Showing papers on "Pore water pressure published in 1985"

Pore Pressure Diffusion and the Mechanism of Reservoir-Induced Seismicity

Abstract: The study of reservoir-induced seismicity offers a controlled setting to understand the physics of the earthquake process. Data from detailed investigations at reservoirs in South Carolina suggested that the mechanism of transmission of stress to hypocentral locations is by a process of diffusion of pore pressure (Pp). These results were compared with available worldwide data. The ‘seismic’ hydraulic diffusivity, α s , was estimated from various seismological observations, and was found to be a good estimate of the material hydraulic diffusivity, α. Application of these results to a dedicated experiment to understand RIS at Monticello Reservoir, S.C., suggested that the diffusing Pp front plays a dual role in the triggering of seismicity. The spatial and temporal pattern of RIS can be explained by the mechanical effect of diffusion of Pp with a characteristic hydraulic diffusivity within an order of magnitude of 5 × 104 cm2/s, corresponding to permeability values in the mtl¨¹darcy range. The triggering of seismicity is due to the combined mechanical effect of Pp in reducing the strength and, possibly, the chemical effect in reducing the coefficient of friction between the clays in the pre-existing fractures and the rocks that enclose these fractures.

Wave-induced stresses in unsaturated submarine sediments

Abstract: A simple method to solve the problem of pore pres-sure and effective stresses induced by harmonic waves in a poroelastic sediment is presented by using Biot's equation and a compatibility condition for elasticity. The sediment is assumed to obey the effective stress principle in the normal definition for saturated sedi-ments, although it may contain gas bubbles occluded in the porewater. The governing equation is a fourth-order partial differential equation in terms of pore pressure or effective mean normal stress. The solutions contain Skempton's pore pressure coefficient B. For unsaturated sediments, with B < 1, the pore pressure and effective stresses induced by wave action consist of two parts. One part depends only on the wavelength and the other part depends on both the sediment and the wave characteristics. The second part is out of phase with the wave. When the submarine sediment is unsaturated, the wave-induced negative effective nor- mal stresses become greater under the trough of waves. A zone ...

Organic carbon dynamics and preservation in deep-sea sediments

Abstract: Three methods of estimating the particulate organic carbon fluxes to the sediment-water interface of the deep Pacific Ocean agree to within the error of the measurements at MANOP sites M, H, and C. Sediment trap experiments, pore water results, and surface sediment organic carbon data suggest that a major fraction of the particulate organic carbon raining to abyssal depths at these locations is degraded within the surface sediments rather than at the sediment-water interface or in the nephloid layer. Organic carbon rain rates at the three sites are similar—within a factor of two; however, the preservation rate of organic carbon and the chemistry of sediment pore waters are very different. A model developed to describe the pore water oxygen and sedimentary carbon distributions indicates model developed to describe the pore water oxygen and sedimentary carbon distributions indicates that changes in the rate constant for organic matter degradation and the bioturbation rate may contribute significantly to the observed differences in character of both pore water and sediment chemistry at these locations. The implication with respect to interpreting the sedimentary record is that cycles of organic carbon and redox sensitive metals (i.e., manganese) are not simply related to particulate organic carbon flux or surface water primary productivity. The residence time of organic carbon with respect to degradation in the surface sediments is on the order of 15 to 150 y.

The influence of pore pressure and confining pressure on dynamic elastic properties of Berea Sandstone

Abstract: Compressional- and shear-wave velocities of water-saturated Berea sandstone have been measured as functions of confining and pore pressures to 2 kbar. The velocities, measured by the pulse transmission technique, were obtained at selected pressures for the purpose of evaluating the relative importance of confining pressure and pore pressure on elastic wave velocities and derived dynamic elastic constants. Changes in Berea sandstone velocities resulting from changes in confining pressure are not exactly canceled by equivalent changes in pore pressure. For properties that involve significant bulk compression (compressional-wave velocities and bulk modulus) an incremental change in pore pressure does not entirely cancel a similar change in confining pressure. On the other hand, it is shown that a pore pressure increment more than cancels an equivalent change in confining pressure for properties that depend significantly on rigidity (shear-wave velocity and Poisson's ratio). This behavior (as well as observed wave amplitudes) is related to the presence of high-compressibility clay that lines grains and pores within the quartz framework of the Berea sandstone.

Consolidation around a point heat source

Abstract: When a heat source such as a canister of radioactive waste is buried in a saturated soil the temperature changes that occur will cause the pore water to expand a greater amount than the voids of the soil. The temperature change will thus usually be accompanied by an increase in pore pressure. If the soil is sufficiently permeable these pore pressures will dissipate. This paper develops an analytic solution for the fundamental problem of a point heat source buried deep in a saturated soil.

Settlement analysis of embankments on soft clays

Abstract: Highly realistic analysis of the consolidation of soft clays is possible within the framework of the traditional concepts of soil mechanics. A practical method is described for predicting the magnitude and rate of settlement and pore water pressure dissipation. Detailed interpretation and evaluation of the consolidation properties of soft clays, including the compressibility parameters, preconsolidation pressure and coefficient of permeability are presented. These properties can be determined from the standard oedometer test with direct permeability measurements. Computer procedure ILLICON, using multi-layer analysis, includes any variation of consolidation parameters with depth. The proposed method for evaluating the consolidation parameters, and settlement and pore water pressure analyses is illustrated using case histories of two test fills on soft clays. Excellent agreement is obtained between the computed behavior and field observations.

Stress and subsidence resulting from subsurface fluid withdrawal in the epicentral region of the 1983 Coalinga Earthquake

Abstract: The proximity of the May 2, 1983, Coalinga earthquake to active oil fields on Anticline Ridge led to speculation that the earthquake might have been triggered by oil field operations. Elsewhere, earthquakes have been associated with pore pressure increases resulting from fluid injection and also with subsidence resulting from fluid extraction. Simple calculations show that shales, which underlie the oil producing strata, hydraulically isolate the oil field from the earthquake focal region. The large volumes of fluid extracted from the oil fields caused a 50% decline in reservoir pressures from 1938 to 1983. These observations independently rule out substantial increases in pore pressure at focal depths due to fluid injection. A theoretical method, based on Biot's constitutive theory for fluid-infiltrated elastic media, is used to evaluate the change in stresses acting in the focal region resulting from fluid extraction in the overlying oil fields. As an independent check on the method, the subsidence of the earth's surface in response to fluid withdrawal is calculated and compared with measured elevation changes of Anticline Ridge. The producing horizons are taken to be horizontal permeable layers, bounded above and below by impermeable horizons. Strains within the producing layers are related to extraction-induced changes in pore fluid mass. Contraction of the producing layers causes the free surface to subside and strains the elastic surroundings. The calculated subsidence rate of Anticline Ridge between 1933 and 1972 is 3 mm/yr, in good agreement with the measured subsidence rate of 3.3±0.7 mm/yr. Calculated pore pressure changes in the deepest producing zone also compare well with observed changes in reservoir pressure. Although the shear stresses induced by extraction favor reverse slip on either the northeast or southwest dipping nodal plane, the induced normal stresses are compressive, inhibiting fault slip. The driving stress (shear stress minus frictional resistance) acting across the northeast dipping plane increased by 0.01 MPa (0.1 bar) between 4 and 9 km depth, weakly favoring slip, and decreased by half that amount at depths of 9 to 11 km, weakly inhibiting slip. The driving stress on the southwest dipping plane increased by 0.02 MPa (0.2 bar) at 10 km, slightly favoring slip. The sign and magnitude of the pore pressure and stress changes at hypocentral depths do not support the hypothesis that the earthquake was induced, although knowledge of the rate of tectonic stress accumulation is required to assess properly the significance of the extraction-induced stresses.

Early chemical diagenesis, sediment-water solute exchange, and storage of reactive organic matter near the mouth of the Changjiang, East China Sea

Abstract: A substantial proportion of the material delivered to the modern oceans is supplied by a few large rivers such as the Changjiang. Early diagenetic reactions in surficial bottom sediments determine in large part both the eventual influence of these rivers on the sea and the nature of sedimentary deposits formed. The region off the mouth of the Changjiang exemplifies the interplay between physical, chemical, and biological factors which can produce particular spatial patterns of diagenesis and sediment-water exchange. To examine these patterns measurement of pore water solute profiles, sediment-water solute fluxes, and solute reaction rates in the upper few decimeters of sediment were made at 27 stations near the Changjiang in the East China Sea. Direct measurements of dissolved Si(OH)4, NH4+, and NO3−, fluxes from or into bottom sediments made during summer and autumn periods (15 to 24°C) range from 0.13 to 13.2, −2.6 to 3.4, and −1.4 to 3.2mmol m−2 day−1, respectively. Net solute flux from the sea floor is often lowest from deposits having the highest interstitial solute concentrations. In addition, bottom regions having the highest build up of reaction products or depletion of reactants in pore waters (with respect to overlying water) actually have the lowest rates of reaction. These same areas of elevated (products) or depleted (reactants) pore water solute concentrations, low reaction rates, and low net rate of solute exchange which are located near the mouth of the Changjiang are sites of high sedimentation rates and depauperate benthic communities. High water turbidity and resuspension apparently hinder water column production and input of reactive organic matter or other biogenic material which drive many diagenetic reactions. Rapid sedimentation or disturbance hinders benthic community development, lowers biogenic reworking, and allows build up or depletion of reaction products or reactants in bottom sediments. Offshore areas of lower sedimentation, higher productivity, and active bottom communities are sites of high initial reaction rates and increased sediment-water solute exchange compared with rapid sedimentation regions. A diagenetic paradox resulting from the interaction between benthic communities and the physical environment of sedimentation is that proportionally the greatest storage of diagenetic products related to organic matter decomposition can occur in sediments that are initially the least diagenetically reactive.

Chemistry of calcium carbonate-rich shallow water sediments in the Bahamas

Abstract: The geochemistry of calcium carbonate-rich sediments from a variety of environments throughout the Bahamas was investigated with particular emphasis on the factors that control the pore water chemistry. Most sediments are supersaturated with respect to aragonite, the most abundant carbonate component. Experimental studies indicate that the observed in situ calcium carbonate ion activity products can often be produced as reversible metastable equilibria between the sediments and seawater. This is interpreted as being the result of interactions between the solutions and the minor high Mg-calcite component present in these sediments. Although the overlying waters are more supersaturated than the pore waters, carbonate dissolution, not precipitation, dominates in these sediments as a result of organic matter oxidation and the resulting increase in P/sub CO/sub 2//. The carbonate sediments of the Bahamas are remarkable for their purity, with the exception of special environments such as mangrove swamps and tidal flats with algal mats. Organic matter and heavy metal content is extremely low. Only minor sulfate reduction is occurring in most sediments. Phosphate is undetectable in all pore waters, probably as a result of adsorption on carbonate mineral surfaces. Other dissolved pore water components such as ammonia and DOC are much lower than typicallymore » found in shallow water fine-grained terrigeneous sediments.« less

Relationships of moisture stress and effective stress functions in unsaturated soils

Abstract: The relationships of moisture stresses and consequent effective stress components in unsaturated soils are examined on the basis of a capillary model. The validity of such a model for this purpose is demonstrated. Effective stresses due to suctions in the pore water are shown to be dependent jointly on the pressure deficiency in the pore water and the degree of pore space saturation. An approximate method of computation of such effective stresses is outlined. A simple series of relationships is proposed between a decrease of applied stress, and a consequent increase of pressure deficiency in the pore water, with a resultant change in effective stress.

The effect of tidally induced changes in the creekbank water table on pore water chemistry

Abstract: Water moved into the creekbank sediments in direct response to the changing levels of the water table caused by the tides. The net water loss of the sediments was 3–30% on each low tide and this loss was confined to within 4 m (horizontal) of the creek. The replacement of this water by incoming tidal water could not supply sufficient nutrients for the growth of creekbank Spartina. However, during ebb tide there was a replacement of water in the creekbanks with nutrient-rich water from the marsh interior as demonstrated by the large changes in pore water chemistry over a tidal cycle. The concentration and the range of a chemical parameter depended upon the stage of the tide, the tidal range, the time of year and (for salinity) the rainfall patterns of the month preceding sampling. Over a single tidal cycle the maximum ranges were: salinity ‰, 26–33; alkalinity, 2·5–13·6 med 1−1, ammonia, 2–400 μm, sulfate, 23·5–29 mmol 1−1. Measurable concentrations of sulfide were only found in a few samples. This high nutrient water can supply nitrogen and probably other nutrients to Spartina.

A shock‐tube technique for studying pore‐pressure propagation in a dry and water‐saturated porous medium

Abstract: Wave propagation in a porous column consisting of sand particles is studied by means of a shock‐tube technique. Experimental results are presented for both air and water as pore fluids. Quantitative information on pore‐pressure amplitudes, wave speeds, and damping is compared with theoretical predictions. Good agreement is obtained for the air‐filled pores. For the water‐saturated pores the two‐wave structure is observed, and the corresponding pressure‐amplitude ratio is consistent with the observed first‐wave speed. The transient permeability is approximately one‐third the steady‐state value. Added mass is significant.

Development and application of a combined soil water - slope stability model

Abstract: Summary A general one-dimensional soil water infiltration scheme is coupled to an infinite slope stability analysis model to illustrate the potential of exploring the impact of soil suction and parameter variability in stability analysis. The primary data base used for this investigation is published data for the Mid-Levels area of Hong Kong. The model is used to show that current procedures in the estimation of c′ and φ′ derived from strength tests do not perhaps accurately reflect the strength of the residual soils examined. More generally, the desirability of two-dimensional modelling in the context of both soil suction/pore pressure models and stability analysis is discussed, but the parameterization and validation problems of such a scheme are considered to be immense.

The impact of wave loads and pore-water pressure generation on initiation of sediment transport

Abstract: The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported.

High pore pressure generation in sediments in front of

Abstract: DSDP data for the Barbados Ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches. We model the evolution of pore pressure by using a finite element pro- cedure. Three consecutive stages are recognized: (1) In the oceanic basin, sedimentation rate is low; there is little excess pore pressure. (2) Approaching the trench but before being incorporated into the accretionary prism, sediments are sub- jected to increasing tectonic compression as well as greater sediment overburden. Thus, high pore pressure can be created. An important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone. (3) Arcward of the trench, sediments above the decollement are offscraped while those beneath it are sub- ducted. Fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth. This facilitates the forma- tion of mud volcanoes, even oceanward of the deformation front.

High pore pressure generation in sediments in front of the Barbados Ridge Complex

Abstract: DSDP data for the Barbados Ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches. We model the evolution of pore pressure by using a finite element procedure. Three consecutive stages are recognized: (1) In the oceanic basin, sedimentation rate is low; there is little excess pore pressure. (2) Approaching the trench but before being incorporated into the accretionary prism, sediments are subjected to increasing tectonic compression as well as greater sediment overburden. Thus, high pore pressure can be created. An important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone. (3) Arcward of the trench, sediments above the decollement are offscraped while those beneath it are subducted. Fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth. This facilitates the formation of mud volcanoes, even oceanward of the deformation front.

Bottomhole Stress Factors Affecting Drilling Rate at Depth

Abstract: The mean formation stress near the bottom of a borehole is reduced by strain relaxation when a well is drilled. This causes a PV increase that can significantly reduce the local pore pressure of impermeable rocks, such as shales, but does not affect the pore pressure of permeable rocks. Since the penetration rate is strongly affected by the difference between the local pore pressure and the borehole fluid pressure, impermeable formations drill slower than adjacent permeable formations.

The causes and effects of aging in quick clays

Abstract: The physicochemical properties of a clay from La Baie, Quebec were found to be typical of Champlain quick clays: very low plasticity, liquidity index greater than 3, sensitivity greater than 500, and pH of about 9.5. The mineralogy is characterized by the abundance of primary or "rock flour" minerals such as feldspar, quartz, amphibole, and calcite, with illite as the principal clay mineral. The pore water chemistry consists predominantly of sodium bicarbonate. Low concentrations of sulfate, calcium, and magnesium and high alkalinity and pH are attributed, in part, to anaerobic sulfate-reduction processes taking place in situ.Quick clay samples stored in the laboratory showed signs of aging regardless of the storage procedure. The remoulded strength and the liquid limit increased with time, whereas the sensitivity, the liquidity index, and the pH decreased. The water content, plastic limit, and undisturbed strength remained practically unchanged. The pore water concentrations of calcium, magnesium, and su...

Measurements of wave‐induced pore pressure in submarine sediments under various marine conditions

Abstract: Wave‐induced pore pressures were measured at four sites around Shimizu Harbor, Central Japan. At Site 1 the measurements of the wave‐induced seafloor pressure and pore pressure at a depth of 0.5 m below the mud line were conducted in about 14 m of water. The sediment was silty sand with a permeability of 4 × 10−5 m/s. The pressures induced by wind waves with about a 10‐s period were transferred through the sediment to the pore pressure with damping of about 4% through 9% and time lag of 0.1 rad (0.2 s) through 0.16 rad (0.3 s). The wave‐induced seafloor pressure and pore pressure changed differently in the breaker sediment. At Site 3 absolute and differential piezometers were installed in sand and gravel sediment at depths of 0.7 m and 0.5 m, respectively, in about 1.7 m of water. The pressure attenuation was about 1% for the tide without time lag. The piezometers used at Sites 1 through 3 were a transformer type. At Site 4 a new probe of semiconductor type was tested in sand and gravel sediment ...

Evaluation of Wave-Induced Pore Pressure in Sand Layer by Wave Steepness

Abstract: The attenuation of wave-induced pore water pressures inside sand beds is investigated experimentally in a wave tank. It is found that the pore water pressure depends on the wave steepness. ...

Effects of pore pressure and mud filtration on drilling rates in a permeable sandstone

Abstract: During laboratory drilling tests in a permeable sandstone, the effects of pore pressure and mud filtration on penetration rates were measured. Four water-based muds were used to drill four saturated sandstone samples. The drilling tests were conducted at constant borehole pressure while different backpressures were maintained on the filtrate flowing from the bottom of the sandstone samples. Bit weight was varied also. Filtration rates were measured while circulating mud during drilling and with the bit off bottom. Penetration rates were found to be related qualitatively to the difference between the filtration rates measured while drilling and circulating. There was no observed correlation between standard API filtration measurements and penetration rate.