Shear-enhanced compaction during non-linear viscous creep of porous calcite-quartz aggregates

In this paper, we extend the previous studies of semi-brittle flow of synthetic calcite-quartz aggregates to a range of temperatures and effective pressures where viscous creep occurs. Triaxial deformation experiments were performed on hot-pressed calcite-quartz aggregates containing 10, 20 and 30 wt% quartz at confining pressure of 300 MPa, pore pressures of 50-290 MPa, temperatures of 673-1073 K and strain rates of 3.0×10⁻⁵/s, 8.3×10⁻⁵/s and 3.0×10⁻⁴/s. Starting porosity varied from 5 to 9%. We made axial and volumetric strain measurements during the mechanical tests. Pore volume change was measured by monitoring the volume of pore fluid that flows out of or into the specimen at constant pore pressure. Yield stress increased with decreasing porosity and showed a dependence on effective pressure. Thus, the yield stress versus effective pressure can be described as a yield surface with negative slope that expands with decreasing porosity and increasing strain hardening, gradually approaching the envelope of strength at 10% strain, which has a positive slope. Creep of porous rock can be modeled to first order as an isolated equivalent void in an incompressible nonlinear viscous matrix. An incremental method is used to calculate the stress-strain curve of the porous material under a constant external strain rate. The numerical simulations reproduce general trends of the deformation behavior of the porous rock, such as the yield stress decreasing with increasing effective pressure and significant strain hardening at high effective pressure. The drop of yield stress with increasing porosity is modeled well, and so is the volumetric strain rate, which increases with increasing porosity.     

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer‐piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to ?ow events and their effects on bank stability. The saturated/unsaturated ?ow was modelled using a ?nite element seepage analysis, for the main ?ow events occurring during the four‐year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern‐Price), using pore water pressure distributions obtained by the seepage analysis. The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of ?ow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the con?ning pressure of the river during the initial drawdown were responsible for triggering the mass failure. Results deriving from the seepage and stability analysis of nine selected ?ow events were then used to investigate the role of the ?ow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.     

Streambank dewatering for increased stability

Streambank erosion is often the dominant source of sediment leaving modified watersheds. Mass failure of high, steep banks is one of the most serious forms of streambank erosion. The risk of a given bank experiencing mass failure is a function of bank height, angle, and soil strength, which is governed by soil moisture. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low‐cost pump system and subsurface horizontal drains. Pore water pressures (both positive and negative pressures, or matric suction) were continuously monitored for 2 years at the pumped site, at an adjacent untreated control section, and for 1 year at the site stabilized with horizontal drains. Resulting data were used to calculate a time series of the factor of safety using a computer model. Over the course of two wet seasons, average bank retreats for the control and pumped plots were 0·43 and 0·21 m, respectively. More limited monitoring revealed that the site with passive drains retreated about 0·23 m. At the pumped site pore water pressure was 3–4 kPa lower than at the control site during the most critical periods. Accordingly, computed factors of safety were above the failure threshold at the pumped site, but fell below unity at the control site on 11 occasions over the period of observation. Similarly, the drained site displayed generally lower pore water pressure and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. These results suggest, but do not prove, that bank dewatering promoted lower rates of bank retreat and higher levels of stability since the three sites had slight differences in soils, geometry and boundary conditions. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater. Published in 2009 by John Wiley & Sons, Ltd.     

A partial explanation of the dependency of hydraulic conductivity on positive pore water pressure in peat soils

Part of the relationship between positive pore water pressures and hydraulic conductivity in peat soils may be explained by accumulations of methane bubbles. We show how compression and expansion of gas bubbles with changes in pore water pressure could cause changes in hydraulic conductivity and thus help to explain some observations of dependency of hydraulic conductivity in peats on pore water pressure. Consideration is also given to the effect on hydraulic conductivities of methane gas going into solution with increase in pore water pressure.     

Dynamic pressures on rigid cantilever walls retaining poroelastic soil media. Part I. First method of solution

The problem of the determination of dynamic pressures and the associated forces on a rigid, vertical cantilever wall retaining a semi-infinite, uniform, fully-saturated poroelastic layer of soil is solved analytically under conditions of plane strain. Hysteretic damping in the soil skeleton may also be present. The rigid wall and the base of the soil layer are both excited by an acceleration harmonically varying with time and spatially invariant. The governing partial differential equations of motion, after separation of variables and the simplifying assumption of zero vertical normal stresses, reduce to a system of two ordinary differential equations for the amplitudes of the horizontal solid skeleton displacement and the pore water pressure, which are easily solved. Soil displacements and stresses, wall pressures and resultant forces as well as the pore water pressure are explicitly expressed. Their variation with frequency, hysteretic damping, porosity and permeability is numerically computed in order to assess the relative importance of the various parameters on the response.     

On modelling installation designs for soil water instrumentation,with particular reference to casagrande type piezometer systems

Appropriate installation design of soil water monitoring equipment is necessary to ensure the meaningful interpretation of the resultant data. For example, although piezometers are widely used in monitoring positive pore water pressures, the limits on the shape factor of the system in terms of the ceramic tip and filter design have not been fully studied. The filter design boundary conditions are investigated here using a finite element analysis, in which threshold ceramic tip and filter characteristics (permeability and dimensions) are identified. From the modelling results, it is possible to select the most suitable design installation.     

Poisson's ratio at high pore pressure

Laboratory investigations suggest that a precise relationship exists between Poisson's ratio, pore pressure and fluid type. Values of Poisson's ratio for dry samples are significantly smaller than those for fluid-saturated samples. The values are anomalously high for high pore pressure, with the possibility of differentiating between gas-saturated, brine-saturated and oil-saturated porous rocks.
The present study considers two overpressure models, based on oil/gas conversion and disequilibrium compaction, to obtain Poisson's ratio versus differential pressure (confining pressure minus pore pressure). The model results are in good agreement with experiments. Poisson's ratio is approximately constant at high differential pressures and increases (decreases) for saturated (dry) rocks at low differential pressures. Fluid type can be determined at all differential pressures from Poisson's ratio. The analysis is extended to the anisotropic case by computing the three Poisson's ratios of a transversely isotropic rock versus differential pressure. While one of them is practically independent of effective pressure, the others increase with increasing pore pressure. Experiments performed on cores under different pressure conditions, and calibration of the models with these data, provide a tool for inverting pore pressure from seismic data.     

Non-equilibrium compaction and abnormal pore-fluid pressures: effects on rock properties1

Knowledge of pore pressure using seismic data will help in planning the drilling process to control potentially dangerous abnormal pressures. Various physical processes cause anomalous pressures on an underground fluid. Non-equilibrium compaction is a significant process of overpressure generation. This occurs when the sedimentation rate is so rapid that the pore fluids do not have a chance to 'escape' from the pore space.
The model assumes a closed system and that the pore space is filled with water and hydrocarbon in a liquid state. Balancing mass and volume fractions yields the fluid pressure versus time of deposition and depth of burial. Thermal effects are taken into account. The pore pressure, together with the confining pressure, determines the effective pressure which, in turn, determines the bulk moduli of the rock matrix.
We assume a sandstone saturated with hydrocarbons and water, for which calibration of the model with experimental data is possible. The seismic velocities and attenuation factors are computed by using Biot's theory of dynamic poroelasticity and the generalized linear solid. The example shows that the formation can be overpressured or underpressured depending on the properties of the saturating fluid. Wave velocities and quality factors decrease with decreasing differential pressure. The effect is important below approximately 20 MPa. The model is in good agreement with experimental data for Berea sandstone and provides a tool for predicting pore pressure from seismic attributes.     

Pore pressure in deep bore-holes: Its measurement and relation with seismic activities

The method of measuring the pore pressure of stratum fluid in deep bore-holes is presented. Taking the sedimentary basins in North China as an example, the variation law of pore pressure measured in strata within the depth range of 0 to 4 km is analyzed. Furthermore, the relation between the regional distribution of abnormal pore pressures in 3 000 m depth and seismic activities is discussed in connection with the earthquakes of M _S5.0 that occurred after 1 900 in the studied region. The study result indicates that the strata pore pressure P ₀ measured in deep bore-holes is not exactly the same as the hydrostatic pressure PH. Ultra pore pressure does exist, though not commonly. The relation that the pore pressures (P ₀₁, P ₀₂ and P ₀₃) measured in exploration, evaluation and exploitation wells in Oil-fields and the hydrostatic pressuree (P _H) of the above wells is as follows: 1 P ₀₁>P ₀₂>P _H>P ₀₃ in areas with ultra pore pressures; 2 P _H>P ₀₁>P ₀₂>P ₀₃ in areas where the pore pressures are normal or on the lower side. The relation between the regional distribution of ultra pore pressures and the activity of earthquakes of M _S5.0 has been analyzed. The result shows that, with latitudes 36.0°–36.5°N as a demarcation, pore pressures are ultra-high in the south and are normal or lower in the north. In the south, the measured pore pressure is obviously higher than the hydrostatic pressure below the depth of about 2 000 m and it increases as a power function with increasing depth; meanwhile, the earthquake activity there is weaker. In the north, however, the measured pore pressure increases as a linear function with increasing depth; meanwhile, the earthquake activity there is stronger. This project sponsored by the Chinese Joint Seismological Science Foundation.     

Three-dimensional poro-elasto-plastic model for wave-induced seabed response around submarine pipeline

The evaluation of the wave-induced excess pore pressure around a buried pipeline is particularly important for pipeline engineers involved in the design of offshore pipelines. Existing models for the wave-induced seabed response around submarine pipeline have been limited to poro-elastic soil behavior and de-coupled oscillatory and residual mechanisms for the rise in excess pore water pressure. To overcome the shortcoming of the existing models, in this study a three-dimensional poro-elasto-plastic soil model with submarine pipeline is established, in which both oscillatory and residual mechanisms can be simulated simultaneously. With the proposed model, a parametric study is conducted to investigate the relative differences of the predictions of the wave-induced pore pressure with poro-elasto-plastic model. Based on numerical examples, it can be concluded that the poro-elasto-plastic behaviors of soil have more significant influence on wave-induced pore pressure of seabed around submarine pipeline. As the seabed depth increases, the normalized pore pressures decrease rapidly at the upper part of seabed, and then change slightly at the lower part of the seabed. Soil permeability and wave period have obvious influence on the wave-induced normalized pore pressure.     

Fluid flow from pore pressure measurements off La Palma, Canary Islands

In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary Islands, are presented. The data obtained with a Pop Up Pore Pressure Instrument (PUPPI) provide constraints on the fluid circulation and its causes in a very special context: The sediment piles near an intraplate oceanic island built on the continental rise of the Northwest African Margin. The ambient pore pressures estimated from 2 to 4 days long record are negative in almost all cases with values, at depths of a few meters below sea floor, usually on the order of −10 to −70 Pa. Excess pore pressures develop only in the distal most areas. The permeabilities and compressibilities obtained respectively from the decay of the insertion pressures and the amplitude of the tidally induced pore pressure variations range between 2.5×10⁻¹⁸ and 6.6×10⁻¹⁶ m² and, 6.2×10⁻⁹ and 1.5×10⁻⁷ Pa⁻¹. According to these permeabilities fluid flow is estimated to be mostly downward and usually on the range between 0 and −0.3 mm y⁻¹. However, from the excess pore pressure profile a complex pattern of fluid circulation is inferred where horizontal fluid motion cannot be neglected. Horizontal flow is probably controlled by significant contrasts in the permeability of the different layers. The prevailing downward fluid flow is abnormal for a classical passive margin. We thus interpret these results as the superposition to the loss of fluids by sediment compaction (on the continental rise), of a large-scale flow system stimulated by thermal buoyancy (100 km wide) related to the volcanic activity on the island of La Palma.     

Energy approach to unsaturated cyclic strength of sand

The mechanical response to cyclic loading of saturated cohesionless soils is usually investigated by means of effective stress method considering pore water pressure changes that lead to reduced strength and stiffness. On the other hand, the behavior of partially saturated sands is different from the behavior of saturated sand deposits. The development of negative pore water pressures in particular makes it difficult to estimate the behavior of partially saturated sands. The response of partially saturated sands, however, can be examined in a physically understandable manner by investigating their energy characteristics independently of pore pressure behavior. To establish a general framework for understanding the behavior of partially saturated sand, a total of 52 resonant column and dynamic torsional shear tests were conducted under undrained conditions. The effects of factors such as the amplitude of shear strain, relative density, saturation ratio and confining pressure on the dynamic characteristics of the sand and on energy dissipation were studied. The use of the energy concept in the evaluation of partially saturated soils is shown to be a promising method for the evaluation of the cyclic behavior of partially saturated sands.     

Investigation into dynamic response of regional sites to seismic waves using shaking table testing

This study addresses the changes in acceleration,pore water pressure and Fourier spectrums of different types of seismic waves with various amplitudes via large-scale shaking table tests from two sites:a sand-containing regional site and an all-clay site.Comparative analyses of the test results show that the pore water pressures in sand-soil layers of the regional site initially increase and then decrease as the amplitudes of the seismic accelerations increase.The actions of the vertical and vibrational seismic waves contribute to greater pore water pressures.The amplification coefficient of the sand-layer regional site becomes smaller as the seismic waves grow stronger,so that both sites are capable of filtering high frequencies and amplifying low frequencies of seismic waves.This is more apparent with the increase in the peak value of the acceleration,and the natural vibration frequencies of both sites decrease with the transmission of the seismic waves from the basement to the ground surface.The decreasing frequency value of the sand-containing regional site is smaller than that of the all-clay site.     

孔隙压力对含随机缺陷岩石破坏过程及全部变形特征的影响

对于平面应变压缩条件下含有随机缺陷的岩样,利用FLAC研究了孔隙压力对岩样破坏过程、全部变形特征及前兆的影响。以前编写的若干FISH函数,被用于生成缺陷和计算轴向、侧向、体积应变及侧向应变与轴向应变比值的负值（计算得到的泊松比）。在峰值应力之后,密实岩石单元服从线性应变软化行为及随后的理想塑性行为,而材料缺陷呈现理想塑性行为。当孔隙压力较高时,应力-侧向应变曲线具有一平台;破坏的前兆更明显;变形后岩样的体积总是大于原始体积;在初始加栽阶段、均匀变形阶段及峰后变形阶段,由于明显的侧向膨胀,计算得到的泊松比远大于0．5。当孔隙压力较低时,在峰值应力之前,变形后岩样的体积小于原始体积,体积扩容出现于峰值应力之后,引起了负的体积应变。利用广义虎克定律,解释了平面应变弹性状态下数值结果的合理性。对岩样进行带状区域扫描后,确认随机缺陷的初始分布与岩样的最终破坏形态紧密相关。     

A New System for Ground Water Monitoring

This paper describes a new system for ground water monitoring, "the BAT System," which includes the following functions: (a) sampling of ground water in most types of soils, (b) measurement of pore water pressure, and (c) in situ measurement of hydraulic conductivity. The system can also be used for tracer tests. The system utilizes a permanently installed filter tip attached to a steel or PVC pipe. Installation is normally performed by pushing the tip down to the desired depth. The filter tip can also be buried beneath a landfill. The primary feature of the new system is that the filter tip contains a self-sealing quick coupling unit, which makes it possible to temporarily connect the filter tip to adapters for various functions, e.g. water sampling and for measurement of pore pressure and hydraulic conductivity. The new technique makes sampling of both pressurized water and gas possible. Samples are obtained directly in hermetically sealed, pre-sterilized sample cylinders. Sampling of ground water and measurement of pore pressure can be repeated over a long period of time with undiminished accuracy. This technique is also well-adapted for taking water samples from different strata in a soil profile, in both the saturated and unsaturated zones. Actual installations range from 0.5 to 60m depth.     

Learning of pore pressure response and dynamic soil behavior from downhole array measurements

Downhole arrays are deployed to measure motions at the ground surface and within the soil profile, with some arrays instrumented to also record the pore pressure response within soft soil profiles during excitation. The measurements from these arrays have typically been used in conjunction with parametric and nonparametric inverse analysis approaches to identify soil constitutive model parameters for use in site response analysis or to identify averaged soil behavior between locations of measurement. The self-learning simulations (SelfSim) inverse analysis framework, previously developed and applied under total stress conditions, is extended to effective stress considerations and is employed to reproduce the measured motions and pore pressures from downhole arrays while extracting the underlying soil behavior and pore pressure response of individual soil layers. SelfSim is applied to the 1987 recordings from the Imperial Valley Wildlife Liquefaction Array. The extracted soil behavior suggests a new functional form for modeling the degradation of the shear modulus with respect to excess pore pressures. The extracted pore pressure response is dependent on the number and amplitude of shear strain cycles and has a functional form similar to current strain-based pore pressure generation models.     

油藏水驱开采时移地震监测岩石物理基础测量

岩石物理测量是油藏水驱开采时移地震监测的基础.在实验室对来自胜利油田的5块岩石样品模拟储层条件进行了水驱和气驱动态岩石物理弹性测量,重点分析了流体替换、温度、孔隙压力对岩石纵、横波速度的影响.实验表明,在水驱情形下,由于流体替换和温度、孔隙压力变化所引起的岩石纵横波速度的变化均很小,实施时移地震监测具有较大的风险性.相比之下,气驱可能引起较为明显的纵波速度变化,有利于时移地震监测的实施.进一步完善实验方法、丰富实验内容、是今后时移地震岩石物理实验研究的主要任务.     

Velocity anisotropy and attenuation of shale in under- and overpressured conditions

The seismic velocity and attenuation of fully saturated shales were measured for the first time under overpressured conditions, using the ultrasonic reflection technique. Shale cores from naturally overpressured horizons in the North Sea were tested in the laboratory, at confining and pore pressures relevant to in situ conditions.
A single-frequency tone-burst pulse wave was used to determine the seismic wave velocities and quality factors of the shale samples, with errors less than 0.3% and 0.1 dB/cm, respectively, at a frequency of 0.75 MHz. Sample length changes with varying confining and pore pressure were measured and the pore pressure equilibration time was monitored for each sample.
The anisotropy of the seismic attributes ( V _p, V _s, Q _p and Q _s) was determined over a range of differential pressures from 5 to 60 MPa, with respect to the predominant foliation. The ultrasonic velocity data followed a transversely isotropic pattern depending on the direction of wave propagation with respect to the laminations. The Poisson's ratio was found to rise by 5% as the shale material progressed from a normally pressured to an overpressured state. The quality factor ( Q ) characteristics were interpreted in terms of pore geometry and connectivity as well as the directional permeability of the transversely isotropic shale material. The results were converted to bulk and shear loss modulus defects, and a positive bulk loss was observed for waves propagating perpendicular to the lamination plane even above differential pressures of 20 MPa. This indicates different levels of Biot-flow and squirt-flow attenuation mechanisms acting within the shale structure, depending on the wave propagation and vibration directions.     

An Integrated Study on Physical Properties of a KTB Gneiss Sample and Marble from Portugal: Pressure Dependence of the Permeability and Frequency Dependence of the Complex Electrical Impedance

— Pressure-induced variations in pore geometry were studied on dry- and fluid- saturated samples by means of electrical impedance spectroscopy and permeability measurements. Hydrostatic pressures (up to 120 MPa) and uniaxial pressures (up to failure) were applied. Hydrostatic pressures reduce the aspect ratio of cracks and thus cause a decrease of permeability and electrical bulk conductivity. The opposite was observed in uniaxial pressure experiments where new cracks were formed and consequently permeability and electrical conductivity were increased. More specific informations of these generated observations were derived from the interpretation of the frequency dispersion of the complex electrical conductivity. This least-squares-refinement considers electrochemical interactions between the fluid pore electrolyte and the inner surface of the sample, thus providing informations on the pore geometry and pressure-induced variations. Consequently changes in aspect ratio, size and geometry of the pore system can be detected by means of impedance spectroscopy.     

Production-induced Normal Faulting in the Valhall and Ekofisk Oil Fields

—?In situ stress and pore pressure data from the Valhall and Ekofisk oil reservoirs indicate that at the onset of production in both fields an incipient state of normal faulting existed in the crest of the anticlinal structures. In contrast, on the flanks of the structures the initial least principal stress values indicate an almost isotropic state of stress. Oil production from both fields caused marked pore pressure reductions as well as poroelastic reductions of the least principal stress in both the crest and flanks of the two structures. We demonstrate that as a result of production-induced pore pressure and stress changes, normal faulting appears to have spread out from the crests of the structures on to the flanks. Further evidence of a normal faulting stress state at Valhall has been found using data from a passive seismic monitoring experiment. Numerous microearthquakes were recorded during a six week monitoring period that are located at the very top of the reservoir or in the shale caprock immediately above it. An inverse/composite focal plane mechanism of these microearthquakes is consistent with a normal faulting stress regime.