Overpressure

About: Overpressure is a research topic. Over the lifetime, 3236 publications have been published within this topic receiving 34648 citations.

The nature and diversity of pressure transition zones

Abstract: Pressure transition zones occur where the rate of pressure increase or decrease exceeds a fluid gradient. Transition zones are found between intervals of permeable rock and, in each case, fluid movement is impeded by a 9seal9, which acts as a temporary barrier over geological time. The composite profile of pressure vs. depth is a function of three phenomena: (1) the mechanism responsible for abnormal pressure, (2) redistribution of pressure due to fluid movement during and after the mechanism is occurring, and (3) the lithological profile of the rock succession. Pressure profiles in some overpressured systems can reveal which of the mechanisms is causing the overpressure. Once overpressure has been created, pressure decay and transference away from the interval of generation can strongly influence the nature of transition zones. Transition zones are controlled in a fundamental way by the permeability of the rocks in which the abnormal pressure is found.

Blast Injuries and Blast-Induced Neurotrauma: Overview of Pathophysiology and Experimental Knowledge Models and Findings

Abstract: Explosions are physical phenomena that result in the sudden release of energy; they may be chemical, nuclear, or mechanical. This process results in a near-instantaneous pressure rise above atmospheric pressure. The positive pressure rise (“overpressure”) compresses the surrounding medium (air or water) and results in the propagation of a blast wave, which extends outward from the explosion in a radial fashion. As the front or leading edge of the blast wave expands, the positive phase is followed by a decrease in pressure and the development of a negative wave (“underpressure”) before subsequently returning to baseline. Figure 45.1 shows an idealized form of a shock wave (Friedlander wave) (Friedlander, 1955) generated by a spherical, uncased explosive in the air in free field conditions. The extent of damage from the blast wave mainly depends on five factors: (1) the peak of the initial positive-pressure wave (an overpressure of 690–1,724 kPa, for example, 100–250 psi, is considered potentially lethal) (Champion et al., 2009); (2) the duration of overpressure; (3) the medium of explosion; (4) the distance from the incident blast wave; and (5) the degree of focusing because of a confined area or walls. Intensity of an explosion pressure wave declines with the cubed root of the distance from the explosion. Thus, a person 3 m (10 ft) from an explosion experiences nine times more overpressure than a person 6 m (20 ft) away. Additionally, explosions near or within hard solid surfaces can be amplified two to nine times because of shock wave reflection (Rice and Heck, 2000). Indeed, it was observed that victims positioned between a blast and a building often suffer injuries two to three times the degree of the injury of a person in an open space. People exposed to explosion rarely experience the idealized pressure-wave form. Even in open-field conditions, the blast wave reflects from the ground, generating reflective waves that interact with the primary wave, thus changing its characteristics. In a closed environment (such as a building, an urban setting, or a vehicle), the blast wave interacts with the surrounding structures and creates multiple wave reflections, which, interacting with the primary wave and between each other, generate a complex wave (Ben-Dor et al., 2001; Mainiero and Sapko, 1996).Blast injuries are characterized by interwoven mechanisms of systemic, local, and cerebral responses to blast exposure (Cernak, 2010). When a blast generated by explosion strikes a living body, part of the shock wave is reflected and another fraction is absorbed becoming a tissue-transmitted shock wave. The transferred kinetic energy causes low-frequency stress waves that accelerate a medium from its resting state, leading to rapid physical movement, displacement, deformation, or rupture of the medium (Clemedson, 1956; Clemedson and Criborn, 1955). Thus, a militarily relevant blast injury model should be able to capture and measure these phenomena based on sufficient knowledge of shock wave physics, the characteristics of the injurious environment generated by an explosion, and the clinical manifestations and sequelae of the injuries. The purpose of this chapter is to outline the pathophysiology of blast-body/blast-brain interactions and to summarize the scientific evidence to date for the selection of appropriate experimental models for characterizing and understanding these interactions.

A Two-Dimensional Model of Overpressure Development and Gas Accumulation in Venture Field, Eastern Canada (1)

Abstract: We used a basin-scale model to reconstruct the development of overpressures in a sedimentary basin. The area of application is the Venture gas field, Nova Scotia, Canada. It is in an overpressured zone in which sedimentation rates have been relatively low since 90 Ma, and where large horizontal stresses have been evidenced. We investigated the importance of overpressure mechanisms using the Institut Francais du Petrole Temispack model, which integrates compaction, hydraulic fracturing, fluid flow, heat transfer, and the formation and migration of hydrocarbons. We first constructed a reference model using geological, geochemical, and physical data. Second, we studied sensitivity of the model to several input parameters and, from this, deduced additional constraints. The modeled permeability along faults and of shaly beds interlayered in the reservoirs must be very low to enable both the local and regional distributions of overpressure and gas to be reproduced. Several runs of the reference model studying the relative importance of possible overpressuring mechanisms and the relative contribution of plausible source rocks to gas accumulations showed that gas generation and accumulation, and lateral compression contribute little to overpressuring, which is mainly accounted for by compaction disequilibrium in spite of low sedimentation rates. Faults and shaly beds play an important role in maintaining the overpressure. Gas generation in the reservoir units or close to them does not contribute significantly to the field gas accumulations. Gas sources are likely to be found in the underlying formations.

Wind-tunnel investigation of the validity of a sonic-boom-minimization concept

Abstract: The Langley unitary plan unitary plan wind tunnel was used to determine the validity of a sonic-boom-minimization theory. Five models - two reference and three low-boom constrained - were tested at design Mach numbers of 1.5 and 2.7. Results show that the pressure signatures generated by the low-boom models had significantly lower overpressure levels than those produced by the reference models and that small changes in the Mach number and/or the lift caused relatively small changes in the signature shape and overpressure level. Boundary-layer effects were found in the signature shape and overpressure level. Boundary-layer effects were found to be sizable on the low-boom models, and when viscous corrections were included in the analysis, improved agreement between the predicted and the measured signatures was noted. Since this agreement was better at Mach 1.5 than at Mach 2.7, it was concluded that the minimization method was definitely valid at Mach 1.5 and was probably valid at Mach 2.7, with further work needed to resolve the uncertainty.