times larger (240 mm3). It appeared that the coal sample was prone to fracturing in defined directions. Comprehensive research has demonstrated the legitimacy of using the proposed methodology to evaluate changes in the sample structure during the triaxial test.

Three-dimensional numerical solution and stress cage analysis of high conductive fractures pressure sealing

Aiming at the main problems in the design and analysis of well wall strengthening for fractured formations, a three-dimensional (3D) numerical simulation scheme is proposed. The three-dimensional finite element software is used to analyze the mechanical behavior of fractures and the pressure sealing process, and evaluate the stress cage effect. The main features of the model are as follows: (1) The equivalent fractures in the analytical model represent the function sum of the mechanical behavior of all fractures on the well wall, which is a functionally equivalent crack.(2) When evaluating the stress cage effect, the shape of the crack wedge filled with the plugging agent particles is formed by simulating the fractures opening process under the injection pressure, not a given regular shape.(3) In the model of calculating bull heading of block agent, the liquid pressure on the well wall is the injection pressure, which is a variation increased with time. The fluid pressure on the well wall in the stress cage calculation model is generated by the initial pore pressure. (4) The numerical evaluation of the stress cage effect is achieved by calculating the increase amplitude FX of the minimum hoop stress on the well wall. Using this model, several sets of injection pressure design values can be used for pressure plugging numerical simulation and stress cage effect evaluation calculation, and then the optimal and accurate quantitative value of “injection pressure, block agent particle size, safe mud window upper bound” are found through comparison. Finally, through an engineering example of horizontal well drilling pressure sealing in a shale gas reservoir developed by a fracture, we use the above theoretical tools to introduce the process and results of the numerical analysis of the extended mud window drilled in the shale gas fissure reservoir.

Analytical expression for isolated pore shape in solid

Algebraic expressions for the pore shape after a bubble completely entrapped in solid are provided in this study. Properties of functional materials in micro-or nano-technologies strongly depend on pore shapes in solid. The model is based on a previous work by accounting for transient gas pressure in the pore affected by solute transfer for different cases, and balance of gas, capillary and hydrostatic pressures, and physico-chemical equilibrium at the bubble cap. The results find that algebraic prediction of the pore shapes with exact expressions of the maximum pore radii in different cases can be accomplished by dividing the pore length into three segments with inclination angles greater than initial contact angle, between initial contact angle and 90 degrees, and less than 90 degrees. The shape of an isolated pore is thus delineated by the bottom spherical cap, radius and length at the initial contact angle and maximum pore radius and length at inclination angle of 90 degrees, and top spherical cap. Dimensionless independent parameters controlling the pore length include initial contact angle, mass transfer coefficient, supersaturation ratio, product of Bond number with initial solute concentration in liquid, Henry's law constant (in Cases 1-3), and partition coefficient (in Cases 3 and 4). The predicted lengths in three segments and shapes of the isolated pore agree with numerical computations confirmed by experimental data.

Influence of Trapped Gas on Pore Healing under Hot Isostatic Pressing in Nickel-Base Superalloys

Under the typical hot isostatic pressing (HIP) processing conditions, plastic deformation by dislocation slip is considered the primary mechanism for pore shrinkage, according to experimental observations and deformation mechanism maps. In the present work, a crystal plasticity model has been used to investigate the influence of applied pressure and holding time on porosity reduction in a nickel-base single crystal superalloy. The influence of trapped gas on pore shrinkage is modeled by coupling mechanical deformation with pore–gas interaction. In qualitative agreement with experimental investigations, we observe that increasing the applied pressure or the holding time can effectively reduce porosity. Furthermore, the effect of pore shape on the shrinkage is observed to depend on a combination of elastic anisotropy and the complex distribution of stresses around the pore. Simulation results also reveal that, for pores of the same shape, smaller pores (radius < 0.1 μm) have a higher shrinkage rate in comparison to larger pores (radius ≥ 0.1 μm), which is attributed to the increasing pore surface energies with decreasing pore sizes. It is also found that, for smaller initial gas-filled pores (radius < 0.1 μm), HIP can result in very high gas pressures (on the order of GPa). Such high pressures either act as a driving force for argon to diffuse into the surrounding metal during HIP itself, or it can result in pore re-opening during subsequent annealing or mechanical loading. These results demonstrate that the micromechanical model can quantitatively evaluate the individual influences of HIP processing conditions and pore characteristics on pore annihilation, which can help optimize the HIP process parameters in the future.

Distinct Undrained Cyclic Behavior of Fine-Grained Gassy Soil at Various Initial Pore Water Pressures

Undissolved gas bubbles, which are widely formed in marine sediments, could modify the behavior of the soil. To date, the cyclic behavior of fine-grained gassy soil has been rarely studied, limiting any reliable analysis of cyclically loaded offshore foundations in fine-grained gassy seabed. This study presents a series of undrained cyclic triaxial tests on normally consolidated (NC) fine-grained gassy soil. Each sample was charged with the same amount of N2 gas before consolidation. Then, samples were consolidated to an identical mean operative stress (total stress minus pore water pressure) of 200 kPa, but at different initial pore water pressures (uw0=0–600 kPa) before cycling. Compared with a saturated sample, the inclusion of a small amount of gas bubbles can either significantly decrease the level of cyclic stiffness degradation when the initial pore water pressure uw0 is lower than a threshold value (150 kPa in this study), or vice versa. This behavior is associated with two competing mechanisms during the cyclic shearing: (1) collapse of gas-filled pores, which increases cyclic pore water pressure (CPWP), and therefore stiffness degradation; and (2) partial water drainage into gas cavities, which reduces CPWP and stiffness degradation. The former and the latter are likely to dominate at relatively high and low uw0, respectively. This implies that water depth would significantly influence the cyclic deformation and stiffness degradation of offshore foundations in fine-grained gassy sediments.

Effects of frying temperature and pore profile on the oil absorption behavior of fried potato chips

In present study, total oil (TO), surface oil (SO), structural oil (STO), penetrated surface oil (PSO), and oil distribution during frying were analyzed. Results showed STO (53.10–75.89%) fraction made up the largest part of TO followed by PSO (36.26–58.28%) and SO (2.59–3.50%), and the proportion of STO in TO decreased with the increasing frying time, while PSO elevated, indicating the higher frying temperature facilitated the formation of less and smaller pore in samples, and thus led to the less oil content. Therefore, effect of pore on oil absorption was further investigated. Results showed there was no significant difference in oil content of samples with initial pore diameter of 0–0.2 mm. While, TO (mainly STO) increased with the increasing initial pore diameter of 0.3–1.2 mm. The bigger initial pore diameter induced bigger pore volume and porosity. Taken together, this study provided new ideas to clarify oil absorption based on pore profiles.

Phase field simulation of grain growth in Al2O3-based composite ceramic cutting tool materials containing second phase nanoparticles and pores

A phase field model was established to simulate the grain growth of Al2O3-based composite ceramic tool materials containing second phase nanoparticles and pores. The free energy parameters of the model are directly related to the surface and grain boundary energies of Al2O3 such that the grain growth process can be quantitatively analysed. The model was used to investigate the effects of the volume fraction and size of second phase particles on grain growth in Al2O3 with a certain initial pore volume fraction. Findings show that pores and second phase particles jointly hinder grain growth. When the radii of second phase nanoparticles are the same, as the number of particles increase, most of the grain boundaries occur as straight lines in the microstructure, and grain size decreases. It is beneficial to obtain fine and uniform microstructure. However, when the particles content reaches a certain level, the second phase particles tend to agglomerate at the grain boundary of matrix grains, which will result in a decrease in the properties of nanocomposite ceramic tool materials. The study also found that, at a constant volume fraction of particles, with the decrease of particle radius, the grain size decreases. When the second phase particle size is reduced to 50 nm, relatively more intragranular microstructure was observed, which is considered to be the main reason for increasing the toughness of nano composite ceramic tool materials. Through analyzing the simulation results, the optimal combination of the content and size of the second phase particles can provide theoretical guidance for Al2O3 nano composite ceramic tool materials design and preparation.

Salinity and inundation effects on productivity of brackish tidal marsh plants in the San Francisco Bay-Delta Estuary

Plant productivity is central to numerous ecosystem functions in tidal wetlands. We examined how productivity of brackish marsh plants in northern California responded to abiotic stress gradients of inundation and salinity using two experimental approaches. In a greenhouse study with varying salinity, shoot production and biomass of Juncus balticus, Schoenoplectus acutus and S. americanus all declined monotonically with higher salinity, with evidence of differences in sensitivity among species by their varied functional responses. Salinity also negatively affected fecundity for the one species (S. americanus) that produced enough inflorescences during the experiment for analysis. In a field manipulation of inundation and initial pore water salinity, total end-of-season biomass and other metrics of growth in the high marsh species, J. balticus, had unimodal relationships with inundation. Root production tended to be greater strongly impacted by greater inundation than shoot production. The salinity treatment quickly dissipated for treatments that were flooded more frequently but persisted at a higher marsh elevation where it suppressed plant growth. These results suggest that both increased flooding and salinity associated with climate change and sea-level rise may negatively impact productivity of brackish marsh species, but with variable effects by species and stressor.

A Three-Phase Study on Preflush Stage in Sandstone Acidizing: Experimental and Modeling Analysis of Evolved Carbon Dioxide in a Hydrocarbon and Aqueous Environment

SummaryLimited studies have been conducted in understanding the mechanics of preflush stages in sandstone-acidizing processes. Among those conducted in this area, all efforts have been directed toward singular aqueous-phase scenarios. Encountering 100% water saturation (Sw) in the near-wellbore region is seldom the case because hydrocarbons at residual or higher saturations can exist. Carbonate-mineral dissolution, being the primary objective of the preflush stage, results in carbon dioxide (CO2) evolution. This can lead to a multiphase presence depending on the conditions in the porous medium, and this factor has been unaccounted for in previous studies under the assumption that all the evolved CO2 is dissolved in the surrounding solutions. The performance of a preflush stage changes in the presence of multiphase environments in the porous media.A detailed study is presented on the effects of evolved CO2 caused by carbonate-mineral dissolution, and its ensuing activity during the preflush stages in matrix acidizing of sandstone reservoirs. Four Carbon Tan Sandstone cores were used toward the purpose of this study, of which two were fully water saturated and the remaining two were brought to initial water saturation (Swi) and residual oil saturation to waterfloods (Sorw) before conducting preflush-stage experiments. The preflush-stage fluid, 15 wt% hydrochloric acid (HCl), was injected in the concerning cores while maintaining initial pore pressures of 1,200 psi and constant temperatures of 150°F.A three-phase-flow numerical-simulation model coupled with chemical-reaction and structure-property modeling features is used to validate the conducted preflush-stage coreflood experiments. Initially, the cores are scanned using computed tomography (CT) to accurately characterize the initial porosity distributions across the cores. The carbonate minerals present in the cores, namely calcite and dolomite, are quantified experimentally using X-ray diffraction (XRD). These measured porosity distributions and mineral concentrations are populated across the core-representative models. The coreflood effluents' calcium chloride and magnesium chloride, which are acid/carbonate-mineral-reaction products, as well as spent-HCl concentrations were measured. The pressure drop across the cores was logged during the tests. These parameters from all the conducted coreflood tests were used for history matching using the numerical model. The calibrated numerical model was then used to understand the physics involved in this complex subsurface process.In fully water-saturated cores, a major fraction of unreacted carbonate minerals still existed even after 40 pore volumes (PV) of preflush acid injection. Heterogeneity is induced as carbonate-mineral dissolution progresses within the core, creating paths of least resistance, leading to the preferential flow of the incoming fresh acid. This leads to regions of carbonate minerals being untouched during the preflush stimulation stage. A power-law trend, P = aQb, is observed between the stabilized pressure drops at each sequential acid-injection rate vs. the injection rates, where P is the pressure drop across the core, Q is the sequential flow rate, and a and b are constants, with b < 1. An ideal maximum injection rate can be deduced to optimize the preflush stage toward efficient carbonate-mineral dissolution in the damaged zone. An average of 25% recovery of the oil in place (OIP) was seen from preflush experiments conducted on cores with Sorw. In cores with Swi, the oil saturation was reduced during the preflush stage to a similar value as in the cores with Sorw. The oil-phase-viscosity reduction caused by CO2 dissolution in oil and the increase in saturation and permeability to the oil phase resulting from oil swelling by CO2 are inferred as the main mechanisms for any additional oil production beyond residual conditions during the preflush stage. The potential of evolved CO2, a byproduct of the sandstone-acidizing preflush stage, toward its contribution in swelling the surrounding oil, lowering its viscosity, and thus mobilizing the trapped oil has been depicted in this study.

Improving sciatic nerve regeneration by using alginate/chitosan hydrogel containing berberine.

Peripheral nerve injuries are the common results of trauma that lead to pain and handicap in patients. Berberine due to its properties like antibiotic, immunostimulant, antitumor, antimotility, and positive effect on neurological disorders can be used to enhance peripheral nerve injuries. In this study, alginate/chitosan hydrogel containing different concentrations of berberine (0, 0.1, 1, 10% (w/v)) was created, evaluated, and applied as a scaffold for sciatic nerve regeneration. To prepare hydrogel, sodium alginate was dissolved in distilled water and cross-linked with CaCl2, and chitosan was dissolved in acetic acid and cross-linked with β-glycerol phosphate. The structure, release, swelling, weight loss, cytocompatibility, and hemocompatibility of the prepared hydrogels were assessed. The sciatic nerve crush was created in rats and fabricated hydrogels were injected, and functional analysis was used to evaluate their effectiveness. The results of physical characterization of the hydrogel indicated that the initial average pore size was about 39 μm and about 70% of the main weight of hydrogels was lost after incubation for 21 days and hemocompatibility of hydrogels was also confirmed. The MTT assay showed the cytocompatiblity of hydrogels and also indicated that berberine has dose-dependence effect on cell proliferation. The in vivo results showed the positive effect of berberine especially the hydrogel contained 1% of berberine on regeneration of sciatic nerve. Based on this study, Alg/Chit hydrogel can be applied as a treatment to heal peripheral nerve injuries. Graphical abstract.

A Mechanistic Pore-Scale Analysis of the Low-Salinity Effect in Heterogeneously Wetted Porous Media

Low-salinity (LS) waterflooding has been a topic of substantial recent interest in the petroleum industry. Studies have shown that LS brine injection can increase oil production relative to high-salinity (HS) brine injection, but contradictory results have also been reported and a mechanistic explanation of these findings remains elusive. We have recently developed a pore-scale model of LS brine injection in uniformly wetted networks (Watson et al. in Transp Porous Med 118:201–223, 2017), and we extend this approach here to investigate the low-salinity effect (LSE) in heterogeneously wetted media. We couple a steady-state fluid displacement model to an innovative tracer algorithm and track the evolving salinity front as oil and HS brine are displaced from the network. The wettability of the pore structure is locally modified where water salinity falls below a critical threshold, and simulations show that this can have significant consequences for oil recovery. Our results demonstrate that, for heterogeneously wetted networks, the oil-wet (OW) pores are the only viable source of incremental oil by LS brine injection. Moreover, we show that a LS-induced increase in the displaced OW pore fraction is a necessary, but not sufficient, condition to guarantee additional oil production. Simulations further suggest that the initial OW pore fraction, the average network connectivity and the initial HS brine saturation are factors that can determine the extent of incremental oil recovery following LS brine injection. This study clearly highlights that the mechanisms of the LSE can be markedly different in uniformly wetted and in non-uniformly wetted porous media.

Hydrostatic contraction and anisotropic contraction effects on oxygen molecule nanorods

We study the effects of both hydrostatic and anisotropic contractions on the molecular condensation of oxygen molecules (O2) physisorbed to nanosized pores, termed “O2 nanorods”, through the magnetization measurements. Multisteps of O2 solidification accompany the reduction in structural symmetry with decreasing temperature, such that the structural change by external stress varies the stability of O2 solidification. For initial pore diameters of D = 8.5, 14.5, and 24.0 nm, anisotropic compression for nanorods (preferential compression along radial direction of the pores) occurred, and molecule solidification is suppressed at the lower temperature side compared with that under hydrostatic compression. For the smallest D = 6.5 nm, a hydrostatic contraction almost occurred, and the high adsorption capability enabled the detection of both the melting transition and change in crystal structure within the β phase, in addition to α–β and β–γ transitions.

Behaviour of discrete piles used to stabilise a tree-covered railway embankment

Many clay railway embankments in the UK are well over 100 years old, and now suffer from a range of stability and serviceability problems. A common means of remediating earthworks that have either suffered, or are assessed as being at risk of, deep-seated slope failure is to install a row of discrete piles along the mid-slope. This paper presents field data from a 9 m high, pile-stabilised embankment at Mill Hill, north London. Initial pore water pressure measurements had shown that the trees present on the earthwork maintained low pore water pressures during winter; thus the trees were left in place after piling to aid stability. The piles initially bent upslope, as a result of inward shrinkage of the embankment over a period of dry weather, before then being loaded by shallower downslope movements of the clay. Later cycles of seasonal movement caused a small but gradual ratcheting upward of pile bending moments. The largest bending moments measured over 6 years of monitoring were those resulting from the initial inward shrinkage of embankment, which reached about 25% of the design capacity of the piles. At the end of the monitoring period, the measured bending moments resulting from shallower downslope ground movements were about 20% of the pile design capacity.

In situ radiographic and ex situ tomographic analysis of pore interactions during multilayer builds in laser powder bed fusion

Porosity and high surface roughness can be detrimental to the mechanical performance of laser powder bed fusion (LPBF) additive manufactured components, potentially resulting in reduced component life. However, the link between powder layer thickness on pore formation and surface undulations in the LPBF parts remains unclear. In this paper, the influence of processing parameters on Ti-6Al-4 V additive manufactured thin-wall components are investigated for multilayer builds, using a custom-built process replicator and in situ high-speed synchrotron X-ray imaging. In addition to the formation of initial keyhole pores, the results reveal three pore phenomena in multilayer builds resulting from keyhole melting: (i) healing of the previous layers' pores via liquid filling during remelting; (ii) insufficient laser penetration depth to remelt and heal pores; and (iii) pores formed by keyholing which merge with existing pores, increasing the pore size. The results also show that the variation of powder layer thickness influences which pore formation mechanisms take place in multilayer builds. High-resolution microcomputed tomography images reveal that clusters of pores form at the ends of tracks, and variations in the layer thickness and melt flow cause irregular remelting and track height undulations. Extreme variations in height were found to lead to lack of fusion pores in the trough regions. It is hypothesised that the end of track pores were augmented by soluble gas which is partitioned into the melt pool and swept to track ends, supersaturating during end of track solidification and diffusing into pores increasing their size.

Studying the Geotechnical Properties of Clayey Soil Contaminated by Kerosene

This study focuses on investigating the impacts of kerosene on the physical, mechanical, and chemical characteristics of clay soil. The soils specimens are contaminated artificially with six ratios of kerosene (5, 10, 20, 30, 40, and 50) % calculated according to the dry weight of soil. The artificial contamination includes air drying of the disturbed soil, then placed in plastic containers and mixed with the field water content and the specified concentration of kerosene to ensure getting homogenous contaminated soil specimens. The contaminated soil specimens left for 30 days in plastic containers covered by nylon sheets to control the water content and prevent volatility of contaminant. The results of tests proved that different ratios of kerosene have different impacts on the engineering and chemical characteristics of soil specimens. The specific gravity, percentages of fine particles, optimum water content, the initial and final void ratio, coefficient of consolidation, swelling index, permeability, the undrained shear strength, effective shear strength parameters, and the rate of reduction of initial pore water pressure are reduced significantly with increasing the content of kerosene in soil. Generally, the concentration of kerosene less than 10% has slight impacts on the studied characteristics of soil specimens.

Mechanical properties of soft soils experiencing lateral unloading under initial excess pore water pressure

The mechanical properties of soft soils were investigated through triaxial lateral unloading tests under different initial excess pore water pressures and confining pressures. The lateral unloading stress–strain, strength parameters, and initial unloading moduli of soft soils are presented and discussed in this study. The results show that soft soils abruptly undergo unloading failure. The lower confining pressure and the higher initial excess pore water pressure can result in intensive unloading failure behavior. The unloading strength of soft soils linearly decreases as the initial excess pore water pressure increases. The stress–strain curve is characterized as strain hardening. The cohesion and initial unloading moduli of soft soils drastically reduce when the initial excess pore water pressure exceeds 20 kPa. The conclusions obtained in this study can provide a theoretical reference for the design and numerical analysis of underground spaces in soft soils.

Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

To understand the evolution of stress-induced elastic wave anisotropy, three triaxial experiments were performed on sandstone specimens with bedding orientations parallel, perpendicular, and oblique to the maximum principal stress. P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen's anisotropy equation. The results show that the elastic wave anisotropy is very sensitive to mechanical loading. Under hydrostatic loading, the magnitude of anisotropy is reduced in all three specimens. However, under deviatoric stress loading, the evolution of anisotropic characteristics (magnitude and orientation of the symmetry axis) is bedding orientation dependent. Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress. P-wave anisotropy ε ′ is linearly related to volumetric strain S v and dilatancy, indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy. The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy. However, opening of new cracks, aligned in the maximum principal direction, accounts for the increase of the anisotropy. The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.