Showing papers on "Overpressure published in 2012"

An analytical model for gas overpressure in slug-driven explosions:insights into Strombolian volcanic eruptions

Abstract: Strombolian eruptions, common at basaltic volcanoes, are mildly explosive events that are driven by a large bubble of magmatic gas (a slug) rising up the conduit and bursting at the surface. Gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals. We present a theoretical investigation of slug overpressure based on magma-static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts, slug length at burst, and the depth at which the burst process begins. We find that burst overpressure is controlled by two dimensionless parameters: V', which represents the amount of gas in the slug, and A', which represents the thickness of the film of magma that falls around the rising slug. Burst overpressure increases nonlinearly as V' and A' increase. We consider two eruptive scenarios: (1) the "standard model," in which magma remains confined to the vent during slug expansion, and (2) the " overflow model," in which slug expansion is associated with lava effusion, as occasionally observed in the field. We find that slug overpressure is higher for the overflow model by a factor of 1.2-2.4. Applying our model to typical Strombolian eruptions at Stromboli, we find that the transition from passive degassing to explosive bursting occurs for slugs with volume >24-230 m(3), depending on magma viscosity and conduit diameter, and that at burst, a typical Strombolian slug (with a volume of 100-1000 m(3)) has an internal gas pressure of 1-5 bars and a length of 13-120 m. We compare model predictions with field data from Stromboli for low-energy " puffers," mildly explosive Strombolian eruptions, and the violently explosive 5 April 2003 paroxysm. We find that model predictions are consistent with field observations across this broad spectrum of eruptive styles, suggesting a common slug-driven mechanism; we propose that paroxysms are driven by unusually large slugs (large V').

Offshore sediment overpressures of passive margins: Mechanisms, measurement, and models

Abstract: [1] Fluid pressure in excess of hydrostatic equilibrium, or overpressure, in offshore environments is a widespread phenomenon that contributes to the migration and storage of fluids, solutes, and energy and to the potential mechanical instability of these sediments. Overpressure exists in deep and shallow systems and is most likely to be found where low-permeability ( mm/yr), tectonic loading, and lateral fluid transfer) and thermal and chemical processes (e.g., aquathermal expansion, hydrocarbon generation, mineral diagenesis, and organic maturation). In systems where near-lithostatic overpressures are generated, potentially unstable sediments are created. Failures of these sediments can create large-scale natural disasters, generate fractures, and damage seafloor and subseafloor infrastructure. Detailed characterization of overpressured systems has been accomplished through geological and geotechnical analyses, including investigation of physical-mechanical properties (mainly porosity, consolidation state, and shear strength), inversion of geophysical data (e.g., compressional and/or shear velocities), measurement of in situ properties, and postevent analyses. Process-based models have been developed to explain the origin of overpressure in terms of rate of overpressure genesis. This allows identification of potentially unstable zones and assessment of the potential for failure. Future development in measurements and in coupling of models will lead to more accurate analysis and prediction of fluid pressure in offshore sediments, which in turn will facilitate better hazard analyses and will enable safer and more cost-effective offshore drilling practices and other offshore infrastructure development.

A Multiscale Approach to Blast Neurotrauma Modeling: Part I – Development of Novel Test Devices for in vivo and in vitro Blast Injury Models

Abstract: The loading conditions used in some current in vivo and in vitro blast-induced neurotrauma models may not be representative of real-world blast conditions. To address these limitations, we developed a compressed-gas driven shock tube with different driven lengths that can generate Friedlander-type blasts. The shock tube can generate overpressures up to 650 kPa with durations between 0.3 and 1.1 ms using compressed helium driver gas, and peak overpressures up to 450 kPa with durations between 0.6 and 3 ms using compressed nitrogen. This device is used for short duration blast overpressure loading for small animal in vivo injury models, and contrasts the more frequently used long duration/high impulse blast overpressures in the literature. We also developed a new apparatus that is used with the shock tube to recreate the in vivo intracranial overpressure response for loading in vitro culture preparations. The receiver device surrounds the culture with materials of similar impedance to facilitate the propagation of a single overpressure pulse through the tissue. This method prevents pressure waves reflecting off the tissue that can cause unrealistic deformation and injury. The receiver performance was characterized using the longest helium-driven shock tube, and produced in-fluid overpressures up to 1500 kPa at the location where a culture would be placed. This response was well correlated with the overpressure conditions from the shock tube (R2 = 0.97). Finite element models of the shock tube and receiver were developed and validated to better elucidate the mechanics of this methodology. A demonstration exposing a culture to the loading conditions created by this system suggest tissue strains less than 5% for all pressure levels simulated, which was well below functional deficit thresholds for strain rates less than 50 s-1. This novel system is not limited to a specific type of culture model and can be modified to reproduce more complex pressure pulses.

Using a gel/plastic surrogate to study the biomechanical response of the head under air shock loading: a combined experimental and numerical investigation

Abstract: A combined experimental and numerical study was conducted to determine a method to elucidate the biomechanical response of a head surrogate physical model under air shock loading. In the physical experiments, a gel-filled egg-shaped skull/brain surrogate was exposed to blast overpressure in a shock tube environment, and static pressures within the shock tube and the surrogate were recorded throughout the event. A numerical model of the shock tube was developed using the Eulerian approach and validated against experimental data. An arbitrary Lagrangian-Eulerian (ALE) fluid–structure coupling algorithm was then utilized to simulate the interaction of the shock wave and the head surrogate. After model validation, a comprehensive series of parametric studies was carried out on the egg-shaped surrogate FE model to assess the effect of several key factors, such as the elastic modulus of the shell, bulk modulus of the core, head orientation, and internal sensor location, on pressure and strain responses. Results indicate that increasing the elastic modulus of the shell within the range simulated in this study led to considerable rise of the overpressures. Varying the bulk modulus of the core from 0.5 to 2.0 GPa, the overpressure had an increase of 7.2%. The curvature of the surface facing the shock wave significantly affected both the peak positive and negative pressures. Simulations of the head surrogate with the blunt end facing the advancing shock front had a higher pressure compared to the simulations with the pointed end facing the shock front. The influence of an opening (possibly mimicking anatomical apertures) on the peak pressures was evaluated using a surrogate head with a hole on the shell of the blunt end. It was revealed that the presence of the opening had little influence on the positive pressures but could affect the negative pressure evidently.

Devolatilization‐induced pressure build‐up: Implications for reaction front movement and breccia pipe formation

Abstract: Generation of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction-induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build-up: (i) The efficiency of the devolatilizing reaction front (pressure build-up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic-rich shales owing to methane generation in the contact aureole.

Elasto-plastic and hydromechanical models of failure around an infinitely long magma chamber

Abstract: [1] Surface displacements solutions of elastic deformation around an inflating magma chamber generally assume that the associated internal overpressure is limited by the bedrock tensile strength. When considering stress equilibrium in the bedrock adjacent to a spherical or infinitely long cylinder, the gravity body force actually resists tensile failure, thus leading to a much larger pressure threshold. And when considering a Coulomb failure criterion, analytical and numerical models predict that shear failure develops instead of tensile failure. Here, three numerical codes are used to compare elasto-plastic solutions of surface displacements and patterns of failure in plane-strain. Shear failure propagates independently from the surface downward, then from the chamber walls upwards, and finally the two plasticized domains connect. Another test with internal underpressure (simulating source deflation) fits standard solutions from tunneling engineering. The effect of pore fluid pressures is also explored. In case of lithostatic fluid pore pressure in the bedrock, the gravity effect cancels out, and tensile failure is enabled for an overpressure close to the tensile strength. Coupled hydromechanical models in undrained conditions indicate that the initial bedrock porosity modifies the evolution of fluid pressure, volumetric strain and effective normal stress, and consequently also the pressure threshold for the onset of failure. We show that a bedrock of low porosity is more prone to fail than a bedrock of high porosity. In summary, our elasto-plastic and hydromechanical models illustrate the contexts for either tensile or shear failure around magmatic bodies, at the same time complementing and delimiting predictions deduced from elasticity.

Burial and Thermal History of the Haynesville Shale: Implications for Overpressure, Gas Generation, and Natural Hydrofracture

Abstract: The Haynesville Shale is an organic rich sedimentary rock found in northwestern Louisiana, eastern Texas, and southwestern Arkansas. It was deposited during the Late Jurassic in a marine environment. Average thickness varies from 200 to 300 ft (60–90 m). The Haynesville Shale is typically found at depths of 10,000 ft (3 km) or more and is characterized by ultra low permeability. It is an area of active exploration and development for natural gas especially in northwestern Louisiana. Results from an earlier thermal-mechanical model suggest that Jurassic temperature gradients were more than twice the current regional value of 0.0135 to 0.02°F/ft (25 to 35°C/km). Thus, Jurassic age sediments have been close to their current temperatures for the last 100 m.y. Using subsurface data, a simple model of heat transport by advection and conduction and fluid flow by compaction was used to estimate temperature, maturation, and fluid pressure through time for the Haynesville Shale. High heat flow in the Early Cretaceous contributed to high temperature gradients and early maturation of hydrocarbons. Rapid sedimentation in the Early Cretaceous resulted in generation of significant overpressure within the Haynesville Shale. This overpressure cannot be maintained over geologic time because the unit is too thin and there was subsequent uplift and erosion. Hydrocarbon generation produced additional overpressure in the Late to mid-Cretaceous and the Late Paleogene. However, under most conditions, model overpressures do not exceed the fracture gradient.

Analysis of reflected blast wave pressure profiles in a confined room

Abstract: To understand the blast effects of confined explosions, it is necessary to study the characteristic parameters of the blast wave in terms of overpressure, impulse and arrival time. In a previous study, experiments were performed using two different scales of a pyrotechnic workshop. The main purpose of these experiments was to compare the TNT equivalent for solid and gaseous explosives in terms of mass to define a TNT equivalent in a reflection field and to validate the similitude between real and small scales. To study the interactions and propagations of the reflected shock waves, the present study was conducted by progressively building a confined volume around the charge. In this way, the influence of each wall and the origins of the reflected shock waves can be determined. The purpose of this paper is to report the blast wave interactions that resulted from the detonation of a stoichiometric propane-oxygen mixture in a confined room.

The regional pressure impact of CO2 storage: a showcase study from the North German Basin

Abstract: A regional scale, showcase saline aquifer CO2 storage model from the North German Basin is presented, predicting the regional pressure impact of a small industrial scale CO2 storage operation on its surroundings. The intention of the model is to bridge the gap between generic and site-specific, studying the role of fluid flow boundary conditions and petrophysical parameters typically found in the North German Basin. The numerical simulation has been carried out using two different numerical simulators, whose results matched well. The most important system parameters proved to be the model’s hydrological boundary conditions, rock compressibility, and permeability. In open boundary aquifers, injection-induced overpressures dissipate back to hydrostatic level within a few years. If a geological flow barrier is present on at least one side of the aquifer, pressure dissipation is seriously retarded. In fully closed compartments, overpressures can never fully dissipate, but equilibrate to a compartment-wide remnant overpressure. At greater distances to the injection well, maximum fluid pressures are in the range of a few bar only, and reached several years to decades after the end of the actual injection period. This is important in terms of long-term safety and monitoring considerations. Regional pressure increase impacts the storage capacities of neighbouring sites within hydraulically connected units. It can be concluded that storage capacities may be seriously over- or underestimated when the focus is on a single individual storage site. It is thus necessary to assess the joint storage capacities and pressure limitations of potential sites within the same hydraulic unit.

Chemical compaction of mudrocks in the presence of overpressure

Abstract: In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress. In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.

Pressure and fluid dynamic characterisation of the Dutch subsurface

Abstract: This paper presents and discusses the distribution of fluid and leak-off pressure data from the subsurface of onshore and offshore Netherlands in relation to causes of formation fluid overpressure and the permeability framework. The observed fluid pressure conditions demonstrate a clear regional difference between the southern and the north and north-eastern part of the study area. In the southern area, formation fluid pressures are close to normal and well below measured leak-off pressures. In the north, formation fluids are overpressured and may locally even approach the measured leak-off pressures. The regional differences in fluid overpressure can, in large part, be explained by differences in geologic framework and burial history. In the south, relatively low rates of sedimentary loading and the presence of relatively permeable sedimentary units have led to the currently observed normally pressured conditions. In the northern area, relatively rapid Neogene sediment loading plays an important role in explaining the observed overpressure distributions in Cenozoic mudstones, Cretaceous Chalk and Rijnland groups, and probably also in Jurassic units. The permeability framework of the northern and north-eastern area is significantly affected by Zechstein and Triassic salt deposits and structures. These units are characterised by very low permeability and severely restrict fluid flow and pressure dissipation. This has created hydraulically restricted compartments with high overpressures (for example overpressures exceeding 30 MPa in the Lower Germanic Trias Group in the Terschelling Basin and Dutch Central Graben).

Flame acceleration of premixed methane/air explosion in parallel pipes

Abstract: The explosion propagation characteristics in parallel pipes have been studied for the first time in two different types of pipe. It was found that flame speed and overpressure in two branches of equal length were close when the ignition was at the pipe head. The explosion violence was strengthened after the flame and blast wave were superimposed. When the ignition was acted in the corner of one branch, the peak overpressure near the meeting point was higher than that at the two adjacent points, while the flame speed showed a downtrend. When the parallel pipe was not full of gas, the peak overpressure in the two branches followed similar trends and showed an obvious downtrend. However, when the blast wave reached the meeting point, the peak overpressure also showed an obvious upward trend until the end of the pipe. The flame accelerated in the two different branches of unequal length, but it slowed down when traveling to the meeting point, and the peak overpressure evolution was contrary to the flame speed. The results suggested that the violence of the underground gas explosion near the meeting point was more serious, such that the equipment and people in this area should be paid more attention.

How to Accurately Measure Low Sonic Boom or Model Surface Pressures in Supersonic Wind Tunnels

Abstract: R 3/4 scaling. Methodology being developed for general acoustic (linear) ray spreading was applied to sonic boom and (non- linear: required for large initial sonic boom overpressure) aging was applied separately. Acoustic conical ray spreading meant a slower amplitude decay that varies with R ½ scaling; therefore, the rest of the observed amplitude decay was due to non-linear aging or stretching of the signature. Measurement distance in a wind tunnel is affected by both aging and acoustic propagation concerns.

Experimental Study on Characteristics of Methane–Coal-Dust Mixture Explosion and Its Mitigation by Ultra-Fine Water Mist

Abstract: This paper presents the results of an experimental investigation on the characteristics of methane–coal-dust mixture explosion and its mitigation by ultra-fine water mist. Four E12-1-K-type fast response thermocouples, two printed circuit board (PCB) piezotronic pressure transducers were used to obtain the temperature and pressure history, while a GigaView high-speed camera was used to visualize the processes. Different methane concentrations, coal-dust concentrations, diameters of coal particles, and volumes of ultrafine water mist were considered to investigate their effects on methane–coal-dust mixture explosion. The temperature of explosion flame, the maximum explosion overpressure, the maximum rate of overpressure rise, and the critical volume flux of ultra-fine water mist were experimentally determined. The results show that the characteristics of the methane–coal-dust mixture explosion and the mitigating effectiveness by ultra-fine water mist are influenced by the methane concentration, the coal-dust concentration, the coaldust diameter and the applied volume flux of ultra-fine water mist. For example, both the maximum explosion overpressure and rate of overpressure rise increased with increasing of coal-dust concentrations and methane concentrations. All of the test cases indicate that ultra-fine water mist can mitigate the mixture explosion and suppress the flame propagation efficiently from the images recorded by the high-speed video camera. [DOI: 10.1115/1.4005816]

Main controlling factors and formation models of natural gas reservoirs with high-temperature and overpressure in Yinggehai Basin

Abstract: There are three main factors that control the formation of productive lithologic gas-reservoirs with high-temperature and overpressure in the first member of the upper Miocene Huangliu Formation in Yinggehai Basin,South China SeaThese three factors are ①a gravity-flow driven sandstone reservoir with high quality;②a gas-reservoir located in the west side of the eastern and western provenance conjunction and overlain by high-quality neritic-facies mudstones as the cap rock;and ③a gas-reservoir located in the diapir outskirt,under which there exists a fault/fissure system connecting with underlying overpressure high-maturity source rocks of the mid-Miocene Meishan Formation and lower Miocene Sanya FormationHigh-temperature and overpressure gas reservoirs in Yinggehai Basin have two main gas-accumulating models,one is the lithologic pattern,in which gas reservoirs are distributed in wings and outskirts of the diapir anticline,even in the syncline base,and characterized by the earlier formation of lithologic traps that are consequently uncontrolled by diaper structures,and the other is the anticline pattern,in which gas reservoirs are distributed on the diapir anticline,formed by structures and consequently controlled by diapir structuresCompared with the structural gas reservoir,the lithologic gas reservoir is of good permeability,large-scale reserves and productive capacity

How Do ∼2° Slopes Fail in Areas of Slow Sedimentation? A Sensitivity Study on the Influence of Accumulation Rate and Permeability on Submarine Slope Stability

Abstract: Overpressure generation due to rapid sediment deposition can result in low effective stresses within the sediment column. It has been proposed that these large overpressures are the main preconditioning factor for causing large-scale submarine slope failure on passive continental margins, such as those in the Gulf of Mexico and offshore Norway. The rate of overpressure generation depends on the sedimentation rate, sediment compressibility and permeability. The Gulf of Mexico and the Norwegian continental slope have experienced comparatively high sediment input, but large-scale slope failure also occurs in locations with very low sedimentation rates such as the Northwest African continental margin. Here we show results from 2D numerical modelling of a 2° continental slope subjected to deposition rates of 0.15 m/ka. These results do not indicate any evidence for significant overpressure or slope instability. We conclude that factors other than overpressure must be fundamental for initiating slope failure, at least in locations with low sedimentation rates.

Safety of atmospheric storage tanks during accidental explosions

Abstract: The occurrence of a chain reaction from blast on atmospheric storage tanks in oil and chemical facilities is hard to predict. The current French practice for SEVESO facilities ignores projectiles and assumes a critical peak overpressure value observed from accident data. This method could lead to conservative or dangerous assessments. This study presents various simple mechanical models to facilitate quick effective assessment of risk analysis, the results of which are compared with the current practice. The damage modes are based on experience of the most recent accidents in France. Uncertainty propagation methods are used in order to evaluate the sensitivity and the failure probability of global tank models for a selection of overpressure signatures. The current work makes use of these evaluations to demonstrate the importance of a dynamic analysis to study domino effects in accidents. L'occurrence de reaction en chaine, dite reaction par effets dominos, sur les reservoirs de stockage atmospherique suit...

Ambient pressure dependence of the ultra-harmonic response from contrast microbubbles.

Abstract: Sub-harmonic response from ultrasound contrast agent microbubbles has been demonstrated to be an effective modality for noninvasive pressure measurement In the present study, the dependence of ultra-harmonic response on the ambient overpressure was investigated by both experimental measurements and simulations In the measurements, the microbubbles were exposed to Gaussian pulses with varied driving frequencies and pulse lengths, at an acoustic pressure of 03 MPa The amplitudes of sub- and ultra-harmonic components were measured when the ambient overpressures varied from 0–25 kPa At the driving frequency of 133 MHz, the ultra-harmonic energy decreased but the sub-harmonic energy increased with the increasing overpressure; while at the driving frequency of 4 MHz, both the sub- and ultra-harmonic components showed the same tendency that the corresponding energy decreased as the overpressure was increased A 4-MHz Gaussian pulse with 64 cycles could provide an ultra-harmonic response with both good ambient pressure sensitivity and high linearity Furthermore, the effects of shell parameters of a microbubble on the generation of ultra- and sub-harmonic responses were discussed based on simulations using Marmottant’s model This study suggests that the ultra-harmonic response from contrast microbubbles might be applicable for noninvasive pressure measurement

Investigation on the Interactions of Gas Explosion Flame and Reflected Pressure Waves in Closed Pipes

Abstract: In the present research work, the basic concepts of the coupling interaction mechanisms of gas explosion–reflected over pressure and flame were outlined by experiments and theory analysis. During the test processes, high speed cameras were employed for shooting the flame images, and pressure and flame sensors were set along the pipes for the signals detection. The research results show that the reflected reverse pressure wave has a significant influence on the transmission properties of gas explosion flame in the closed pipes. Under the effects of reversed pressure waves, gas explosion flames also present several discontinuous characteristics, such as flame ruptures or extinguishing. Generally, it can be inferred that the main effects of reflected explosion pressure on the flame propagations in the closed pipes are inhibitions. The present research results of these studies are shown to allow for a significant improvement in our ability to address practical problems of coal mine gas explosion mitigations.