Showing papers on "Pore water pressure published in 1990"

Micromechanics of pressure-induced grain crushing in porous rocks

Abstract: The hydrostatic compaction behavior of a suite of porous sandstones was investigated at confining pressures up to 600 MPa and constant pore pressures ranging up to 50 MPa. These five sandstones (Boise, Kayenta, St. Peter, Berea, and Weber) were selected because of their wide range of porosity (5–35%) and grain size (60–460 μm). We tested the law of effective stress for the porosity change as a function of pressure. Except for Weber sandstone (which has the lowest porosity and smallest grain size), the hydrostat of each sandstone shows an inflection point corresponding to a critical effective pressure beyond which an accelerated, irrecoverable compaction occurs. Our microstructural observations show that brittle grain crushing initiates at this critical pressure. We also observed distributed cleavage cracking in calcite and intensive kinking in mica. The critical pressures for grain crushing in our sandstones range from 75 to 380 MPa. In general, a sandstone with higher porosity and larger grain size has a critical pressure which is lower than that of a sandstone with lower porosity and smaller grain size. We formulate a Hertzian fracture model to analyze the micromechanics of grain crushing. Assuming that the solid grains have preexisting microcracks with dimensions which scale with grain size, we derive an expression for the critical pressure which depends on the porosity, grain size, and fracture toughness of the solid matrix. The theoretical prediction is in reasonable agreement with our experimental data as well as other data from soil and rock mechanics studies for which the critical pressures range over 3 orders of magnitude.

Friction, overpressure and fault normal compression

Abstract: More than twenty-five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thickness of fault gouge. For example, a clay-filled fault zone about 1 km wide at the base of the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near-zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near-lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.

Calculated Volume and Pressure Changes During the Thermal Cracking of Oil to Gas in Reservoirs (1)

Abstract: The rising temperature that accompanies increasing burial depth converts oil in a reservoir into thermal gas. A consideration of hydrogen balance during this cracking shows that approximately 3000 cubic ft (85 cubic meters) of gas (at standard temperature and pressure) is generated from each barrel of oil. In addition, a graphitic residue is precipitated. If the volume relationships among oil, thermal gas, and the graphitic residue are combined with data for gas solubility in pore water and gas nonideality (Z factor), then pressure can be calculated for any degree of thermal cracking. These calculations show that in an effectively isolated system, pressures would become very high and could considerably exceed the rock load, so that fracturing must occur causing pressure b eed off and loss of gas. The lithostatic gradient (1.0 psi/ft or 22.6 kPa/m) is reached after only about 1.0% of the oil is cracked. If the reservoir system remains open (i.e., at hydrostatic pressure) and is initially filled with oil that is subsequently cracked to gas, then roughly 75% of the gas will be lost or the reservoir volume must effectively increase in size, for example, by moving the gas-water contact downward.

Permeabilities, fluid pressures, and flow rates in the Barbados Ridge Complex

Abstract: Recent measurements from Ocean Drilling Program leg 110 and Deep Sea Drilling Project leg 78a indicate that pore pressures near the toe of the Barbados accretionary prism may be close to lithostatic and that the decollement is a zone with relatively high rates of fluid flow and methane transport. We used a numerical model of fluid flow to estimate intrinsic permeabilities, pore pressures, and flow velocities that are consistent with these observations. Model results suggest that the permeability of the decollement may be 3–5 orders of magnitude greater than that of adjacent prism sediments. If permeabilities in the prism vary with depth in a manner similar to those in sedimentary basins, the average intrinsic permeability of the decollement, kd, must be about 10−14 m2. When kd is 10−13 m2, high pore pressures do not develop near the deformation front in the model. If kd is 10−15 m2, simulated pressures are unrealistically high in both the prism and underthrust sediments arcward of the deformation front. Water originating from compaction in the decollement and underthrust sediments flows laterally seaward, while water expelled from prism sediments flows upward to the ocean floor. However, flow velocities are small, and the net motion of pore water in prism and underthrust sediments is arcward relative to the deformation front because of tectonic transport. Pore water migrates seaward in spite of tectonic transport only in discrete zones with higher permeability, in this case the decollement.

The influence of tidal marshes on upland groundwater discharge to estuaries

Abstract: We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.

An integrated approach to quantify groundwater transport of phosphorus to Narrow Lake, Alberta

Abstract: An integrated approach was used to quantify groundwater phosphorus flux to Narrow Lake, a smallglacial-terrain lake in central Alberta. Data from a drilling program, major ion concentrations, environmental isotopes, and computer simulations indicated that the lake gains water through the nearshore region from a small, shallow groundwater flow system; at deep offshore regions, water moves from the lake to the groundwater flow system. Seepage flux was quantified by water budget, Darcy’s equation with data from wells near the lake, Darcy’s equation with data from minipiezometers in the lake, and seepage meters. Whole-lake seepage flux determined from minipiezometer data (30 mm yr-I) was only lO-25% of the other estimates (mean, 221 mm yr-I; range, 133-332 mm yr- l from seepage meter and water budget data, respectively). Groundwater contributed - 30% of the annual water load to the lake. The P concentration, [PI, in pore water from lake sediments (mean, 175 mg m-‘) was 8 times higher than groundwater from wells near the lake (mean, 2 1 mg m-3). Thus, if well water was used to estimate the [P] of the seepage water, the rate of groundwater P loading to the lake would be underestimated. The rate of groundwater P loading to the lake computed from average seepage flux and average pore-water [P] was 39 mg m-2 yr-I, and groundwater may be the largest single source of P to epilimnetic water in the lake.

Pore pressure response during failure in soils

Abstract: Three experiments were performed on natural slopes to investigate variations of soil pore-water pressure during induced slope failure. Two sites in the Wasatch Range, Utah, and one site in the San Dimas Experimental Forest of southern California were forced to fail by artificial subsurface irrigation. The sites were instrumented with electronic piezometers and displacement meters to record induced pore pressures and movements of the slopes during failure. Piezometer records show a consistent trend of increasing pressure during the early stages of infiltration and abrupt decreases in pressure from 5 to 50 minutes before failure. Displacement meters failed to register the amount of movement, due to location and ineffectual coupling of meter pins to soil. Observations during the experiments indicate that fractures and macropores controlled the flow of water through the slope and that both water-flow paths and permeability within the slopes were not constant in space or time but changed continually during the course of the experiments.

Immobile water during solute transport in unsaturated sand columns

Abstract: Solute transport experiments were carried out to investigate the presence of immobile water in an unsaturated fine sand under different flow regimes. Both steady and unsteady flow conditions were imposed, but the average pore water velocity and water content were the same in each. Evidence for the presence of an immobile water fraction was sought by fitting the concentration distributions using models both with and without an explicit term accounting for solute exchange with immobile water. Strong evidence was found for the presence of an immobile water fraction affecting the solute concentration distribution under steady flow conditions but not for unsteady flow. These apparently conflicting results are explained in terms of different water flow patterns arising from the two flow regimes.

The influence of macropores on debris flow initiation

Abstract: Soil structure affects the movement of water in hillslope soils and therefore exerts a strong influence on slope stability. A debris flow is analysed within the confines of an instrumented catchment on the South Island, New Zealand, in order to examine the influence of soil macropores on slope stability. Tensiometric and slope throughflow data for nearby slope areas show that vertical cracks conduct rainfall at rates well in excess of the mineral soil matrix conductivity. The presence of a well connected pipe system at the soil-bedrock interface distributes this water quickly downslope. Under exceptionally high rainfall intensities, however, this crack-pipe system may induce slope instability by increasing the rate of infiltration over lateral pipeflow rates. This results in a build-up of pore pressure at the soilI–rock interface and subsequent slope failure.

A simplified finite element analysis of wave-induced effective stresses and pore pressures in permeable sea beds

Abstract: The wave-induced pore pressures and effective stresses in a saturated submarine sediment are treated by means of the finite element method. By using the general theory of behaviour of the saturated poro-elastic media presented by Biot (1972, 1973), field equations of motion are established. By introducing a valid approximation for very slow phenomena, the full formulation is simplified and the well-known equations of consolidation are obtained. Stability conditions and accuracy of the process of solution are discussed. A verification of the written finite element program is made by a comparative study with the infinite depth solution given by Yamamoto (1978) and Madsen (1978). The effects of bed thickness, permeability and soil stiffness on the wave-induced pore pressures, effective normal stresses, shear stresses, horizontal and vertical displacement at the mud line and with depth are investigated by various examples. The response of a heterogeneous sea bed to a harmonic wave loading is compared with a h...

Flow to a heated borehole in porous, thermoelastic rock: Analysis

Abstract: Exact solutions are obtained for fluid flow induced by the heating of a borehole. The rock is modeled as a fluid-saturated, porous, thermoelastic medium. The temperature and pore pressure fields are governed by a pair of diffusion equations, which are coupled through a source term in the pressure equation proportional to the temperature rate. The pressure profile exhibits a maximum that grows in magnitude and propagates away from the borehole. For a constant heat flux applied as an instantaneous step, the fluid flux to the borehole takes a finite initial value, and decays monotonically. When the heat flux exhibits a finite rise time, the fluid flux is initially zero, rises to a maximum, and then decays. At late time, the inverse of the fluid flux is linear in ln t; this observation can be exploited to estimate the permeability and fluid diffusivity of low-permeability rock. Sample calculations are shown for Westerly granite.

Mineralization, pore water chemistry and phosphorus release from peaty sediments in the eutrophic Loosdrecht lakes, The Netherlands

Abstract: SUMMARY. 1. Pore water chemistry in peaty sediment was monitored for a year at two representative locations of the eutrophic shallow Loosdrecht lakes. The Netherlands. Phosphorus fluxes over the sediment-water interface were calculated using measured concentration gradients in the pore water and compared to fluxes measured under laboratory conditions. Results were analysed with Redundancy Analysis to detect patterns of variation in pore water chemistry and in measured and calculated fluxes, that could be ascribed to environmental variables. 2. It was demonstrated that phosphorus fluxes measured in long-term laboratory incubations were not correlated to any of the pore water characteristics. 3. Initial phosphorus fluxes measured in sediment columns, which varied between −7.7 and 1330 μmol m−2: day−1, were correlated significantly to the calculated phosphorus flux over the sediment-water interface. 4. The high correlation between calculated fluxes of ammonia, phosphorus and methane and measured initial flux of phosphorus, conclusively pointed to mineralization of organic matter as the driving force for phosphorus release from the sediment. 5. Redundancy Analysis demonstrated that the rates of mineralization and phosphorus release were only weakly related to temperature. They appeared to be especially stimulated by the autumnal decrease in temperature which was probably related to an extra input of organic matter.

Positive feedback fracture process induced by nonuniform high-pressure water flow in dilatant granite

Abstract: During forced water infiltration into a dry granite rock in the dilatant state, migration of a water front and a clear positive feedback between acoustic activity and water flow into microcracks were revealed by means of AE hypocenter locations and velocity tomography. Water was injected into a cylindrical granite sample subjected to 40 MPa confining pressure and constant axial stress equal to about 70% of short-term dry strength. The injection water pressure was held constant at 17 MPa until failure occurred. Soon after water injection into the bottom end of the sample, AE hypocenters began near the bottom end. Subsequently, they moved up with a strong preference for the region where a weak concentration of AE events had been observed during a steady state creep stage before the water injection. The P wave velocity structure was determined in the section parallel to the loading axis and crossing the cluster of AE hypocenters. The velocity structure, reconstructed by seismic tomography, showed a nonsymmetric distribution of the velocity with regard to the loading axis during the injection. The strong concentration of AE hypocenters suggests that water flow channels are preferentially formed in a very limited region in the dilatant rock. The concentration of AE events and formation of these water channels marks a positive feedback process in which the local strength of the rock is reduced by the decrease of the effective confining pressure caused by increasing pore pressure which, in turn, triggers more microfracturing.

A simple pore-water sampler for coarse, sandy sediments of low porosity

Abstract: A simple, inexpensive, and highly efficient centrifuge tube allows nearly complete extraction of interstitial water from coarse, sandy sediments with porosities as low as 32%. The method is fast (5 min), efficient (55–92% of the available water is obtained), reliable, and can be peformed on board ship at only 1,500 × g. The efficiency of the method permits a high sampling resolution (millimeter scale). The centrifuge tube is made of polyethylene and can be used for trace metal determinations after cleaning with acid. Due to a built-in filter, pore waters are clean and need not be filtered afterward, thus reducing the risk of contamination and possible oxidation artifacts.

Pore pressure prediction method

Abstract: A method of predicting pore pressures at a proposed drilling location using Interval Transit Times derived from seismic data and Interval Transit Times from a calibrated normal geopressure trend. Seismic Interval Transit Times derived pore pressures and actual pore pressures derived from logs in a drilled well at an offset location are used to correlate graphically or analytically the seismic Interval Transit Times to normal geopressure interval transit times from a proposed drilling location to be used in the prediction of pore pressures at the proposed location.

Groundwater flow fields in infinite slopes.

Abstract: This Technical Note presents an analytical description of all posssible groundwater flow fields in infinite slopes. These flow fields may have any spatial orientation and may be affected by hydraulic anisotropy and heterogeneity, but they may produce pore pressures or seepage forces that vary only in the co-ordinate direction normal to the slope surface. An equation is presented that together with its specialized forms define hydraulic gradients and hence determine pore pressure distributions and seepage forces in infinite slopes. The practical ramifications of this equation are discussed.

Examination of erosion resistance of clays in embankment dams

Abstract: Internal erosion is a possible cause of failure in embankment dams. Erosion may occur when soil in the vicinity of a Crack disperses into suspension and is transported by the water into the drainage system. An important factor in assessing the potential of a soil for dispersion is the true cohesion defined as the strength at zero effective stress. A new test, the cylinder dispersion test, has been developed to investigate true cohesion and dispersion in soils. Tests carried out on a number of different soils demonstrated the major influence of the pore water chemistry on the true cohesion and dispersion properties, and hence erosion resistance.