Showing papers on "Pore water pressure published in 1988"

Two types of reservoir-induced seismicity

Abstract: The temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response. At some reservoirs, seismicity begins almost immediately following the first filling of the reservoir. At others, pronounced increases in seismicity are not observed until a number of seasonal filling cycles have passed. These differences in response may correspond to two fundamental mechanisms by which a reservoir can modify the strength of the crust—one related to rapid increases in elastic stress due to the load of the reservoir and the other to the more gradual diffusion of water from the reservoir to hypocentral depths. Decreased strength can arise from changes in either elastic stress (decreased normal stress or increased shear stress) or from decreased effective normal stress due to increased pore pressure. Pore pressure at hypocentral depths can rise rapidly, from a coupled elastic response due to compaction of pore space, or more slowly, with the diffusion of water from the surface.

Fault stability changes induced beneath a reservoir with cyclic variations in water level

Abstract: The stress and pore pressure changes produced by a steady periodic variation of water level on the surface of a uniform porous elastic half-space are evaluated using the fully coupled (Biot) equations of elastic deformation and pore fluid flow. Diverse choices of material properties all give a coupled stress field differing from the elastic stress field by at most 0.035 po, where po is the water pressure at the bottom of the reservoir. Peak coupled pore pressure change can lag peak water level in the depth range 0 < (ω/2c)1/2z < π, where ω is frequency of the cyclic change in water level, c is diffusivity, and z is depth. The maximum lag increases as B decreases, where B is the ratio of pore pressure increase to mean compressive stress increase under undrained conditions. Directly beneath the reservoir, for example, peak pore pressure in an annual cycle can lag peak water level by at most 10 days if B = 0.80, but can lag by up to 122 days if B = 0.11. When cyclic water level changes are superimposed on the steady state reservoir level, the time during the cycle at which a fault is most destabilized depends on whether the weight of the reservoir stabilizes or destabilizes the fault, which, in turn, depends on its orientation and location relative to the reservoir. B and c also influence the timing of the greatest destabilization. If B and c are low, maximum destabilization at low water level is possible for faults that are stabilized by the weight of the reservoir; this mechanism may have operated at Lake Mead. The analysis suggests that induced seismic events should be separated into groups having a common focal mechanism and occurring in similar locations relative to the reservoir before studying the time at which the events occur relative to the water level. The fully coupled solution is compared with an uncoupled solution, with a solution that is coupled but which assumes incompressible solid and fluid constituents (consolidation) and with a decoupled solution in which the difference between the pore pressure field and B times the elastic mean compressive stress obeys a homogeneous diffusion equation. The uncoupled and consolidation solutions respectively underestimate and overestimate pore pressure during short-term reservoir level fluctuations as well as at times short compared to that required to achieve steady state. In contrast, the decoupled solution agrees closely with the fully coupled solution for the problem studied here.

Comparison of truly undrained and constant volume direct simple shear tests

Abstract: The Paper describes a chamber for the Norwegian Geotechnical Institute's direct simple shear device that enables undrained tests with pore pressure measurements to be run. The results of undrained ...

Dipping Seismic Reflectors, Electrically Conductive Zones, and Trapped Water in the Crust Over a Subducting Plate

Abstract: The LITHOPROBE program across the northern Cascadia subduction zone at Vancouver Island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust. Land and marine multichannel seismic reflection profiles show several bands of reflectors. The primary one at a depth of about 30 km is about 5 km thick, dips inland, and appears to coincide with an electrically conductive layer defined by magnetotelluric data. The downgoing oceanic crust is imaged as a thin reflector dipping at about 15° and 10 km deeper than the primary reflector, a location that is consistent with seismicity and seismic refraction data. Both the reflectors and the conductor have been modeled by near-horizontal lamellae with 1–4% saline water porosity. Detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400°–500°C. It is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier. Low vertical permeability is required to keep the water from separating to the top of the layer. This can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry, although very small effective grain size is necessary. An abrupt transition from high permeability to very low permeability is predicted at a several percent porosity, in agreement with the modeled porosity. Such pore geometry with low permeability may be restricted to temperatures above about 400°C, i.e., in greenschist to amphibolite facies conditions, where grains will deform to equilibrium geometry and cracks are annealed. The region above the reflective and conductive band is estimated to be within blueschist conditions, consistent with the inferred high seismic velocity from refraction data and high density from gravity data.

A conceptual model for soils containing large gas bubbles

Abstract: Offshore site investigations have indicated the presence of undissolved gas bubbles in the soils forming the sea-bed at various locations throughout the world. The gas bubbles are typically much larger than the normal void spaces, so they cannot be considered as occluded bubbles within the pore water which simply change the compressibility of the pore fluid. The effect of these large bubbles on the engineering properties of the soil must be understood if offshore construction is to take place in areas of gas-bearing sediments. This Paper describes a conceptual model for soils containing large gas bubbles. The model consists of a matrix of saturated soil surrounding isolated gas-filled cavities. The cavities forming the bubble sites are fixed within the soil, so that gas drainage occurs only by the movement of dissolved gas. The behaviour of the model is complicated by the processes of localized consolidation and bubble flooding. Des recherches conduites en mer ont indique qu'il y a partout dans le monde d...

Generation of high pore pressures in accretionary prisms: Inferences from the Barbados Subduction Complex

Abstract: Recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values. Indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes. In this study the mechanisms for generating excess pore pressures is investigated by focusing on the Barbados Ridge Complex which is an end-member of sediment-rich accretionary prisms. Modeling is carried out using a two-dimensional finite element procedure, with constraints provided by the following data: Deep Sea Drilling Project results on stratigraphy and lithology, seismic results on sediment structure and thickness, regional geology on tectonic history, and laboratory measurements on mechanical properties of sediments. Coupling among the equations for fluid flow, heat transfer, and sediment deformation is made throughout the computation. The results of the modeling predict that the principal decollement beneath the Barbados Ridge Complex is a zone of high pore pressure, high porosity, and reversal in seismic velocity. These characteristics may be responsible for the mechanical weakness of the decollement, which allows continuous subduction of sediments below it, and for its seismic signatures. Results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms. The direct effect of tectonic compression is of secondary importance, and the effect due to temperature increase with subduction is of little significance. Thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs. Rich sediment supply, low permeability, high subduction angle, and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms.

Experimental “sytectonic” dehydration of serpentinite under conditions of controlled pore water pressure

Abstract: We have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine + talc + water, at temperatures ranging between 300° and 600° and at total pressures from 100 to 300 MPa. Constant stress, stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed. Effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system. Stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction. At 500° and 600°C, however, the strength was linearly dependent on the inverse of the deformation rate, and independent of effective confining pressure over the range investigated. Microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine-grained (0.1 µm olivine produced in the breakdown reaction. The enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion-accommodated grain boundary sliding. These experimental results suggest that metamorphic transformations affect the deformability of rocks, and demonstrate enhancement of deformability through the transient existence of fine-grained reaction products. The results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain-size reduction by dynamic recrystallization in plastically deforming rocks.

Stabilization of rapid frictional slip on a weakening fault by dilatant hardening

Abstract: Frictional slip is often accompanied by dilatancy due to uplift in sliding over asperities and micro-cracking in the adjacent material. If dilatancy occurs more rapidly than pore fluid can flow into the newly created void space, the local pore pressure is reduced and the effective normal stress is increased in compression, tending to inhibit further slip. This dilatant hardening is analyzed for a simple model. One surface of a slab is loaded by compressive stress and shear displacement and connected to a reservoir of pore fluid held at constant pressure. The other boundary is a frictional surface, assumed to have formed at peak stress, on which the shear stress decreases from a peak value τp to a residual value τr as slip increases from zero to δ0. In the absence of pore fluid effects an instability corresponding to an unbounded slip rate occurs when the slope of the shear stress versus slip relation is more negative than the unloading stiffness of the surrounding material. Dilatant hardening prevents this instability provided that the pore pressure in the reservoir is high enough. If the pressure in the reservoir is too low, the pressure at the fault surface can be reduced to the point at which the pore fluid bulk modulus decreases rapidly, eliminating the stabilizing effect. When the analysis is modified to include normal stress changes simulating those in the axisymmetric compression test, the prediction of the critical pressure in the reservoir agrees to within a factor of 2 or 3 with that observed by Martin in tests on Westerly granite. The predictions are also consistent with the trends observed by Martin of decreasing critical reservoir pore pressure with increasing effective confining stress and decreasing nominal strain rate.

Centrifuge liquefaction tests in a laminar box

Abstract: The difficulties associated with instrumenting earthquake sites in order to record pore pressure changes in a future event led to the use of scaled model tests performed in a centrifuge. Both dry and saturated sands were employed, contained in a box constructed of aluminium laminae designed to move freely on each other. This would result in shearing distortions developing in the soil unimpeded by the container. Accelerometers, displacement transducers and pore pressure sensors were attached to the box and embedded in the soil at various elevations so as to record the response of the soil to an earthquake-like excitation supplied to the base of the container. A special apparatus was constructed to imitate earthquake motion. In some tests on saturated sand, the soil profile was liquefied. Test results of accelerations, lateral and vertical displacements and pore pressures against time for typical earthquake inputs are given. The data, obtained under controlled conditions, can be compared with the various calculation methods for dynamically generated pore pressures.

Phosphorus in interstitial water: methods and dynamics

Abstract: Interstitial (pore) water is important for phosphorus dynamics in limnic ecosystems. This review summarizes the most important processes influencing the generation and transport of pore-water P from, to and within the sediment. Sampling methodology is discussed thoroughly in combination with what little information is available on various P species in the pore water.

Laboratory and field evaluation of cone penetrometers

Abstract: Cone penetrometers from 8 organizations have been studied by performing laboratory tests and field tests The testing show that pore pressure and temperature zero shift affect significantly uncorrected cone resistance and sleeve friction The magnitude of the measured pore pressure depends on the location of the porous filter By correcting for temperature zero shift and pore pressure effects, one may significantly reduce the differences in the cone resistances derived from the different cones Differences in sleeve friction values are also reduced by the corrections, but not as much

Wave-induced forces on buried pipelines

Abstract: Ocean waves induce dynamic pressure responses in permeable seabeds which result in dynamic loads on buried pipelines. An analytical model is developed for estimating the pore pressure in the soil and the resulting pressure force on buried pipelines. It is assumed that the seabed is rigid, homogeneous, and porous with isotropic permeability, that the pore water is incompressible, that fluid flow in the soil is modeled by Darcy's Law, and that the seabed is infinitely deep. A solution is developed for a circular, rigid pipeline using conformal mapping techniques. The solution is compared with the results of both small and large‐scale tests; reasonable agreement is obtained for the small‐scale tests. Wave‐induced seepage forces are evaluated by integrating the pressure distribution over the pipe surface. The magnitude of the force remains constant but the direction rotates around the cylinder once with the passage of each wave. This force may be of sufficient magnitude to be an important consideration in the...

The extent of lateral water movement in the sediments of a New England Salt Marsh

Abstract: Models of depth-averaged hydraulic head are used with data from a water balance study in a New England salt marsh to describe horizontal pore water water fluxes near a creek bank. Three hydrologically distinct regions are identified in the sediment. In the marsh studied, semidiurnal tides may drive an oscillating horizontal flux in the narrow region within about 2.5 m of the creek bank. Farther than 15 m from the creek there is essentially no horizontal water movement. In the region between 2.5 and 15 m, drainage to the creek is driven by alternating periods of surface flooding and nonflooding due to the spring neap variation in tidal amplitudes. There is little or no input of fresh groundwater to the marsh sediment at this site. The spatial extent of the drained region depends on the duration of the nonflooded period, the morphology of the sediment surface, and the ratio of hydraulic conductivity to specific storage of the sediment.

Pore pressure formation evaluation while drilling

Abstract: Formation strength and other measurement while drilling parameters are combined in a formation volumetric analysis which produces not only the traditional volumetric components of clay volume, mineral volume, total porosity, and water filled porosity, but also, in shaley formations, an excess or overpressure porosity. The overpressure porosity is then utilized to generate an indication of pore pressure which in turn is used in the drilling process as an aid in determining the lowest optimal drilling mud weight for most efficient drilling without incurring excessive risks of a blowout arising from an overpressured formation.

Overconsolidation ratio of clays from penetration pore pressures

Abstract: A method is proposed for evaluating the overconsolidation ratio in clays based on the distribution of dynamic pore pressure around a penetrating cone. For this purpose the pore-pressure measurement...

Fluid Flow in Low Permeable, Porous Media

Abstract: Migration of hydrocarbons deals with the subsequent movement of petroleum after expulsion from the source rock through water saturated reservoirs or through permeability created by fractures and faults. Although the underlying principles that control the fluid movement in porous media (reservoirs) are well understood by reservoir engineers, less is known about the flow characteristics in low-permeable, porous media, such as clays and shales. For flow considerations, the primary parameters are porosity, permeability and the fluid potential gradients. For clays and shales, these parameters are poorly known; yet these control the time periods during which fluid flow occurs in sedimentary basins (100 years to 100 million years). In this paper, I examine the parametric dependence of the time constantsof fluid flow in low permeability sediments on its porosity and permeability. This is accomplished in two parts. In the first part, a technique is presented to investigate the effect of fluid flow in shales which causes undercompaction and buildup of fluid pressures in excess of normal hydrostatic pressure. The technique is pre-drill in nature; it uses seismic velocity analysis of common depth point gather of surface seismic data and is based on the concept developed by Hottmann and Johnson (1965) and Pennebaker (1968). In the second part of the paper, the flow characteristics are discussed in the basin scale. I develop a model that describes the fluid flow in a continuously accreting and subsiding clastics basins, such as the Gulf of Mexico. I examine the pressure characteristics of such a basin by digital simulations and study the effect of the permeability variation of shales on the geologic time dependence of the fluid flux in the sediments, the basin subsidence rate and the pressure buildup with depth. The model incorporates both mechanical compaction and burial diagenesis involving smectite to illite conversion of shales. The latter is based on a kinetic theory of chemical reaction in which the time-temperature history is provided by the solution of an appropriate heat conduction equation. This model of geopressure has a practical application since it deals with the generation and maintenance of abnormally high fluid pressures over geologic time and impacts the hydrocarbon migration in clastic basins.

Pore pressure changes induced by slip on permeable and impermeable faults

Abstract: Pore pressure changes due to a ramp introduction of slip on permeable and impermeable faults in a fluid-saturated rock mass are calculated for the purpose of evaluating water well level fluctuations. The calculations demonstrate the importance of coupling between deformation and fluid diffusion at observation points less than 5(4ct0)½, where c is the diffusivity and t0 is the rise time. The decay of pore pressure in the results here is due entirely to fluid mass diffusion. An approach that neglects diffusion and assumes that the pore pressure is proportional to the mean normal stress would predict a ramp pore pressure response. At distances greater than 5(4ct0)½ the pore pressure decays so slowly that the neglect of diffusion may be appropriate. For both permeable and impermeable faults, the pore pressure decays more rapidly for shorter slip zone lengths and longer rise times. However, the pore pressure change calculated for the impermeable fault is larger, particularly for observation points near the fault, and decays less rapidly than for the permeable fault. These differences suggest that fault permeability can be a significant factor in the response of water wells near faults and care should be used in inferring details of the slip distribution if hydrologic conditions are not known. These results are applied to a water well level change observed by Lippincott et al. A satisfactory fit to the data is obtained by uniform slip over a fault length of about 1.5 km and a rise time of 8 hours. Although the slip magnitude is not well constrained by the fit, the range of possible values includes the 0.5 to 1.0 cm inferred by Lippincott et al. using a different approach.

New Concept of Effective Stress in Unsaturated Soil and Its Proving Test

Abstract: Triaxial compression tests, including anisotropic consolidation tests, were performed on compacted kaolin clay by an automatically controlled apparatus. Test results were analyzed in terms of suction and (σ - u a ), where σ is total normal stress and u a is pore air pressure, recognizing that when soil is saturated, u a becomes equal to u w , pore water pressure. When suction is kept constant, the relationships among the observed shear strength, stress-strain, and stress-water content change can be formulated in equations similar to those of saturated soil, in which suction can be regarded as a factor contributing to soil constants. When both suction and all-around pressure are varied, the plastic volumetric strain can be expressed by an equation in which (σ - u a ) is multiplied by a function of suction. Therefore suction can be regarded as a factor contributing to soil constants in this case too. These facts mean that the suction need not be regarded as one of the principal stress components such as (σ - u a ) in the stress-strain equations of unsaturated soil.

Chapter 2 Sandstone Diagenesis in Relation to Preservation, Destruction and Creation of Porosity

Abstract: Publisher Summary Diagenetic processes in sandstones are mainly a function of the primary composition of the sandstone, porewater flow, effective or grain-to-grain stress, and temperature. These parameters can, to a large extent, be related to the source rock, depositional facies and stratigraphy, and tectonism, which, in turn, are related to the plate-tectonic setting of the basin. Primary porosity is reduced isochemically by mechanical compaction and pressure solution due to the effective stress. The degree of mechanical compaction is not a function of overburden, but of the effective stress. Consequently, higher pore pressures (lower geostatic pressure/pore pressure ratio) are effective in preserving primary porosity. Pressure solution is enhanced by mica and clay laminae in the fine-grained sandstones. Net cementation of sandstones (i.e., by carbonates or quartz) is a function of total passage of porewater and its degree of supersaturation with respect to the cementing minerals. Very large volumes of porewater flow (10 4 - 10 5 pore volumes) are required to precipitate one pore volume (1 cm 3 ) of cement. Porewater must be flowing from areas of higher solubility to areas of lower solubility due to variations in pH, porewater composition, and/or temperature. Secondary porosity may be caused by fracturing of the sandstone due to tectonic stresses or overpressure (hydrofracturing) and chemical dissolution. Leaching of minerals and cement may occur when the sandstone is flushed by meteoric water undersaturated with respect to carbonate cement or feldspar grains.

A study of geotechnical properties of Cochin Marine Clays

Abstract: Most of the Greater Cochin area, which is undergoing rapid industrialisation, consists of extremely soft marine clay calling for expensive deep foundations. This paper presents a study on the physical properties and engeering characteristics of Cochin marine clays. These marine clays are characterised by high Atterberg limits and natural water contents. They are moderately sensitive with liquidity indices ranging over 0.46 to 0.87.The grain size distribution shows almost equal fractions of clay and silt size with sand content varying around 20%. Use of a dispersing agent in carrying out grain size distribution test plays an important role. The fabric of these clays had been identified as flocculant. The pore water has low salinity which results in marginal changes in properties on washing.Consolidation test results showed a preconsolidation pressure of up to about 0.5 kg/cm2 with high compression indices. Compression index vs liquid limit yielded a correlation comparable to that of published data. The undisturbed samples have a much larger coefficient of secondary consolidation as a result of flocculant fabric. These clays have very low undrained shear strength.

Dewatering and extensional deformation of the Shikoku Basin hemipelagic sediments in the Nankai Trough

Abstract: The Shikoku Basin hemipelagic sequence, which underlies the Nankai Trough wedge, S.W. Japan, thins by 50% between the outer edge of the trench wedge and DSDP Site 582, 14 km arcward. A sedimentation model, which incorporates changes in sedimentation rates with time and with distance from the trench wedge toe, indicates that 57% of the total thinning occurs as a result of temporally varying sedimentation rates and a time transgressive facies boundary between the trench wedge turbidites and the underlying hemipelagites. Burial-induced consolidation beneath the wedgeshaped turbiditic overburden, accounts for the remaining 43% of arcward thinning within the hemipelagic unit. Rapid dewatering, modeled as one-dimensional consolidation suggests that the excess pore water pressures are quite low during this progressive dewatering. Thus, high pore water pressures should not be assumed to occur universally in convergent margin settings. Normal faults and vein structures in the hemipelagites suggest near-horizontal extension in addition to vertical consolidation. The estimates of excess pore water pressures together with fault geometries and horizontal extensional strains, determined from the geometry of the subducting oceanic plate, can be used to constrain the stress conditions at failure. The expulsion of hot water from the rapidly dewatering sediments in the Nankai Trough may help to explain anomalously high heat flow in the central part of the trough.

Selenate Transport in Steady‐State, Water‐Saturated Soil Columns

Abstract: Transport of selenate (Se⁶⁺) in steady-state, water-saturated soil columns was evaluated under different pore water velocities, ionic compositions, and two soil bulk densities. The soil was Panoche loam (Typic Torriorthents) saturated with 0.005 M CaSO₄, 0.005 M CaCl₂, or 0.05 M CaCl₂. To examine the movement of selenate in soil relative to water and SO²⁻₄ (a biogeochemically similar anion), breakthrough curves of ⁷⁵SeO²⁻₄, along witH⁺³H₂O or ³⁵SO²⁻₄, were observed. Selenate appeared before elution of one pore volume and always ahead of the ³H₂O and SO²⁻₄ except in 0.05 M CaCl₂, indicating exclusion of selenate by the soil matrix. On the other hand, soil analysis of selenate following the leaching indicated that selenate was retained by the soil, suggesting simultaneous exclusion and adsorption of selenate. Increasing pore water velocity and soil bulk density did not shift the position of the breakthrough curves significantly. Assuming equilibrium linear adsorption-desorption of selenate by soil, the values of the apparent diffusion coefficient (D) and the retardation factor (R) were obtained by fitting the solution of the solute transport equation without sink and source terms to the measured breakthrough curves of selenate. The values of D showed an increasing trend with pore water velocity and the values of R were influenced by ionic composition of the soil solution. Contribution from the University of California, Davis. Funding was provided by the University of California Salinity/Drainage Task Force.