Showing papers on "Pore water pressure published in 1991"

Stability of Earth Slopes

Abstract: The failure of a mass of soil in a downward and outward movement of a slope is called a slide or slope failure. Slides occur in almost every conceivable manner, slowly or suddenly, and with or without any apparent provocation. They are usually caused by excavation, by undercutting the foot of an existing slope, by a gradual disintegration of the structure of the soil, by an increase of the pore water pressure in a few exceptionally permeable layers, or by a shock that liquefies the soil.

Effect of Reservoir Depletion And Pore Pressure Drawdown On In Situ Stress And Deformation In the Ekofisk Field, North Sea

Abstract: Knowledge of in situ stress and how stress changes with reservoir depletion and pore pressure drawdown is important in a multi-disciplinary approach to reservoir characterization, reservoir management, and enhanced oil recovery projects. Over 20 years of petroleum production from the Ekofisk field has resulted in a 21-24 MPa reduction in reservoir pore pressure. The effect of pore pressure drawdown on the minimum horizontal in situ stress in the Ekofisk field has been determined from shut-in pressure data of 32 hydraulic fractures. The effective stresses in the reservoir increase linearly with pore pressure drawdown, but at different rates. The ratio of the change in effective minimum horizontal stress to the change in effective vertical (overburden) stress is approximately 0.20. Laboratory experiments, which simulate the stress path followed by reservoir rock during the production history of the Ekofisk field, clearly indicate that shear failure has occurred during compaction of high porosity chalk as the shear stress increased with pore pressure drawdown. It is suggested that shear failure during primary production has increased fracture density and reduced matrix block dimensions, and has therefore maintained reservoir permeability, which may account for the continued good producibility of the Ekofisk field, in spite of compaction. 9more » refs., 5 figs.« less

Regional Fractures I: A Mechanism for the Formation of Regional Fractures at Depth in Flat-Lying Reservoirs (1)

Abstract: "Regional" fractures may form slowly, at depth, in extension, and parallel to regional horizontal tectonic compression. Such fracture sets can be common in reservoirs despite the absence of flexure of the strata, and are commonly important contributors to reservoir permeability. Such fractures make good targets for deviated wells. High formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs (conditions conducive to fracturing) despite deep burial, but pore pressure does not exceed the least principal stress and does not cause tensile fracturing: the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets. Rather, many regional fractures form duri g far-field compression, initiate at locally induced tensile stresses caused by flaws in the rock, and propagate in the plane of the maximum and intermediate compressive stresses. The differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure. In the presence of high pore pressure, fractures can be held open at depth by a tectonic, basinwide dilatancy of the strata, and the open void space commonly becomes mineralized.

Behaviour of offshore soils containing gas bubbles

Abstract: An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained onshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during con...

Petrography and Reservoir Physics II: Relating Thin Section Porosity to Capillary Pressure, the Association Between Pore Types and Throat Size

Abstract: Porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes. The size, shape, and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures. Each pore type tends to be associated with a limited range of throat sizes. The association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data. To do so, a set of samples is required wherein the association between pore type and throat size is fixed, but where pore type proportions vary between samples. This condition is met by a sample suite representing res rvoir facies from a single core or, in many cases, from a single field. The relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow. Parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type.

Experimental dissolution-precipitation creep in quartz aggregates at high temperatures

Abstract: Rapid compaction creep has been achieved in fine-grained quartz aggregates during hydrothermal isostatic pressing (HTIP) at 1200K, a confining pressure of 300MPa and a pore water pressure of 200 MPa. While raising the pressure and temperature over a period of three hours, the porosity decreased by over 50%. During subsequent HTIP for periods up to 4 hours, further porosity reduction occurred at compaction creep rates between 10−5 and 10−6 s−1. Changes in grain shapes, grainsize distribution, pore geometry and microstructures on grain surfaces, together with an absence of substantial brittle or intragranular plastic deformation, indicate that compaction creep has occurred largely by dissolution-precipitation processes. Interpenetration of grains and grain-shape truncation are well-developed at grain contacts. Arrays of fine-scale ridges, plateaus and flat-bottomed pores on interfaces indicate that the fluid distribution on actively dissolving interfaces is a dynamic structure that is not controlled entirely by equilibrium wetting angles.

Groundwater Flow and Solute Transport in Fractured Lacustrine Clay Near Mexico City

Abstract: A network of piezometers was installed in a surficial lacustrine clay aquitard overlying a thin saline water aquifer of volcanoclastic origin at a study site near Mexico City in the Basin of Mexico. The aquifer is underlain by additional lacustrine sediments which in turn overlie a thick regional freshwater aquifer. The regional aquifer provides approximately 70% of the water supply for 20 million people in the Basin of Mexico. In the study area, major ions, oxygen 18, and deuterium in the pore water of the surficial aquitard exhibit large variations with depth. The nature of these variations suggests that the saline pore water is being displaced downward by infiltrating meteoric water. The infiltration has been induced by strong downward hydraulic gradients imposed two to three decades ago when heavy aquifer pumping of the thin saline water aquifer began. One-dimensional analytical models representing solute transport in both fractured and unfractured porous media were used to simulate the geochemical profiles in the surficial aquitard. The fractured porous medium model, using a realistic mean hydraulic gradient and fracture spacing (1.5 m) and small but significant fracture aperture (30 μm) provide nearly an exact match to the field data. From this we infer that, because of vertical fractures, there is a much greater potential for downward leakage of water and contaminants through the Mexico City clay into underlying aquifers than has been previously thought.

Effects of saturation and back pressure on sand liquefaction

Abstract: The influence of saturation and back pressures on the liquefaction resistance of a sand is investigated by conducting cyclic triaxial tests on four sets of sand samples with different degrees of saturation and on five sets of samples with different back pressures. The tested sand is taken from the site of Tong Jaizhi Water Power Station in China. Test results show that the conspicuous effect of saturation on the liquefaction resistance of the tested sand cannot be improved by keeping back pressures on samples during the test. On the other hand, the application of back pressures otherwise results in higher liquefaction resistance. The effect of back pressures is further explained by analyzing the internal interaction of soil particles. It is concluded that the back pressure method should not be used in cyclic liquefaction tests for improving the degrees of saturation of sand, and Terzaghi’s effective stress principle is not valid unless additional conditions are included for the actions of interparticle forces in soils.

Cyclic response of calcareous soil from the North-West Shelf of Australia

Abstract: This Paper reports the results from 23 cyclic CIU and CAU triaxial tests on undisturbed samples of calcareous soil. The samples, which include both uncemented and cemented material, have heen subjected to a range of mean and cyclic shear stresses. Results from the tests on the uncemented soil display many of the characteristics of the cyclic response of other cohesionless soils. The more cemented soils sustained higher cyclic stresses and, depending on the load level, showed either elastic behaviour with no pore pressure build-up, eventual stabilization after some pore pressure build-up, or steady state pore pressure build-up and eventual failure. The effect of frequency on the response in the range 0·1 to 10 Hz has heen studied. Since no dramatic differences in performance could he detected, no firm conclusions could be drawn regarding the effect of testing rate. L'article preente les reultats obtenus a partir de 23 essais triaxiaux cycliques de type CIU et CAU effectues sur des echantillons de sol calca...

Analysis of subsidence due to a point sink in an anisotropic porous elastic half space

Abstract: Analytical solutions of the long-term consolidation deformation and excess pore water pressure due to fluid withdrawal from a saturated anisotropic porous elastic half space are presented. In the analysis, both the permeability and the elastic properties are considered cross-anisotropic. Cases of pervious and impervious half-space boundary are studied. The consolidation settlements as effected by the anisotropy and boundary conditions for pore water are illustrated and discussed.

A vacuum-operated pore-water extractor for estuarine and freshwater sediments

Abstract: A vacuum-operated pore-water extractor for estuarine and freshwater sediments was developed and constructed from a fused-glass air stone attached with aquarium airline tubing to a 30 or 60 cc polypropylene syringe. Pore water is extracted by inserting the air stone into the sediment and creating a vacuum by retracting and bracing the syringe plunger. A hand-operated vacuum pump attached to a filtration flask was also evaluated as an alternative vacuum source. The volume and time to extract pore water varies with the number of devices and the sediment particle size. Extraction time is longer for fine sediments than for sandy sediments. Four liters of sediment generally yield between 500 and 1,500 mL of pore water. The sediment that surrounds and accumulates on the air stone acts as a filter, and, except for the first few milliliters, the collected pore water is clear. Because there is no exposure to air or avenue for escape, volatile compounds andin situ characteristics are retained in the extracted pore water.

Geochemistry of uranium in Black Sea sediments

Abstract: The sediment geochemistry of uranium has been studied at four sites in the Black Sea. Samples collected by an in situ pore water sampler show that U is depleted in the upper few centimeters of the sediment column, decreasing from the overlying water value to <0.5 dpm 238U l−1. Removal of U from sediment pore water supports a diffusional flux of U across the sediment-water interface, and these fluxes range from 15 to 52 dpm cm−2 ky−1. Authigenic solid phase U concentrations are found to increase with depth at the central basin stations, reaching activities of 10–14 dpm g−1. An outer shelf site, characterized by a high sediment accumulation rate, shows no apparent U accumulation in the solid phase. Fluxes calculated based on solid phase authigenic U at depth in the sediment and on sedimentation rates derived from 210Pb data range from 47 to 149 dpm cm−2 ky−1. Differences between the fluxes derived from pore water and solid phase data reflect different accumulation processes, sampling constraints and uncertainties associated with each method/calculation. These fluxes are comparable to those reported for other anoxic basin sediments which, together, account for about 20% of the oceanic U balance.

Solute rejection in freezing soils

Abstract: Dissolved compounds in the water phase are rejected during freezing of soils. This phenomenon occurs due to the inability of ice crystals to incorporate most alien molecules. The ice grows only by association with water molecules, and molecules of solutes are rejected into the unfrozen water. A mathematical model for salt rejection in freezing saline soils has been developed by employing the mass balance equations for water and solute in a saturated porous medium, and the conservation of energy equation. Additional relations introduced include a moisture retention curve as a function of ice-water capillary pressure, the Clapeyron equation and expressions for the heat capacity and the effective thermal conductivity of the soil. The resulting set of seven equations in terms of solute concentration, soil temperature, pore water, ice, and capillary pressures, and degrees of water and ice saturation have been solved simultaneously using a Newton-Raphson linearization, with implicit iterative treatment of nonlinearities. Results indicate the development of unfrozen high salt concentration regions trapped in a frozen zone at sufficiently high freezing rates. The numerical results are compared with experimental data obtained by some researchers. Favorable match is obtained between the theoretical and experimental results. Finally, extension of the model to situations where ice lensing and soil deformations are significant is discussed.

Design and Performance of the Imperial College Instrumented Pile

Abstract: This paper describes the design, calibration, and field performance of a number of new instruments that have been used for research into the behavior of displacement piles in clays and sand. The instruments include axial load cells, surface stress transducers, and pore pressure probes. The special features of the surface stress transducers and pore pressure probes are described in detail, and problems associated with measuring pore pressures in strongly dilating soils are discussed. Data recorded at a stiff clay site are used to demonstrate the reliability of measurements obtained with the new instruments.

Control of pore-water chemistry at the base of the Florida escarpment by processes within the platform

Abstract: PORE waters collected from seep sediments hosting active chemosynthetic communities1,2 tend to be rich in sulphide, chloride and ammonium and depleted in sulphate relative to the concentrations in sea water. To investigate the source of the energy-rich compounds and the processes causing low sulphate concentrations in seep-sediment pore waters, we have measured the sulphur isotope composition, δ34S, of pore waters from seep sediments at the base of the West Florida escarpment. The isotopic composition of pore-water sulphate remains approximately constant as its concentration is depleted, indicating that processes within the Florida platform, rather than microbial processes at seep sites, control pore-water chemistry in these sediments. The composition of the seep brines, deduced from a linear mixing model, provides information on processes deep within the platform.

Wave-Induced Pore Pressure in Rubble Mound Breakwaters

Abstract: Experimental results from large scale tests are presented for the distribution of wave-induced pore pressure within rubble mound breakwaters, for the reflection coefficients and the damping of wave motion through the armour and under-layer. The results are then discussed with respect to application and further research.

The Effect of Quadratic Gradient Terms on the Borehole Solution in Poroelastic Media

Abstract: When compressible fluid continuity in a fluid-saturated compressible porous medium under transient conditions is considered, we cannot obtain a standard linear diffusion equation in terms of pressure unless we ignore the quadratic terms in the pressure gradient expression, for example [∂p/∂r]2 for cylindrical plane strain coordinates. They are assumed to be so small that their contribution can be ignored in pressure analysis. Thereby, a nonlinear equation can be avoided. During hydraulic fracturing, rapid drawdown, or slug testing, the pressure difference can reach a high value “instantly.” An extremely steep pressure gradient is generated, and it may not be appropriate to neglect quadratic terms. In this paper, an analytical solution for pore pressure coupling with the deformation in a porous medium is developed by taking the quadratic term into account. By Laplace transformation, we obtain a solution for a nonlinear diffusion equation by setting up a fluid continuity equation according to the mass conservation law rather than from energy principles in terms of volume. Deviations from existing solutions are identified in cases of high pressure gradients, and these deviations are related to the compressibility of the pore and injected fluids. It would seem that the new solution gives a more correct early time response in these cases. To calculate the effective stresses and pore pressure, we need to carefully define hydraulic diflfusivity. We have related the coefficients of Biot and Geertsma to those of mass conservation, which are commonly used in hydrology.

Pore Pressure Diffusion and the Hydrologic Response of Nearly Saturated, Thin Landslide Deposits to Rainfall

Abstract: Previous workers have correlated slope failures during rainstorms with rainfall intensity, rainfall duration, and seasonal antecedent rainfall. This note shows how such relationships can be interpreted using a periodic steady-state solution to the well-known linear pressure diffusion equation. Normalization of the governing equation yields a characteristic response time that is a function of soil thickness, saturated hydraulic conductivity, and pre-storm effective porosity, and which is analogous to the travel time of a piston wetting front. The effects of storm frequency and magnitude are also successfully quantified using dimensionless attenuation factors and lag times.