Showing papers on "Pore water pressure published in 1995"

Pore Pressure Estimation From Velocity Data: Accounting for Overpressure Mechanisms Besides Undercompaction

Abstract: A new method for estimating pore pressure from formation sonic velocity data is presented. Unlike previous techniques, this method accounts for excess pressure generated by both undercompaction, and fluid expansion mechanisms such as aquathermal pressuring, hydrocarbon maturation, clay diagenesis, and charging from other zones. The method is an effective stress approach; the effective stress is computed from the velocity, and the result is subtracted from the overburden stress to obtain pore pressure. to include multiple sources of overpressure, a pair of velocity-vs.-effective-stress relations are introduced. One relation accounts for normal pressure and overpressure caused by undercompaction. The second is applied inside velocity reversal zones caused by fluid expansion mechanisms. Example applications of the method are presented from the U.S. gulf coast, the Gulf of Mexico, and the Central North Sea. some other pore pressure estimation approaches are also examined to demonstrate how these techniques have unknowingly accounted for overpressure mechanisms other than undercompaction. It is also explained how velocity-vs.-effective-stress data can be used to identify the general cause of overpressure in an area. For instance, the empirical correlation of Hottman and Johnson indicates that overpressure along the US gulf coast cannot be due only to undercompaction.

Frictional slip of granite at hydrothermal conditions

Abstract: Sliding on faults in much of the continental crust likely occurs at hydrothermal conditions, i.e., at elevated temperature and elevated pressure of aqueous pore fluids, yet there have been few relevant laboratory studies. To measure the strength, sliding behavior, and friction constitutive properties of faults at hydrothermal conditions, we slid laboratory granite faults containing a layer of granite powder (simulated gouge). Velocity stepping experiments were performed at temperatures of 23° to 600°C, pore fluid pressures PH2O of 0 (“dry”) and 100 MPa (“wet”), effective normal stress of 400 MPa, and sliding velocities V of 0.01 to 1 μm/s (0.32 to 32 m/yr). Conditions were similar to those in earlier tests on dry granite to 845°C by Lockner et al. (1986). The mechanical results define two regimes. The first regime includes dry granite up to at least 845° and wet granite below 250°C. In this regime the coefficient of friction is high (μ = 0.7 to 0.8) and depends only modestly on temperature, slip rate, and PH2O. The second regime includes wet granite above ∼350°C. In this regime friction decreases considerably with increasing temperature (temperature weakening) and with decreasing slip rate (velocity strengthening). These regimes correspond well to those identified in sliding tests on ultrafine quartz. We infer that one or more fluid-assisted deformation mechanisms are activated in the second, hydrothermal, regime and operate concurrently with cataclastic flow. Slip in the first (cool and/or dry) regime is characterized by pervasive shearing and particle size reduction. Slip in the second (hot and wet) regime is localized primarily onto narrow shear bands adjacent to the gouge-rock interfaces. Weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide, or from a dependence of strength on gouge particle size. Major features of the granite data set can be fit reasonably well by a rate- and temperature-dependent, three-regime friction constitutive model (Chester, this issue). We extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural, slipping faults for sliding rates as low as 31 mm/yr. We do this for two end-member cases. In the first case, pore pressure is assumed hydrostatic at all depths. Shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data, while at depths below about 9–13 km (depending on slip rate), strength becomes less sensitive to depth but sensitive to slip rate. In the second case, pore pressure is assumed to be near-lithostatic at depths below ∼5 km. Strength is low at all depths in this case (<20 MPa, in agreement with observations of “weak” faults such as the San Andreas). The predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm/yr, in rough agreement with the seismic-aseismic transition depth observed on mature continental faults. These results highlight the importance of fluid-assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry, large displacements, higher concentrations of phyllosilicates, and time-dependent fault healing.

Lotung Downhole Array. II: Evaluation of Soil Nonlinear Properties

Abstract: The characteristics of soil response during earthquake excitations, at a site in Lotung, Taiwan are identified using the Lotung large scale seismic test (LSST) data. A technique is developed to evaluate soil shear stress-strain histories directly from the free-field downhole accelerations. These histories are used to estimate variation of soil shear moduli and material damping characteristics with shear strain amplitude, and to assess the effects of pore pressure buildup. Soil stiffness properties are found to compare satisfactorily with those obtained through laboratory tests conducted by the University of California, Davis; the National Taiwan University; and the University of Texas at Austin. Pore pressure buildup appears to be accompanied by a reduction in soil stiffness. The information obtained in this study demonstrates that downhole accelerometer and pore pressure arrays offer a direct effective means of evaluating seismic soil properties.

Magnetic and electric fields associated with changes in high pore pressure in fault zones : Application to the Loma Prieta ULF emissions

Abstract: We determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels. Exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent. Rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high-pressure compartment, along the fault face. Nonuniform fluid flow results from pressure decrease in the fracture from crack-generated dilatancy, partial blockage by silica deposition, and clearing as pressure increases. A direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic, piezomagnetic, and magnetohydrodynamic effects. Models of these processes for fault geometries with 1-km-high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms. The electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ML 7.1 Loma Prieta earthquake of October 18, 1989, provided fracture lengths are less than 200 m.

Identification of the permeability distribution in soil by hydraulic tomography

Abstract: A tomographical method is presented that allows the reconstruction of the structure of soil layers between two boreholes. If the soil is water saturated, the transient field equation for pore water pressure S(x) delta tu(x,t)- Del .(k(x) Del u(x,t))=Q can be used to reconstruct the soil structure from its permeability k(x). The transient potential field u(x, t) is controlled by hydraulic dipoles (point source and point sink). The positions of source and sink are varied over both boreholes. Pore water pressure observations are used as data for the inverse problem. Uniqueness problems are discussed. The forward problem is discretized in a FE algorithm, the inverse problem solved by a standard output least-squares method.

Analysis of coupled heat, moisture and air transfer in a deformable unsaturated soil

Abstract: A new theoretical formulation for the analysis of coupled heat, moisture and air transfer is presented, which is applicable to a deformable unsaturated soil. The approach proposed extends previous analyses of the coupled transport of heat, pore water and pore air to take account of the deformation behaviour of partially saturated soil. A numerical solution of the governing differential equations is achieved by the use of the finite element method as a spatial discretization technique coupled with a finite difference recurrence relationship to describe transient behaviour. Three isothermal test problems are then considered to model the behaviour of unsaturated soil under varying stress and suction conditions. Both heave and collapse due to wetting are simulated. The model is applied to experimental work performed by others for the case of heating of an unsaturated montmorillonite clay and is shown to be capable of producing quantitatively physically correct results. These indicate that strong interactions ...

Seismic pore space compressibility and Gassmann’s relation

Abstract: The pore space compressibility of a rock provides a robust, model-independent descriptor of porosity and pore fluid effects on effective moduli. The pore space compressibility is also the direct physical link between the dry and fluid-saturated moduli, and is therefore the basis of Gassmann's equation for fluid substitution. For a fixed porosity, an increase in pore space compressibility increases the sensitivity of the modulus to fluid substitution. Two simple techniques, based on pore compressibility, are presented for graphically applying Gassmann's relation for fluid substitution. In the first method, the pore compressibility is simply reweighted with a factor that depends only on the ratio of fluid to mineral bulk modulus. In the second technique, the rock moduli are rescaled using the Reuss average, which again depends only on the fluid and mineral moduli.

Rate laws for water‐assisted compaction and stress‐induced water‐rock interaction in sandstones

Abstract: Mineral-water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones, phase relations in metamorphic rocks, mass redistribution and replacement reactions during diagenesis, and loss of porosity in deep sedimentary basins. As a step toward understanding the fundamental geochemical processes involved, using naturally rounded St. Peter sand, we have investigated the kinetics of pore volume loss and quartz-water reactions under nonhydrostatic, hydrothermal conditions in flow-through reactors. Rate laws for creep and mineral-water reaction are derived from the time rate of change of pore volume, sand-water dissolution kinetics, and (flow rate independent) steady state silica concentrations, and reveal functional dependencies of rates on grain size, volume strain, temperature, effective pressure (confining minus pore pressure), and specific surface areas. Together the mechanical and chemical rate laws form a self-consistent model for coupled deformation and water-rock interaction of porous sands under nonhydrostatic conditions. Microstructural evidence shows a progressive widening of nominally circular and nominally flat grain-grain contacts with increasing strain or, equivalently, porosity loss, and small quartz overgrowths occurring at grain contact peripheries. The mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts (termed pressure solution or solution transfer), with a lesser component of time-dependent crack growth and healing. The magnitude of a stress-dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy-induced supersaturations, favoring instead the normal stress dependence of molar Gibbs free energy associated with grain-grain interfaces.

Climate-driven flushing of pore water in peatlands

Abstract: NORTHERN peatlands can act as either important sources or sinks for atmospheric carbon1,2. It is therefore important to understand how carbon cycling in these regions will respond to a changing climate. Existing carbon balance models for peatlands assume that fluid flow and advective mass transport are negligible at depth3,4, and that the effects of climate change should be essentially limited to the near-surface. Here we report the response of groundwater flow and porewater chemistry in the Glacial Lake Agassiz peat-lands of northern Minnesota to the regional drought cycle. Comparison of field observations and numerical simulations indicates that climate fluctuations of short duration may temporarily reverse the vertical direction of fluid flow through the peat, although this has little effect on water chemistry5. On the other hand, periods of drought persisting for at least 3–5 years produce striking changes in the chemistry of the pore water. These longer-term changes in hydrology influence the flux of nutrients and dissolved organic matter through the deeper peat, and therefore affect directly the rates of fermentation and methanogenesis, and the export of dissolved carbon compounds from the peatland.

Water Recharge and Solute Transport Through the Vadose Zone of Fractured Chalk Under Desert Conditions

Abstract: This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four dry-drilling holes were bored through the vadose zone. Core and auger samples, collected at 30- to 50-cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6–11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth. Close to land surface, the pore water is strongly evaporated (δ18O = +5.94‰) and highly concentrated (∼29 meq Cl/100 g rock), but changes gradually with depth to amore dilute concentration (∼4 meq Cl/100 g rock) and isotopically depleted composition (δ18O = −4.4‰), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a hydrologic barrier.

Dike intrusion as a trigger for large earthquakes and the failure of volcano flanks

Abstract: Pore fluid pressures induced through dike intrusion have the capability to trigger large earthquakes and to initiate and sustain massive and catastrophic failure of volcano flanks. Suprahydrostatic pore pressures are generated as a result of both mechanical and thermal straining of the rock-fluid medium. Mechanical strains and resulting pore pressures are described using an analogy to a moving volumetric dilation within a porous elastic medium. Thermal pore fluid pressures are simply represented by a static diffusive model subjected to uniform temperature rise at the dike interface. Resulting excess pore pressure distributions acting along the base of a wedge-shaped slide block are used to define the stability of the flanks when subjected to the magma pressures that accompany intrusion. The destabilizing influence of mechanically induced pore pressures is maximized as the intruded width, or corresponding overpressure, of the dike is increased. For realistic parameter magnitudes in volcaniclastic materials, induced pore pressures are capable of initiating failure; however, the mechanical influence is restricted to the proximity of the intrusion, and by itself may not be capable of sustaining sliding motion once it is initiated. The destabilizing influence of thermally induced pore pressures is conditioned by the severity of thermal forcing, ratios of thermal and hydraulic diffusivities, and the time available for the combined thermal and pressure disturbance to propagate outward from the dike. Thermal pressurization of pore fluid extends more slowly than mechanical straining but is shown to be capable of developing massive uplift forces that could initiate and possibly sustain failure along downslope-dipping failure planes. Deviatoric stress-induced generation of pore pressures and frictional heating of pore fluids may act following slide initiation to maintain the impetus of flank failure and enable long runout over compressible marine sediments. In addition to the development of shallow failure, limited deep-seated sliding may also result from the mechanical or thermal pore pressure loading mechanisms. In addition, pore pressures generated through intrusion or frictional heating may trigger, or amplify, large earthquakes, and these in turn may aid shallower flank slip through seismic ground accelerations and vibration-induced pore pressure generation. These phenomenological models of pore pressure rise and stability control may aid comprehension of the cyclic growth, lateral expansion, and subsequent destruction of shield and stratovolcano flanks. Examples in this work refer particularly to oceanic volcanoes typified by the Hawaiian Islands and Reunion Island.

Hydraulic Continuity In Large Sedimentary Basins

Abstract: Regional hydraulic continuity is a phenomenological property of the rock framework. This property is quantified as the ratio of an induced change in pore pressure at a point of observation to an inducing change at a point of origin. A subsurface rock body is considered to be hydraulically continuous on a given time scale if a change in pressure at any of its points can cause a change at any other point, within a time interval measurable on the specified time scale. The hydraulically continuous behaviour of the rock framework may be masked in large sedimentary basins by large distances, relatively short periods of observations, and sharp contrasts in flow-sensitive properties of subsurface fluids, such as temperature and chemical composition. Due probably mainly to these masking effects, the concept of “hermetically” sealed hydraulic compartments in the subsurface is still used as a working hypothesis by some earth scientists, particularly in petroleum geology.

Soil Mechanics: Principles and Practice

Abstract: Soil Formation and Nature Soil Description and Classification Permeability and Seepage Effective Stress and Pore Pressure Contact Pressure and Stress Distribution Compressibility and Consolidation Shear Strength Shallow Foundations: Stability Shallow Foundations: Settlements Pile Foundations Lateral Earth Pressures and Retaining Structures Slope Stability Earthworks and Soil Compaction Site Investigation

Black‐box modeling of the subglacial water system

Abstract: Measurements of water pressure beneath Trapridge Glacier, Yukon Territory, Canada, yield the following generalizations about subglacial conditions in the studied region: (1) Even over short distances the subglacial water system is highly heterogeneous. (2) The subglacial water system consists of at least two distinct components which we refer to as the “connected” and “unconnected” water systems. (3) Regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system. (4) Large spatial pressure gradients can exist within the unconnected water system, and between the connected and unconnected systems. (5) Rapid pressure variations can occur in the unconnected water system. (6) Pressure variations in the unconnected water system do not match those in the connected system and can, in fact, be strongly anticorrelated with pressure variations in the connected system. If the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing, the input-output relation between forcing and response can be efficiently represented as a low-order nonlinear ordinary differential equation. The response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of ∼1.7 hours and ∼7.4 hours that we believe are associated with process rates for substrate compression and pore water diffusion, respectively.

Nonequilibrium Transport of Atrazine through Large Intact Soil Cores

Abstract: Preferential flow in heterogeneous soils may result in more rapid leaching of pollutants through soils than would be predicted using transport models based on the local equilibrium assumption (LEA). Our objectives were to evaluate nonequilibrium processes important to the transport of tritiated water ( 3 H 2 O) and atrazine under varying pore water velocities and soil water contents, and to distinguish between transport-related nonequilibrium (TNE) and sorption-related nonequilibrium (SNE). Column experiments were performed using a 3 H 2 O- 14 C-labeled atrazine pulse through intact soil cores (15.24-cm diam., 30-cm length) at pore water velocities of 0.12, 0.69, and 2.16 cm h -1 (θ v 0.39) and at 0.74 cm h -1 (θ v 0.44). The asymmetrical shape and the left-handed displacement of 3 H 2 O breakthrough curves (BTCs) as a function of pore water velocity and soil water content (θ v ) indicated that 3 H 2 O was subject to TNE at only the 0.74 (θ v =0.44) and 2.16 cm h -1 pore water velocities. The asymmetrical shape and increased tailing of atrazine BTCs at all pore water velocities indicated that atrazine was influenced primarily by SNE at pore water velocities of 0.12 and 0.69 cm h -1 , and a combination of both TNE and SNE at pore water velocities of 0.74 (θ v 0.44) and 2.16 cm h -1 . The convection-dispersion equation based on the LEA was unable to predict atrazine BTCs at any pore water velocity. Although the nonequilibrium bicontinuum (two-site/two-region) model provided excellent fit to all atrazine BTCs, fits to the model cannot be used to separate between TNE and SNE when both mechanisms are operative. Results of this study confirm that TNE and SNE are important transport processes in naturally structured soils under conditions of relatively high pore water velocities and volumetric water contents

Effects of ion exclusion and isotopic fractionation on pore water geochemistry during gas hydrate formation and decomposition

Abstract: The effects of ion exclusion and isotopic fractionation associated with gas hydrate formation and decomposition in continental margin sediments are examined using simple mass balance calculations. In a closed system pore fluid salinity can be increased to brine levels and detectable changes in interstitial waterδ18O can be caused by formation of significant amounts of interstitial gas hydrate. Time- and mass-dependent models indicate that given appropriate geometries, the diffusion of dissolved salts is sufficiently rapid and their supply is large enough to establish dissolved ion gradients that can be measured in sediments obtained from piston cores or boreholes.

Comparison of methods for conducting marine and estuarine sediment porewater toxicity tests—extraction, storage, and handling techniques

Abstract: A series of studies was conducted to compare different porewater extraction techniques and to evaluate the effects of sediment and porewater storage conditions on the toxicity of pore water, using assays with the sea urchin Arbacia punctulata. If care is taken in the selection of materials, several different porewater extraction techniques (pressurized squeezing, centrifugation, vacuum) yield samples with similar toxicity. Where the primary contaminants of concern are highly hydrophobic organic compounds, centrifugation is the method of choice for minimizing the loss of contaminants during the extraction procedure. No difference was found in the toxicity of pore water obtained with the Teflon® and polyvinyl chloride pressurized extraction devices. Different types of filters in the squeeze extraction devices apparently adsorbed soluble contaminants to varying degrees. The amount of fine suspended particulate material remaining in the pore water after the initial extraction varied among the methods. For most of the sediments tested, freezing and thawing did not affect the toxicity of porewater samples obtained by the pressurized squeeze extraction method. Pore water obtained by other methods (centrifugation, vacuum) and frozen without additional removal of suspended particulates by centrifugation may exhibit increased toxicity compared with the unfrozen sample. The toxicity of pore water extracted from refrigerated (4°C) sediments exhibited substantial short-term (days, weeks) changes. Similarly, sediment pore water extracted over time from a simulated amphipod solid-phase toxicity test changed substantially in toxicity. For the sediments tested, the direction and magnitude of change in toxicity of pore water extracted from both refrigerated and solid-phase test sediments was unpredictable.

Temporal variations in dissolved methane deep in the Lake Agassiz Peatlands, Minnesota

Abstract: A study (August 1990 to July 1991) of profiles of dissolved CH4 concentrations, diffusive flux of CH4, and CH4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH4 deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH4 concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH4. At one site (2.5 m depth) the concentration of dissolved CH4 in the peat pore-water was 140 mg L−1. In July 1991, at no site did the concentration of dissolved CH4 in the peat pore water exceed 40 mg L−1 in the peat profile. The average calculated diffusive flux of CH4 decreased from 95 to 45 mg m−2 d−1 between 1990 and 1991. Gaseous CH4 was more in evidence in 1990 than in 1991. In 1990, CH4 at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H2CO3, CO2, HCO3− and CO32− decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L−1 to 38 mg C L−1 in 1991. A dynamic mechanism must exist which traps CH4 deep in the peat column allowing gaseous CH4 to build up, increasing dissolved CH4. Other times, CH4 passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH4 bubbles forms at depth in the peat, trapping gaseous CH4. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the water table elevation.

Analysis of Induced Seismicity for Stress Field Determination and Pore Pressure Mapping

Abstract: The focal mechanisms of some one hundred microseismic events induced by various water injections have been determined Within the same depth interval, numerous stress measurements have been conducted with the HTPF method When inverted simultaneously, the HTPF data and the focal plane solutions help determine the complete stress field in a fairly large volume of rock (about 15×106 m3) These results demonstrate that hydraulically conductive fault zones are associated with local stress heterogeneities Some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field They preclude the determination of the regional stress field from the sole inversion of focal mechanisms In addition to determining the regional stress field, the integrated inversion of focal mechanisms and HTPF data help identify the fault plane for each for each of the focal mechanisms These slip motions have been demonstrated to be consistent with Terzaghi's effective stress principle and a Coulomb friction law with a friction coefficient ranging from 065 to 09 This has been used for mapping the pore pressure in the rock mass This mapping shows that induced seismicity does not outline zones of high flow rate but only zones of high pore pressure For one fault zone where no significant flow has been observed, the local pore pressure has been found to be larger than the regional minimum principal stress but no hydraulic fracturing has been detected there

Pressure and temperature conditions for methane hydrate dissociation in sodium chloride solutions.

Abstract: The pressure and temperature conditions were experimentally determined for methane hydrate dissociation in sodium chloride solutions, and an empirical equation of the conditions was obtained in the pressure range up to 18 MPa. The present results indicate that the maximum depth of oceanic sediments where methane hydrate is stable increases as water depth to seafloor increases, and that the maximum depth in saline water is smaller than that in pure water. The difference in the depth between saline and pure waters increases with decreasing the water depth, indicating that salinity of pore water affects significantly the amount of methane hydrate in oceanic sediments, in particular, beneath seafloor at relatively shallow depths.