Showing papers on "Pore water pressure published in 1986"

Compressibility of porous rocks

Abstract: Four compressibilities are defined for porous rock, relating changes in the bulk and pore volumes to changes in the pore and confining pressures. Using a micromechanical theory based on classical linear elasticity, three relations are found between these compressibilities. Two of these relations are verified experimentally for Berea and Bandera sandstone. Bounds are derived for these compressibilities, involving only the porosity and the elastic moduli of the rock-forming minerals. For the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures. This dependence is verified for Berea, Bandera, and Boise sandstone. While the strains cannot be expressed as functions of an “effective stress,” regardless of how it is defined, the (elastic) volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure.

Ideality of clay membranes in osmotic processes; a review

Abstract: Clays can act as osmotic membranes and thus give rise to osmotically induced hydrostatic pressures. The magnitude of generated osmotic pressures in geologic systems is governed by the theoretical osmotic pressure calculated solely from solution properties and by value of the membrane's three phe- nomenological coefficients: the hydraulic permeability coefficient, Lv; the reflection coefficient, tr; and the solute permeability coefficient, 60. Generally, low values of Lp correspond to highly compacted membranes in which ~ is near unity and r approaches zero. Such membrane systems should give rise to initially high osmotic fluxes and gradual dissipation of their osmotic potentials. The high fluid pressures in the Dunbarton Triassic basin, South Carolina, are a good example of osmotically induced potentials. A unique osmotic cell is created by the juxtaposition of fresh water in the overlying Cretaceous sediments against the saline pore water housed within the membrane-functioning sediments of the Triassic basin. Because wells penetrating the saline core of the basin show anomalously high heads relative to wells penetrating the basin margins, the longevity of this osmotic cell is probably dictated by the rate at which salt diffuses out into the overlying fresh water aquifer.

Pore pressure generation in sedimentary basins: Overloading versus aquathermal

Abstract: The principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade. Among the various mechanisms proposed, mechanical overloading and aquathermal pressuring have probably received the greatest attention. Most previous discussions, however, have been based upon qualitative conceptual models. Here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and, second, by comparing the relative importance of the overloading versus the aquathermal mechanisms. The coupling between pore pressure and compaction as well as the nonlinear and path-dependent material properties are found to be critical in the present analysis. We find that although changes in stress and temperature both contribute to pressure generation, mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles.

Fluid pressure response to undrained compression in saturated sedimentary rock

Abstract: The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, B = (dPf/dPclld,"f~O' where Pf is fluid pressure, P, is confining pressure, and mf is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite are near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high-compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure results. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure characterized by low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer-reservoir systems at very low effective stress states, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.

Acoustic and resistivity measurements on rock samples containing tetrahydrofuran hydrates: Laboratory analogues to natural gas hydrate deposits

Abstract: In this paper we report laboratory acoustic velocity and electrical resistivity measurements on Berea Sandstone and Austin Chalk samples saturated with a stoichiometric mixture of tetrahydrofuran (THF) and water. THF and water is an excellent experimental analogue to natural gas hydrates because THF solutions form hydrates similar to natural gas hydrates readily at atmospheric pressures. Hydrate formation in both the chalk and sandstone samples increased the acoustic P wave velocities by more than 80% when hydrates formed in the pore spaces; however, the velocities soon plateaued and further lowering the temperature did not appreciably increase the velocity. In contrast, the electrical resistivity increased nearly 2 orders of magnitude upon hydrate formation and continued to increase slowly as the temperature was decreased. In all cases resistivities were nearly frequency independent to 30 kHz, and the loss tangents were high, always greater than 5. The dielectric loss showed a linear decrease with frequency suggesting that ionic conduction through a brine phase dominates at all frequencies, even when the pores are nearly filled with hydrates. We find that resistivities were strongly a function of the dissolved salt content of the pore water. Pore water salinity also influenced the sonic velocity, but this effect is much smaller and only important near the hydrate formation temperature.

Flux of dissolved organic carbon and pore water through the substrate of a Spartina alterniflora marsh in North Carolina

Abstract: The export of dissolved organic carbon (DOC) from a sandy substrate Spartina alterniflora marsh in North Carolina was studied from August 1982 to July 1983. DOC of the pore water was determined using a total organic carbon analyzer and was reliably estimated by measuring the absorbance of the pore water. DOC ranged between 1 and 6 mg l −1 and was highest during summer and lowest in winter. Based on hydraulic conductivity calculations, over 34 000 1 year −1 of pore water was exported through the 1×25 m transect perpendicular to a tidal creek. Most of this flux (over 90%) occurred within 2 m of the creek bank. Based on this pore water flux, belowground DOC export was only 1·52 g Cm −2 year −1 .

Thermal expansion and pore pressure generation in oil sands

Abstract: The prediction of stress changes and deformations arising from ground heating requires the coupled solution of the heat transfer and consolidation equations. Heat consolidation as a class of proble...

Direct indication of pore-water advection from pore pressure measurements in Madeira Abyssal Plain sediments

Abstract: The difference in pressure between the sea floor and a point in the underlying sediment is an indicator of the velocity of pore-water flow. This differential pore pressure has now been measured at five sites using a free-fall piezometer (pop-up pore pressure instrument, PUPPI) in a distal turbidite province of the Madeira Abyssal Plain. We found that at one of the five sites the pressure was slightly positive, at two others it was effectively zero, but at the remaining two sites the pore pressure was significantly negative (−450 and −120 Pa at 4 m depth). The implication is that at these last two sites water is moving downwards at rates of 3.1 and 0.8 mm yr−1 (calculated using laboratory-measured hydraulic conductivities from core samples). If this downward movement is part of a hydrogeological system which includes upward movement of pore water elsewhere, there are obvious implications for schemes to dispose of highly radioactive waste by burial in deep-sea sediments.

Coral Reef Hydrology: Field Studies of Water Movement within a Barrier Reef

Abstract: Water movement through the framework of Davies Reef, a coral reef in the central Australian Great Barrier Reef, was studied using field and laboratory determinations of permeability, tide gauge measurements of water levels, dye tracers, and pore water chemistry. Flow is driven by current, wind-induced, or tide-induced water level differences which were shown to occur between reef front and lagoon. The reef is hydraulically very heterogeneous with bulk flow occurring through high permeability zones (voids and rubble) at a velocity on the order of 10 m/d. Pore water exchange in less permeable zones occurs at a much slower rate. Vertical components of flow are significant. Chemical data indicate that carbonate precipitation and solution occur so that porosities, permeabilities, and flow paths may change with time. Implications for nutrient transfer through the benthic sediments and for fresh water resources on reef islands are discussed.

Undrained deep penetration, II: pore pressures

Abstract: Undrained deep penetration causes significant excess pore pressures that dominate other soil stresses and hold the key to the understanding of penetration mechanisms. On the basis of the strain pat...

Influence of a Critical Gradient on the Consolidation of Clays

Abstract: Several studies in the laboratory have shown the existence of a permeability law which is different from Darcy's law and which defines the flow of the pore water in certain natural clays. Consolidation tests in the laboratory, including the measurement of permeability at different stages of loading, that were carried out on Saint Herblain clay at the E.N.S.M. Soil Mechanics Laboratory prove the existence of major critical gradients (12 < i 0 < 20). According to this observation, a consolidation model worked out with a non-Darcian permeability law has been produced by means of a finite-difference computer program. On the one hand, this model is compared with Terzaghi's consolidation model and, on the other hand, the law that has been established is tested next to the laboratory tests.

Method for determining pore pressure and horizontal effective stress from overburden and effective vertical stresses

Abstract: The porosity-effective stress relationship, which is a fuction of lithology, is used to calculate total overburden stress, vertical effective stress, horizontal effective stress and pore pressure using well log data. The log data can be either real time data derived from measurement-while-drilling equipment or open hole wireline logging equipment.

The Transition from Sediment Suspension to Settling Bed

Abstract: The dynamic behavior of sediment in water is very complicated. At low concentrations, the behavior is basically that of a fluid, becoming increasingly non- Newtonian with increasing concentration, until the sediment particles or floes are able to establish an interacting framework. As the concentration or density increases further, the characteristics of the system are controlled more and more by the characteristics of the soil matrix, with increasing strength and erosion resistance as a bed forms and consolidates. This paper examines the transition from fluid supported sediment where the total stress and water pressure are equal, to an interacting framework, where the water pressure is less than the total vertical stress. It is shown that this transition point is not associated with a unique concentration or density, but is a function of the history of the system (for example, the rate of sedimentation or the initial concentration) and varies also with time. There is therefore not a simple criterion to determine when a suspension behavior begins to change to that of a settling bed. Rather, an understanding of the basic physical processes is required.

Changes in complex resistivity during creep in granite

Abstract: A sample of Westerly granite was deformed under constant stress conditions: a pore pressure of 5 MPa, a confining pressure of 10 MPa, and an axial load of 170 MPa. Pore volume changes were determined by measuring the volume of pore fluid (0.01M KClaq) injected into the sample. After 6 days of creep, characterized by accelerating volumetric stain, the sample failed along a macroscopic fault. Measurements of complex resistivity over the frequency range 0.001–300 Hz, taken at various times during creep, showed a gradual increase in both conductivity and permittivity. When analysed in terms of standard induced polarization (IP) techniques, the changing complex resistivity resulted in systematic changes in such parameters as percent frequency effect and chargeability. These results suggest that it may be possible to monitor the development of dilatancy in the source region of an impending earthquake through standard IP techniques.

Pore-Water Chemistry of Carbonate-Rich Sediments, Lord Howe Rise, Southwest Pacific Ocean

Abstract: About 200 pore-water samples were recovered and analyzed from eight sites of DSDP Leg 90. At all sites on the carbonate-rich Lord Howe Rise, Ca + concentrations increase and Nig"" concentrations decrease with increasing sub-bottom depth. The value of ΔCa/ΔMg averages -0.45 mol/mol at these sites, an unusually small negative value in comparison with sites on basaltic crust. This supports the argument that the crust of the Lord Howe Rise is siliceous. Li concentrations increase and K concentrations decrease with depth. Both of these constituents are affected by reactions within the sediment column. Microbial sulfate reduction occurs to a small extent at all sites. In the upper 200 m of Site 594 on the south side of the Chatham Rise, sulfate reduction and alkalinity production are more pronounced. Carbonate recrystallization is indicated by large increases in Sr + concentrations with depth at all sites. The exact value of the Sr maximum at each site is determined by equilibrium with respect to celestite (SrSO4). The greater the degree of microbial sulfate reduction, the higher is the pore water Sr concentration. The diffusive flux of Sr out of the pore waters is consistent with an initial rate of carbonate recrystallization of 10%/m.y. This recrystallization must alter the isotopic and trace elemental compositions of some carbonate components.