Showing papers on "Pore water pressure published in 2014"

Experimental investigation of matrix permeability of gas shales

Abstract: Predicting long-term production from gas shale reservoirs has been a major challenge for the petroleum industry. To better understand how production profiles are likely to evolve with time, we have conducted laboratory experiments examining the effects of confining stress and pore pressure on permeability. Experiments were conducted on intact core samples from the Barnett, Eagle Ford, Marcellus, and Montney shale reservoirs. The methodology used to measure permeability allows us to separate the reduction of permeability with depletion (because of the resultant increase in effective confining stress) and the increase in permeability associated with Knudsen diffusion and molecular slippage (also known as Klinkenberg) effects at very low pore pressure. By separating these effects, we are able to estimate the relative contribution of both Darcy and diffusive fluxes to total flow in depleted reservoirs. Our data show that the effective permeability of the rock is significantly enhanced at very low pore pressures ( []) because of the slippage effects. We use the magnitude of the Klinkenberg effect to estimate the effective aperture of the flow paths within the samples and compare these estimates to scanning electron microscopy image observations. Our results suggest effective flow paths to be on the order from tens of nanometers in most samples to 100–200 nm, in a relatively high-permeability Eagle Ford sample. Finally, to gain insight on the scale dependence of permeability measurements, the same core plugs were crushed, and permeability was again measured at the particle scale using the so-called Gas Research Institute method. The results show much lower permeability than the intact core samples, with very little correlation to the measurements on the larger scale cores.

Application of a SPH depth-integrated model to landslide run-out analysis

Abstract: Hazard and risk assessment of landslides with potentially long run-out is becoming more and more important. Numerical tools exploiting different constitutive models, initial data and numerical solution techniques are important for making the expert’s assessment more objective, even though they cannot substitute for the expert’s understanding of the site-specific conditions and the involved processes. This paper presents a depth-integrated model accounting for pore water pressure dissipation and applications both to real events and problems for which analytical solutions exist. The main ingredients are: (i) The mathematical model, which includes pore pressure dissipation as an additional equation. This makes possible to model flowslide problems with a high mobility at the beginning, the landslide mass coming to rest once pore water pressures dissipate. (ii) The rheological models describing basal friction: Bingham, frictional, Voellmy and cohesive-frictional viscous models. (iii) We have implemented simple erosion laws, providing a comparison between the approaches of Egashira, Hungr and Blanc. (iv) We propose a Lagrangian SPH model to discretize the equations, including pore water pressure information associated to the moving SPH nodes.

Continual in-situ monitoring of pore water stable isotopes in the subsurface

Abstract: . Stable isotope signatures provide an integral fingerprint of origin, flow paths, transport processes, and residence times of water in the environment. However, the full potential of stable isotopes to quantitatively characterize subsurface water dynamics is yet unfolded due to the difficulty in obtaining extensive, detailed, and repeated measurements of pore water in the unsaturated and saturated zone. This paper presents a functional and cost-efficient system for non-destructive continual in situ monitoring of pore water stable isotope signatures with high resolution. Automatic controllable valve arrays are used to continuously extract diluted water vapor in soil air via a branching network of small microporous probes into a commercial laser-based isotope analyzer. Normalized liquid-phase isotope signatures are then obtained based on a specific on-site calibration approach along with basic corrections for instrument bias and temperature dependent isotopic fractionation. The system was applied to sample depth profiles on three experimental plots with varied vegetation cover in southwest Germany. Two methods (i.e., based on advective versus diffusive vapor extraction) and two modes of sampling (i.e., using multiple permanently installed probes versus a single repeatedly inserted probe) were tested and compared. The results show that the isotope distribution along natural profiles could be resolved with sufficiently high accuracy and precision at sampling intervals of less than four minutes. The presented in situ approaches may thereby be used interchangeably with each other and with concurrent laboratory-based direct equilibration measurements of destructively collected samples. It is thus found that the introduced sampling techniques provide powerful tools towards a detailed quantitative understanding of dynamic and heterogeneous shallow subsurface and vadose zone processes.

Past methane release events and environmental conditions at the upper continental slope of the South China Sea: constraints by seep carbonates

Abstract: Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ18O of seawater (δ18Osw) and the δ18O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ18O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.

Modeling of Coupled Thermo-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonite-Backfilled Repository Tunnels in Clay Formations

Abstract: This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS- rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in hor- izontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geo- mechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our ini- tial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low- permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribu- tion. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabil- ization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a pos- sible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.

Detection of overpressure zones and a statistical model for pore pressure estimation from well logs in the Krishna-Godavari Basin, India

Abstract: Abnormally high pressures, measured by repeat formation tester (RFT) and detected by well log data from 10 wells in the Krishna-Godavari (K-G) Basin, occur in the Vadaparru Shale of Miocene and Raghavapuram Shale of Early Cretaceous age. Overpressures generated by disequilibrium compaction, and pore pressures have been estimated using the conventional Eaton sonic equation with an exponent of 3.0. The observed abnormal pore pressure gradient ranges from 11.85 to 13.10 MPa/km, whereas fracture pressure gradient varies from 17.40 to 19.78 MPa/km. The magnitude of vertical stress (Sv) has a gradient from 21.00 to 23.10 MPa/km. The minimum horizontal principal stress (Sh) magnitude is found to vary from 64 to 77% of the Sv in normally pressured to overpressured sediments. A multiple linear regression model with a squared multiple correlation coefficient (R2) of 0.94 is proposed for pore pressure prediction from gamma ray, density and sonic logs to focus on efficient drilling operations and to prevent borehole instability. The statistical model has been calibrated with the RFT data from five wells covering about 3400 sq. km area of the onshore K-G Basin. The model predicted pore pressure values are in close agreement with the actual RFT data for another four wells including a well in the offshore K-G Basin. Hence, the proposed regression model may be useful for predicting pore pressure from other well logs in the K-G Basin.

A simple analytical solution to one-dimensional consolidation for unsaturated soils

Abstract: SUMMARY This paper presents a simple analytical solution to Fredlund and Hasan's one-dimensional (1-D) consolidation theory for unsaturated soils The coefficients of permeability and volume change for unsaturated soils are assumed to remain constant throughout the consolidation process The mathematical expression of the present solution is much simpler compared with the previous available solutions in the literature Two new variables are introduced to transform the two coupled governing equations of pore-water and pore-air pressures into an equivalent set of partial differential equations, which are easily solved with standard mathematical formulas It is shown that the present analytical solution can be degenerated into that of Terzaghi consolidation for fully saturated condition The analytical solutions to 1-D consolidation of an unsaturated soil subjected to instantaneous loading, ramp loading, and exponential loading, for different drainage conditions and initial pore pressure conditions, are summarized in tables for ease of use by practical engineers In the case studies, the analytical results show good agreement with the available analytical solution in the literature The consolidation behaviors of unsaturated soils are investigated The average degree of consolidation at different loading patterns and drainage conditions is presented The pore-water pressure isochrones for two different drainage conditions and three initial pore pressure distributions are presented and discussed Copyright © 2013 John Wiley & Sons, Ltd

Soil strength estimation and pore pressure dissipation for free-fall piezocone in soft clay

Abstract: This paper considers centrifuge modelling of free-fall piezocones in soft clay, and interprets the test data to estimate the undrained shear strength and coefficient of consolidation. Data from the free-fall piezocones, which involved dynamic embedment in a normally consolidated kaolin clay sample from various drop heights, were compared with equivalent data from piezocone tests. The undrained shear strength, as interpreted from the acceleration trace of the free-fall piezocone, was found to be in good agreement with that derived from the piezocone penetration tests. The free-fall piezocone is also found to produce identical pore pressure dissipation behaviour as for a statically penetrated piezocone, despite a significant rise in the pore pressure at the start of the dissipation phase.

Modeling the hydro-mechanical responses of strip and circular punch loadings on water-saturated collapsible geomaterials

Abstract: A stabilized enhanced strain finite element procedure for poromechanics is fully integrated with an elasto-plastic cap model to simulate the hydro-mechanical interactions of fluid-infiltrating porous rocks with associative and non-associative plastic flow. We present a quantitative analysis on how macroscopic plastic volumetric response caused by pore collapse and grain rearrangement affects the seepage of pore fluid, and vice versa. Results of finite element simulations imply that the dissipation of excess pore pressure may significantly affect the stress path and thus alter the volumetric plastic responses.

The Effects of Temperature and Pressure on the Porosity Evolution of Flechtinger Sandstone

Abstract: A porosity change influences the transport properties and the elastic moduli of rock while circulating water in a geothermal reservoir. The static and dynamic elastic moduli can be derived from the slope of stress–strain curves and velocity measurements, respectively. Consequently, the acoustic velocities were measured while performing hydrostatic drained tests. The effect of temperature on static and dynamic elastic moduli and porosity variations of Flechtinger sandstone was investigated in a wide range of confining pressure from 2 to 55 MPa. The experiments were carried out in a conventional triaxial system whereas the pore pressure remained constant, confining pressure was cycled, and temperature was increased step wise (25, 60, 90, 120, and 140 °C). The porosity variation was calculated by employing two different theories: poroelasticity and crack closure. The porosity variation and crack porosity were determined by the first derivative of stress–strain curves and the integral of the second derivative of stress–strain curves, respectively. The crack porosity analysis confirms the creation of new cracks at high temperatures. The porosity variation was increasing with an increase in temperature at low effective pressures and was decreasing with a rise in temperature at high effective pressures. Both compressional and shear wave velocities were increasing with increasing pressure due to progressive crack closure. Furthermore, the thermomechanical behavior of Flechtinger sandstone was characterized by an inversion effect where the sign of the temperature derivative of the drained bulk modulus changes.

Fluid effect on hydraulic fracture propagation behavior: a comparison between water and supercritical CO2‐like fluid

Abstract: The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO2, the pore fluids in deep formations can be changed from oil/saline water to CO2 or CO2 dominated. The type of fluid is important not only because the fluid must fracture the rock, but also because rocks saturated with different pore fluids behave differently. We investigated the influence of fluid properties on fracture propagation behavior by using the cohesive zone model in conjunction with a poroelasticity model. Simulation results indicate that the pore pressure fields are very different for different pore fluids even when the initial field conditions and injection schemes (rate and time) are kept the same. Low viscosity fluids with properties of supercritical CO2 will create relatively thin and much shorter fractures in comparison with fluids exhibiting properties of water under similar injection schemes. Two significant times are recognized during fracture propagation: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are very different for different fluids. Both fluid compressibility and viscosity influence fracture propagation, with viscosity being the more important property. Viscosity can greatly affect hydraulic conductivity and the leak-off coefficient. This analysis assumes the in-situ pore fluid and injected fluid are the same and the pore space is 100% saturated by that fluid at the beginning of the simulation.

Landsliding triggered by reservoir operation: a general conceptual model with a case study at Three Gorges Reservoir

Abstract: The Three Gorge Reservoir, one of the largest civil engineering projects in human history, dams the Yangtze River to form a 660-km-long and 113-km-wide reservoir Today, although the project has been completed and is in normal operation, the on-going landslide initiation and movement in response to the reservoir operating is one of the main geologic hazards The Huangtupo (meaning “yellow soil slope” in Chinese) Slope typifies such on-going landslides along the reservoir Observations from a multi-year monitoring program conducted on this slope indicate that there are multiple slides on the reservoir banks that move episodically into the reservoir and their movements appear to be highly correlated with the initial and seasonal changes in the reservoir pool level A hydro-mechanical numerical model is constructed to investigate the quantitative links among the episodic movements and the variations in pore water pressure, suction stress, hydrostatic reservoir water loading, and slope self-weight induced by the fluctuating water levels Modeling results identify regions within the variably saturated slope where significant changes in stress occur during the periods of the initial impoundment that raised water levels from 68 to 135 m and that occur in response to seasonal fluctuations of the reservoir pool level between 145 and 175 m We find that the rise or decline of reservoir pool level can either increase or decrease the stability of landslide In general, hydrostatic reservoir water loading has positive correlation with the stability; pore water pressure and suction stress have negative correlation with the stability; and the effects of slope self-weight depend on the dip angle and mechanical properties of sliding surface

Swash zone moisture dynamics and unsaturated infiltration in two sandy beach aquifers

Abstract: The intertidal zone of coastal aquifers is a dynamic region of mixing between saline surface water and fresh groundwater. Groundwater circulation in this zone is affected by complex forcing mechanisms that operate on a range of timescales and can regulate chemical fluxes to marine environments. We evaluated wave swash-induced infiltration and associated flow dynamics in the unsaturated region of two sandy beach aquifers with differing wave conditions and beach morphologies. Moisture and pressure sensors were used to measure fluctuations in water content, water table elevation, and hydraulic gradients at high frequencies (5 Hz) in the swash zone at mean lower low water, mean sea level, and mean higher high water. Water content in the unsaturated region of the swash zone responded to wave overtopping and swash infiltration, with a rapid rise in water content followed by a slower decline. Swash-induced unsaturated infiltration rates, calculated from water content response, were lowest near low tide and increased up the beachface with inflow highest near high tide at both sites, consistent with an increase in water table depth up the beach. Unsaturated infiltration was 1.6 m 3 /m per tidal cycle at the wavedominated beach and 0.4 m 3 /m per tidal cycle at the tide-dominated beach. Saturated pore pressure measurements show that a water table mound formed as a consequence of swash infiltration that migrated up the beach during rising tide, leading to divergent seaward/landward groundwater flow. The results demonstrate the significant and spatially variable effects of wave swash on moisture dynamics in the unsaturated zone of beach aquifers, and show that these effects depend on wave conditions and beach characteristics. Results have implications for understanding transport and reaction of solutes in this biogeochemically active zone.

Two-Dimensional Model for Pore Pressure Accumulations in the Vicinity of a Buried Pipeline

Abstract: In this paper, we presented an integrated numerical model for the wave-induced residual liquefaction around a buried offshore pipeline. In the present model, unlike previous investigations, two new features were added in the present model: (i) new definition of the source term for the residual pore pressure generations was proposed and extended from 1D to 2D; (ii) preconsolidation due to self-weight of the pipeline was considered. The present model was validated by comparing with the previous experimental data for the cases without a pipeline and with a buried pipeline. Based on the numerical model, first, we examined the effects of seabed, wave and pipeline characteristics on the pore pressure accumulations and residual liquefaction. The numerical results indicated a pipe with a deeper buried depth within the seabed with larger consolidation coefficient and relative density can reduce the risk of liquefaction around a pipeline. Second, we investigated the effects of a trench layer on the wave-induced seabed response. It is found that the geometry of the trench layer (thickness and width), as well as the backfill materials (permeability K and relative density Dr) have significant effect on the development of liquefaction zone around the buried pipeline. Furthermore, under certain conditions, partially backfill the trench layer up to one pipeline diameter is sufficient to protect the pipelines from the wave-induced liquefaction.

Experimental study of strain localization in sensitive clays

Abstract: For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor.

Shale swelling/shrinkage and water content change due to imposed suction and due to direct brine contact

Abstract: Analyses were performed on nine different preserved shales, representing in situ states of 5–15 % water content and 0.13–0.42 void ratio. Under varying total suction (controlled humidity), each shale shows well-defined relationships among suction, volume change, water content and saturation, with the lower-porosity shales undergoing less volume and water content change than the higher-porosity shales. A decrease in in situ porosity is also associated with a much higher native state suction as well as full saturation extending to suction values beyond 40 MPa. Only part of the high suction is due to capillary tension. Under direct brine exposure, the shales almost always swell, even when the brine has an equivalent suction greater than the shale. This is likely due to the reduction in some component of the matric suction. The shale pore water is found to equilibrate with the solute content of the surrounding brine, due to ion diffusion. Much or all of the swelling, and water increase, appears to take place in the clay-bound water and not in the main (free water) pore space. The swelling magnitude is consistent with the amount of water content increase. Swelling usually corresponds to less than one additional water layer being added between the clays. Swelling, and water increase, is very small for the low-porosity shales. Some osmotic effects are observable in all the shales, and cation exchange on the clays also takes place. Swelling is best inhibited with potassium, followed by sodium, followed by calcium, for brines of equal water activity ranging from 0.8 to 0.9.

Construction, management and maintenance of embankments used for road and rail infrastructure: implications of weather induced pore water pressures

Abstract: Understanding the age and construction quality of embankments used for road and rail infrastructure is critical in the effective management and maintenance of our transport networks, worth £billions to the UK economy. This paper presents for the first time results from full-scale, carefully controlled experiments on a unique model embankment conducted over the 4-year period between 2008 and 2011. It combines point location and spatially distributed measurements of pore water pressures and water content with outputs from hydrological modelling to draw conclusions of significance to both ongoing research in this field and to the asset management practices of infrastructure owners. For researchers, the paper highlights the crucial importance of transient permeability and soil water retention behaviour of fill materials in controlling the magnitude and distribution of pore water pressure in response to climate and weather events. For practitioners, the work demonstrates that there are significant differences in pore water pressure behaviour across the embankment, which is influenced by construction-related issues such as compaction level, aspect and presence of a granular capping material. Permeability was also observed to vary across the embankment both spatially and with depth, being dependent on degree of saturation and macroscale effects, particularly within a ‘near surface zone’. It is proposed that this ‘near surface zone’ has a critical effect on embankment stability and should be the focus of both ongoing scientific research and inspection and monitoring as encompassed by asset management regimes.

Numerical simulation of piezocone dissipation test in clays

Abstract: The piezocone is one of the most widely used in-situ tools with which to characterise the soil profile, distinguishing different types of sediment, and to provide quantitative data on the soil strength and consolidation characteristics. For fine-grained soils where penetration occurs under nominally undrained conditions, consolidation characteristics are obtained from dissipation tests conducted with the piezocone brought to rest. Dissipation testing is interpreted to yield a coefficient of consolidation, by comparison of the excess pore pressure decay with theoretical solutions. It is generally acknowledged that the coefficient of consolidation derived from in-situ dissipation tests will be greater, by a factor of perhaps 2 to 10, than values derived from oedometer tests on samples recovered from the corresponding depth. Apart from potential differences in horizontal and vertical permeability, differences arise from the influence of the rigidity index on the initial excess pore pressure field and the mor...

An NMR-Based Analysis of Soil–Water Characteristics

Abstract: Both direct and indirect methods for determining soil–water characteristic curves rely on determination of some empirical coefficients, which may not necessarily represent real microscopic mechanisms Proton nuclear magnetic resonance (NMR) is a powerful tool for investigating water content and their interaction with solid particles in porous media The NMR technique is widely used in food science and petroleum In the present study, proton NMR spin–spin relaxation time (T 2) distribution measurement is integrated with a Tempe apparatus to characterize the hydraulic processes of unsaturated soils, shedding insights into the microscopic mechanisms of pore water distribution and migration in the soil during hydraulic cycles It is revealed that during a drying process the drainage of pore water occurs sequentially from larger pores to smaller pores, whereas in a wetting process the water invades into the soil sequentially from smaller pores to larger pores A new procedure is developed which can be used to determine the pore size distribution of the soil based on the NMR T 2 distribution measurements; compared to the traditional methods, the new method is rapid and non-destructive The new procedure is validated by comparing the new result with the measurement of the mercury intrusion porosimetry