Showing papers on "Overpressure published in 1998"

Memoir 70, Chapter 2: Mechanisms that Generate Abnormal Pressures: an Overview

Abstract: Normally pressured reservoirs have pore pressures which are the same as a continuous column of static water from the surface. Abnormal pressures occur where the pore pressures are significantly greater than normal (overpressure) or less than normal (underpressure). Overpressured sediments are found in the subsurface of both young basins from about 1.0 to 2.0 km downwards, and in older basins, in thick sections of fine-grained sediments. The main mechanisms considered responsible for most overpressure conditions can be grouped into three broad categories, based on the processes involved: (1) ineffective volume reduction due to imposed stress (vertical loading during burial, lateral tectonic processes) leading to disequilibrium compaction, (2) volume expansion, including porosity increases due to changes in the solid:liquid ratios of the rock, and (3) hydraulic head and hydrocarbon buoyancy. The principal mechanisms which result in large magnitude overpressure are disequilibrium compaction and fluid volume expansion during gas generation. Disequilibrium compaction results from rapid burial (high sedimentation rates) of low-permeability rocks such as shales, and is characterized on pressure vs. depth plots by a fluid retention depth where overpressure commences, and increases downwards along a gradient which can closely follow the lithostatic (overburden) gradient. Disequilibrium compaction is typical in basins with a high sedimentation rate, including Tertiary deltas and some intracratonic basins. In older basins, disequilibrium compaction generated earlier in the basin history may be preserved only in thick, fine-grained sequences, but lost by vertical/lateral leakage from rocks with relatively high permeabilities. Gas generation from secondary maturation reactions, and oil cracking in the deeper parts of sedimentary basins, can result in large fluid volume increases, although the magnitudes are uncertain. In addition, the effect of increased pressures on the reactions involved is unknown. We doubt that any of the other mechanisms involving volume change can contribute significant regional overpressure, except in very unusual conditions. Hydraulic head and hydrocarbon buoyancy are mechanisms whose contributions are generally small; however, they can be easily assessed and may be important when additive to other mechanisms. The effects of transference of overpressure generated elsewhere should always be considered, since the present pressure distribution will be strongly affected by the ability of fluids to move along lateral and vertical conduits. Naturally underpressured reservoirs (as opposed to underpressure during depletion) have not been as widely recognized, being restricted mainly to interior basins which have undergone uplift and temperature reduction. The likely principal causes are hydraulic discharge, rock dilation during erosional unroofing, and gas migration during uplift.

Generation of overpressure and compaction‐driven fluid flow in a Plio‐Pleistocene growth‐faulted basin, Eugene Island 330, offshore Louisiana

Abstract: The complex pressure and porosity fields observed in the Eugene Island (EI) 330 field (offshore Louisiana) are thought to result from sediment loading of low-permeability strata. In this field, fluid pressures rise with depth from hydrostatic to nearly lithostatic, iso-pressure surfaces closely follow stratigraphic surfaces which are sharply offset by growth-faulting, and porosity declines with effective stress. A one-dimensional hydrodynamic model simulates the evolution of pressure and porosity in this system. If reversible (elastic) compaction is assumed, sediment loading is the dominant source of overpressure (94%). If irreversible (inelastic) compaction and permeability reduction due to clay diagenesis are assumed, then thermal expansion of pore fluids and clay dehydration provide a significant component of overpressure (>20%). The model is applied to wells on the upthrown and downthrown sides of the major growth fault in the EI 330 field. Assuming that sediment loading is the only pressure source and that permeability is a function of lithology and porosity, the observed pressure and porosity profiles are reproduced. Observation and theory support a conceptual model where hydrodynamic evolution is intimately tied to the structural and stratigraphic evolution of this progradational deltaic system.

Physical constraints on hydrocarbon leakage and trapping revisited

Abstract: In a water-wet petroleum reservoir with a water-wet seal, a continuous water phase will extend from the reservoir into the seal, and the pressure difference between the water phase in the uppermost pores of the reservoir and the water phase in the lowermost pores of the seal can therefore only be of an infinitesimal magnitude. This implies that any overpressure in a water-wet reservoir will not contribute to pushing the hydrocarbons through a water-wet seal, and overpressured water-wet reservoirs should therefore not be considered more prone to capillary leakage than normally pressured reservoirs. Within a water-wet petroleum reservoirs, the overpressure in the hydrocarbon phase relative to the water phase is balanced by the elastic forces at the fluid interface (interfacial tension). The overpressure in the hydrocarbon phase relative to the water phase therefore does not increase the risk of hydrofracturing the reservoir9s seal. This implies that the risk of hydrofracturing should not be increased as a function of hydrocarbon column height, and should not be considered to be higher for gas than it is for oil. When an upward-directed hydraulic gradient is present from a reservoir unit into the overlying seal, water will continuously move upwards from the reservoir unit and into the seal if both rocks are water-wet. This movement of water may lead to exsolution of gas above the reservoir unit, and the presence of free gas may be detected as gas chimneys on seismic sections. This mechanism will operate regardless of whether or not a hydrocarbon accumulation is present below the gas chimneys, and fracturing of the reservoir unit9s seal or capillary leakage of hydrocarbons are therefore not necessary conditions for the development of gas chimneys.

Attenuation of weak shock waves along pseudo-perforated walls

Abstract: In order to attenuate weak shock waves in ducts, effects of pseudo-perforated walls were investigated. Pseudo-perforated walls are defined as wall perforations having a closed cavity behind it. Shock wave diffraction and reflection created by these perforations were visualized in a shock tube by using holographic interferometer, and also by numerical simulation. Along the pseudo-perforated wall, an incident shock wave attenuates and eventually turns into a sound wave. Due to complex interactions of the incident shock wave with the perforations, the overpressure behind it becomes non-uniform and its peak value can locally exceed that behind the undisturbed incident shock wave. However, its pressure gradient monotonically decreases with the shock wave propagation. Effects of these pseudo-perforated walls on the attenuation of weak shock waves generated in high speed train tunnels were studied in a 1/250-scaled train tunnel simulator. It is concluded that in order to achieve a practically effective suppression of the tunnel sonic boom the length of the pseudo-perforation section should be sufficiently long.

Dynamic Response of Cantilevered Thin-Walled Beams to Blast and Sonic-Boom Loadings

Abstract: The paper deals with the dynamic response of anisotropic cantilevered thin-walled beams exposed to blast and sonic boom loadings. The structural model used in this study incorporates a number of nonclassical effects such as transverse shear and warping inhibition. Moreover, implementation of a specific ply-angle scheme in each constituent lamina results in elastic cross-couplings beneficial from the response behavior point of view. The influence of these effects is highlighted and the efficiency of the tailoring technique toward enhancing the dynamic response to various overpressure signatures is demonstrated.

Influence of pressure-driven gas permeation on the quasi-steady burning of porous energetic materials

Abstract: A theoretical two-phase-flow analysis is developed to describe the quasi-steady propagation, across a pressure jump, of a multi-phase deflagration in confined porous energetic materials. The difference, or overpressure, between the upstream (unburned) and downstream (burned) gas pressure leads to a more complex structure than that which is obtained for an unconfined deflagration in which the pressure across the multi-phase flame region is approximately constant. In particular, the structure of such a wave is shown by asymptotic methods to consist of a thin boundary layer characterized by gas permeation into the unburned solid, followed by a liquid/gas flame region, common to both types of problems, in which the melted material is preheated further and ultimately converted to gaseous products. The effect of gas flow relative to the condensed material is shown to be significant, both in the porous unburned solid as well as in the exothermic liquid/gas melt layer, and is, in turn, strongly affected by the overpressure. Indeed, all quantities of interest, including the burned temperature, gas velocity and the propagation speed, depend on this pressure difference, leading to a significant enhancement of the burning rate with increasing overpressure. In the limit that the overpressure becomes small, the pressure gradient is insufficient to drive gas produced in the reaction zone in the upstream direction, and all gas flow relative to the condensed material is directed in the downstream direction, as in the case of an unconfined deflagration. The present analysis is particularly applicable to those types of porous energetic solids, such as degraded nitramine propellants, that can experience significant gas flow in the solid preheat region and which are characterized by the presence of exothermic reactions in a bubbling melt layer at their surfaces. 7 refs., 6 figs.

Development of serial bio-shock tubes and their application.

Abstract: OBJECTIVE To design and produce serial shock tubes and further examine their application to experimental studies on blast injury. METHODS Bio-medical engineering technique was used for the design and development of the serial shock tubes. One thousand four hundred and fifty nine animals (757 rats, 105 guinea pigs, 335 rabbits, 240 dogs and 22 sheep) were then used to test the wounding effects of the shock tubes. RESULTS Three types of bio-shock tubes, that is, large-, medium- and small-scale shock tubes were made in our laboratory. The large-scale shock tube is 39 meters long; the inner diameter of the test section is 1 meter; and the maximum overpressure in the driving section is 10.3 MPa. A negative pressure could be formed by means of the reflected rarefactive wave produced by the end plate. The medium-scale shock tube is 34.5 meters long; the maximum overpressure in the driving section is 22 MPa; the test section is designed to be a knockdown, showing 5 basic types with inner diameter of 77 to 600 millimeters, which could be used for researches on overpressure, explosive decompression, underwater explosion, and so on. The small-scale shock tube is 0.5 meter long with the maximum endured overpressure of 68.6 MPa. Results from animal experiments showed that this set of shock tubes could induce various degrees of systemic or local blast injury in large or small animals. CONCLUSIONS This set of bio-shock tubes can approximately simulate typical explosive wave produced by nuclear or charge explosion, and inflict various degrees of blast injury characterized by stability and reproducibility. Therefore, they can meet the needs of blast research on large and small animals.

3D modellilng of fault bounded pressure compartments in the North Viking Graben

Abstract: The work presented in this paper involves modelling of the overpressure distribution in Jurassic reservoir and carrier rocks in the North Viking Graben. The main concept used in this study is that faults may form low-permeability barriers to fluid flow in compacting basins, and may thus influence overpressure distribution. The mapped faults in the study area are linked together so that they divide the area into 225 compartments. A model based on Darcy's law and information about offset and burial depth describes the flow conditions across the faults and between the compartments. This model describes a regional fault permeability architecture. A commercial reservoir simulator is used to calculate the fluid flow and the pressure development in all the compartments. The model was calibrated to pressures measured in 16 released exploration wells. The best match to these wells was obtained with a mean deviation of 9.5 bars and a standard deviation of 18. 5 bars between the observed and predicted overpressures for the Brent Group. The results from this pressure modelling can be used to simulate secondary migration of oil and gas.

Some thoughts on porosity reduction: rock mechanics, overpressure and fluid flow

Abstract: Abstract A currently popular paradigm, that porosity reduction occurs as a direct consequence of the effective stress acting on the rock framework grains, is mechanistically incorrect. The commonly observed covariance between porosity and effective stress does not reflect a cause-and-effect relationship. Instead, it arises because both low effective stress and slow porosity reduction are consequences of the inability of compacting rocks to expel their pore fluids quickly enough to maintain normal fluid pressures. The process of porosity loss is here divided into sequential steps, and we argue that the expulsion of pore fluids is the rate-determining step leading to overpressuring. Thus, Darcy’s law assumes equal importance with the relationships describing the mechanical compaction of sediments. In this paper we describe how compaction can be treated as a Coulomb-plastic response that is functionally dependent on effective mean stress, deviatoric stress, and the state of compaction. In the next generation of basin models, a mechanistically correct approach is needed, combining both rock mechanics and hydrogeology.

Blast Wave Mitigation by Water

Abstract: : Safety systems designed to mitigate blast wave effects are absolutely vital in the explosives industry. As a general rule, barricades made of soil, sand or concrete are used, but these systems cannot be moved once they have been constructed. Since plants or installations are frequently required to change location the concept of a mobile barricade is of considerable interest. The effect of a water wall on blast wave mitigation was studied in scale model tests. The influence of different parameters such as the thickness of the wall and the distance between the explosive charge and the water barricade was also calculated. This methodology enabled the use of nomographs giving excess pressure (overpressure) as a function of wall thickness, charge/wall distance and charge/location distance. The results showed the effectiveness of the water wall and confirmed its interest. This study was carried out by performing tests in a reduced scale model plant. The purpose of the tests was to: 1. Confirm the effect of the weight of the water wall on far-field blast mitigation. 2. Measure the effect of the water wall with regard to reflected pressures on rigid walls. In these cases water walls were created in front of the rigid walls.

Memoir 70, Chapter 3: Overpressure Models for Clastic Rocks, Their Relation to Hydrocarbon

Abstract: The purpose of this paper is to review advances made in our understanding of the origin of overpressures in clastic rocks and examine the relationship between overpressuring and hydrocarbon expulsion. This study uses numerical simulations to examine overpressure models in clastic rocks. It is based on a review of previous regional overpressure modeling studies in rapidly subsiding basins (the Mahakam Delta, Indonesia, and the Gulf Coast, U.S.A.), and in slowly subsiding basins (the Williston Basin, U.S.A.-Canada and the Paris Basin, France). We show that compaction models based on effective stress-porosity relations satisfactorily explain overpressures in rapidly subsiding basins. Overpressures appear primarily controlled by the vertical permeability of the shaly facies where they are observed. Vertical permeabilities required to model overpressures in the Gulf Coast and Mahakam basins differ little, they are around 1-10 nanodarcies. Geological evidence and models suggest other causes of overpressure such as aquathermal pressuring or clay diagenesis to be generally small compared with compaction disequilibrium. Hydrocarbon (HC) generation can be a minor additional cause of overpressures in rich, mature source rocks. Shale permeabilities calibrated against observed overpressures appear consistent with direct measurements. Specific surface areas of mineral grains and relationships between effective stress/permeability implied by model calibrations agree with independent experimental determination. The main weakness of mechanical compaction models is that they overestimate the porosity of thick overpressured shales. Unlike in previous studies, we suggest that this mismatch is not caused by fluid generation inside overpressured shales. Instead, we infer that it is a consequence of an inappropriate definition of effective stress. If effective stress is defined as S - aP, instead of S - P, then with a around 0.65-0.85, porosity reversals predicted in overpressured shales are much reduced, and better in agreement with observations. Alpha (a) is known in poro-elasticity as the Biot coefficient. We show that the non-linear distribution of horizontal stress often observed in overpressured shale sequences confirms values of the Biot coefficient in the range indicate above.

Effect of overpressure on dissolution and cavitation of bubbles stabilized on a metal surface

Abstract: Recent experimental evidence indicates that static overpressure dramatically reduces lithotripsy‐induced cell damage or stone comminution [Delius, Ultrasound Med. Biol. 24, 611–617]. The hypothesis is that the damage and comminution are due to cavitation and that overpressure dissolves the small gas bubbles which act as cavitation nuclei. Delius observed, however, that the overpressure to protect cells (1 atm) was significantly lower than for stones (10 atm). In similar experiments cell lysis and pitting of aluminum foil as indicators of cavitation are used; the thresholds were 1 and 30 atm, respectively. In addition, the authors observed that pitting of foils increased for overpressures up to 20 atm. It is proposed that crevices in the foil stabilize cavitation nuclei whereas the bubbles dissolve in fluids. Foil damage increased because overpressure is small relative to the shock wave pressures that drive bubble expansion but is the dominant driving force at bubble collapse. Calculations using the Gilmore equation predict that overpressures up to 30 atm hasten and intensify cavitation collapses and over 60 atm eliminate cavitation, in qualitative agreement with our foil data. The calculated collapse time was confirmed experimentally by passive cavitation detection. If bubbles are stabilized in crevices in kidney stones, overpressure may provide a safer, more effective lithotripsy treatment. [Work supported by NIH‐PO1‐DK43881.]

Prediction of Primary Fragmentation Characteristics of Cased Munitions

Abstract: : Safety to the public and nearby operational personnel during unexploded ordnance (UXO) operations is of the utmost importance. An accidental explosion produces hazards from primary fragments, blast overpressure, ground shock, and noise. The effects used to determine withdrawal distances for UXO operations are predominantly blast overpressure and primary fragmentation. For most ordnance the fragmentation range is much larger than the inhabited building distance (IBD) for blast overpressure. In order to determine the withdrawal distance for primary fragmentation, the fragmentation characteristics of the munition must be determined. The Structural Branch of the U.S. Army Engineering and Support Center, Huntsville (USAESCH) uses methods described in TM 5-1300, "Structures to Resist the Effects of Accidental Explosions" to determine the fragmentation characteristics of cased, cylindrical munitions. These characteristics include initial fragment velocity, weight of the largest fragment, average fragment weight, the total number of fragments, and the fragment weight for a given confidence level. These methods are applicable only for primary fragments resulting from a high-order detonation of a cylindrical casing with evenly distributed explosives in direct contact with the casing. For casings that are not uniform in thickness or diameter along the entire length, the casing must be modeled using a series of equivalent cylinders. The method is a trial-and-error procedure involving iterating on geometry to match the total modeled explosive weight to the actual explosive weight. These calculated fragmentation characteristics are used for a wide variety of purposes such as fragment range, striking energy, areal debris density and fragment penetration. The calculation methods, some modeling tips, and an example are presented.

Optimization of Low Boom Configuration of SST by Genetic Algorithm

Abstract: Sonic boom is one of the environmental problems produced by aircraft, and reduction of its peak pressure is a significant issue in the development of supersonic transport (SST). Recently various low boom configurations are being suggested. In the present study, a Genetic Algorithm (GA) has been applied to an aerodynamic shape optimization problem with low boom configuration. The peak overpressure of sonic boom is obtained from an extrapolation of F-function with an area-balancin g technique for locating shocks. The drag of a designed object is calculated based on the slender body theory and the linear theory. Consequently, it is found that the optimization method proposed here can greatly reduce peak overpressure without increasing drag.

Development of serial bio-shock tubes and their application

Abstract: Objective To design and produce serial shock tubes and further examine their application to experimental studies on blast injury. Methods Bio-medical engineering technique was used for the design and development of the serial shock tubes. One thousand four hundred and fifty nine animals (757 rats, 105 guinea pigs, 335 rabbits, 240 dogs and 22 sheep) were then used to test the wounding effects of the shock tubes. Results Three types of bio-shock tubes, that is, large, medium-and small-scale shock tubes were made in our laboratory. The large-scale shock tube is 39 meters long; the inner diameter of the test section is 1 meter; and the maximum overpressure in the driving section is 10.3 MPa. A negative pressure could be formed by means of the reflected rarefactive wave produced by the end plate. The medium-scale shock tube is 34.5 meters long; the maximum overpressure in the driving section is 22 MPa; the test section is designed to be a knockdown, showing 5 basic types with inner diameter of 77 to 600 millimeters, which could be used for researches on overpressure, explosive decompression, underwater explosion, and so on. The small-scale shock tube is 0.5 meter long with the maximum endured overpressure of 68.6 MPa. Results from animal experiments showed that this set of shock tubes could induce various degrees of systemic or local blast injury in large or small animals. Conclusions This set of bio-shock tubes can approximately simulate typical explosive wave produced by nuclear or charge explosion, and inflict various degrees of blast injury characterized by stability and reproducibility. Therefore, they can meet the needs of blast research on large and small animals.

Computational Study of Water Mitigation Effects On An Explosion Inside A Vented Tunnel System

Abstract: : The effects of water in close contact with detonating high explosives have been studied experimentally by numerous researchers, such as Eriksson (1974), Keenan & Wager (1992) and etc. These tests series had demonstrated that when water was stored closed to the high explosives, both the maximum overpressure and impulse density could be reduced significantly. This reduction has been attributed to the loss of energy from the shock into breaking up the water into droplets and the process of phase change of water from liquid to gas due to shock vaporization which subsequently, reduces the surrounding temperature. The purpose of the present work is to study computationally the mitigation effects of water to an explosion inside a tunnel system with venting. A series of three-dimensional numerical calculations using a Multimaterial Eulerian Finite Element code, MSC-Dytran, has been conducted. In order to capture the behavior of water subjected to shock loading, appropriate equation of state for water has to be determined. This is based on experimental data for shock Hugoniot for water and the Mie-Gruneisen equation of state for water has been chosen. Results from the present study show that water is capable of reducing the peak pressure due to an explosion and the configuration of water surrounding the explosive is important for water mitigation to be effective.

A Combustion Model for the TWA 800 Center-Wing Fuel Tank Explosion

Abstract: In support of the National Transportation Safety Board investigation of the TWA Flight 800 accident, a combined experimental/computational effort was conducted that focused on quarter-scale testing and simulation of the fuel-air explosion in the Boeing 747 center wing fuel tank. This report summarizes the modeling approach used at Sandia National Laboratories. In this approach approximations are introduced that capture the essential physics associated with turbulent flame propagation in multiple compartment fuel tanks. This model efficiently defines the pressure loading conditions during a jet-fuel air explosion in a fuel tank confinement. Modeling calculations compare favorably with a variety of experimental quarter-scale tests conducted in rigid confinement. The modeling describes well the overpressure history in several geometry configurations. Upon demonstrating a reasonable comparison to experimental observations, a parametric study of eight possible ignition sources is then discussed. Model calculations demonstrate that different loading conditions arise as the location of the ignition event is varied. By comparing the inferred damage and calculated impulses to that seen in the recovered tank, it maybe possible to reduce the number of likely sources. A possible extension of this work to better define tank damage includes coupling the combustion model as a pressure loading routine for structural failure analysis.

Application of sound-absorbent plastic to weak-shock-wave attenuators

Abstract: A device for attenuating weak shock waves propagating in a duct has been developed utilizing sound-absorbent plastic which is usually used for attenuating sound waves. The device has a tube made of the sound-absorbent plastic installed coaxially to a surrounding metal tube with a clearance between them. The clearance acts as an air layer to enhance the performance of the shock wave attenuation. When a weak shock wave propagates through this device, the pressure gradient of the shock wave is gradually smeared and hence its overpressure is decreased. The performance of the device was examined using a 1/250-scaled train tunnel simulator which simulated the discharge of weak shock waves created by high-speed entry of trains to tunnels. The overpressure of the shock waves ranged up to 5 kPa. The shock wave overpressure was decreased by 90% with the present attenuator attached. This device can be applied to various industrial noise suppressions which are associated with unsteady compressible flows.

Experiments on Attenuation of Weak Shock Waves in High-Speed Train Tunnels

Abstract: Experimental studies on weak shock wave attenuation in high- speed train tunnels were conducted using a 1/250-scaled train tunnel simulator and a shock tube. Three kinds of porous/perforated walls installed in the test section of the simulator near its exit were examined. Installing aluminum foam or porous plastic on the wall clearly decreased the pressure gradient of the wave front. The processes of the shock wave attenuation were visualized by holographic interferometer. A significant decrease in the overpressure was obtained by installing perforations along the simulato wall. However, with the short perforation section of 0.5 m, the pressure gradient of the shock wave remained steep. Through the series of the experiment and theoretical considerations, it is found that the length of the perforated/porous section is one of the most essential parameters to effectively attenuate the shock wave.

Ground motion and air overpressure study

Abstract: A seismic attenuation and air overpressure study was conducted to determine the attenuation of explosion induced ground motions and air overpressures as a function of distance from shallow subsurface detonated charges, and to derive parameters to predict blast effects at distances beyond the ordinance disposal facility boundary. A total of 210 explosive shots were monitored producing 2048 time histories of ground motions recorded in the vertical, radial, and transverse directions, in addition to recording air overpressures. The data were analyzed for peak particle velocities and peak air overpressures, then plotted versus scaled range. A best fit line was determined for the data to give average, 95% non-exceedance, and upper bound predictive equations which can be used in the disposal operations to avoid damage to adjacent structures.

Pressure-silo with a lower than 1 bar atmospheric overpressure

Abstract: The storage silo A2U has an outlet in the base A3U that feeds the product into a supply line A6U that is supplied with air from a compressor A5U. Air is also supplied through a separate line A8U to pressurise the silo. A pressure sensor A9U monitors the pressure which is controlled so as to be less than one atmosphere gauge pressure. The grain produce is held is pressurised silos