Showing papers on "Overpressure published in 1997"

Mechanisms for Generating Overpressure in Sedimentary Basins: A Reevaluation

Abstract: Overpressure can be produced by the following processes: (1) increase of compressive stress, (2) changes in the volume of the pore fluid or rock matrix, and (3) fluid movement or buoyancy. Loading during burial can generate considerable overpressure due to disequilibrium compaction, particularly during the rapid subsidence of low- permeability sediments. Horizontal stress changes can rapidly generate and dissipate large amounts of overpressure in tectonically active areas. Overpressure mechanisms involving change in volume must be well sealed to be effective. Fluid volume increases associated with aquathermal expansion and clay dehydration are too small to generate significant overpressure unless perfect sealing occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depending upon the kerogen type, abundance of organic matter, temperature history, and rock permeability; however, these processes may be self-limiting in a sealed system because buildup of pressure could inhibit further organic metamorphism. The potential for generating overpressure by hydrocarbon generation and cracking must be regarded as unproven at present. Fluid movement due to a hydraulic head can generate significant overpressure in shallowly buried, "well-plumbed" basins. Calculations indicate that hydrocarbon buoyancy and osmosis can generate only small amounts of localized overpressure. The upward movement of gas in an incompressible fluid also could generate ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 17, 1995; revised manuscript received September 4, 1996; final acceptance January 20, 1997. 2Department of Geological Sciences, Durham University, South Road, Durham DH1 3LE, United Kingdom. Osborne e-mail: M.J.Osborne@ durham.ac.uk; GeoPOP web site http://www.dur.ac.uk/~dgl0zz7/ We wish to thank the companies that support the Geosciences Project on Overpressure (GeoPOP) at the universities of Durham, Newcastle, and Heriot-Watt: Agip, Amerada Hess, Amoco, ARCO, Chevron, Conoco, Elf Exploration, Mobil, Norsk Hydro, Phillips Petroleum UK Company Limited, Statoil, and Total. We also thank Neil Goulty (Durham) for commenting on an earlier draft of this paper. Osborne thanks Gordon Macleod (Newcastle) for help with geochemical modeling.

Secondary Porosity Generation During Deep Burial Associated with Overpressure Leak-Off: Fulmar Formation, United Kingdom Central Graben

Abstract: The overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known. From the resulting effective stress, the maximum potential porosity (MPP) can be calculated. The MPP is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure. Measured rock porosities should be at or below the MPP. We have determined the MPP history for the Fulmar Formation sandstones (Upper Jurassic) of the Central Graben, North Sea, and have compared the predictions to measured core data. We conclude that for the majority of the Fulmar Formation sandstones, the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation. However, in wells sited close to regional overpressure leak-off points, the porosity has been significantly increased from an end-of-Oligocene low (mean 21%) to the present-day values (mean 31%). This porosity increase occurred by feldspar dissolution, with the reaction products being removed from the sandstones. Secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured, although still part of an open hydrogeological system. The generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted.

Sealing efficiency of shales

Abstract: The sealing efficiency of shale layers is studied through the hydraulic parameters which are required for sustaining overpressure during geologically significant periods. Assuming a 1D sedimentary complex composed of a sealing layer overlying a permeable one, we give approximate solutions for the dissipation of a given initial overpressure. The time constant for relaxation involves the thickness, hydraulic conductivity and specific storage of the seal and also those of the permeable layer. The values of the various hydraulic parameters are discussed. It is argued first that the specific storage corresponding to plastic deformation during burial compaction is larger than the one which would correspond to elastic deformation. When taking into account (i) plausible values for specific storage of the upper shale layer and (ii) the storage effect of the lower permeable layer, it is found that a shale layer of several tens of m with an hydraulic conductivity of the order of 10-15 ms-1 maintains overpressure for 1 Myr. For such hydraulic parameter values, in the absence of on-going pressuring forces, initial overpressures would decay with a time constant (corresponding to a decrease by a factor e ∼, 2.7) of 1 Myr. This is interpreted as supporting a dynamic origin for observed abnormal pressures.

Pressure prediction from seismic data: implications for seal distribution and hydrocarbon exploration and exploitation in the deepwater Gulf Of Mexico

Abstract: Pore-pressure prediction before drilling is critical for several reasons. It is required to assess “seal” effectiveness, map hydrocarbon migration pathways and analyze “trap” configuration and geometry of a prospective basin. Furthermore, it aids in the well planning process by providing proper casing and mud program design which can help prevent dangerous “blow-outs”, lost circulation of drilling fluids and stuck pipes. The conventional techniques for pressure prediction are limited by two factors: establishing a “normal” trend of an attribute (usually a porosity indicator) and a set of calibration curves relating “overpressure” to deviation from the normal trend of that attribute. Thus, these techniques cannot be used in rank wildcat areas and areas such as the deep water environment (water depth greater than 330 m) of the Gulf of Mexico where normal compaction trends are usually non-existent. At BP, a new technique for pressure prediction has been developed. The essentials of this technique are as follows. It uses a proprietary transform that relates velocity to effective stress (defined as the difference between overburden and pore-pressure), temperature and gross lithology directly. It takes into account the major causes of overpressure mechanisms: undercompaction, clay dehydration and transformation, buoyancy and charging of fluids in dipping, permeable beds. It does not require local calibration and predicts effective stress directly, which is the most fundamental quantity for pressure prediction. In this paper a brief description of this technology is presented together with several examples from the deepwater environment of the Gulf of Mexico. Applications are made in 1-D, 2-D and 3-D and have enabled explorationists to define “seal” failure risks in deepwater prospects. Drilling experiences have shown that this technology can predict pressures to within 0.5–0.75 pounds per gallon (ppg) at target depths, provided the “low-frequency” trends of seismic interval velocities are of good quality and “close” to well velocities to within 5–10%. The quantitative reliability of the method depends on two factors: availability of high quality seismic velocity data and an understanding of the rock properties. The vertical (temporal) resolution is limited by the available bandwidth of the seismic velocity data whereas the spatial resolution is dictated by the acquisition parameters and the frequency of velocity analysis (CDP spacing and panels for analysis).

Venting Of Deflagrations In Buildings And Equipment : Universal Correlation

Abstract: The universal correlation for gaseous deflagration venting in coordinates "dimensionless reduced pressure turbulent venting parameter" (Molkov, 1995) have been verified on the widened range of experimental data. These included a collection of 39 literature experimental data, processed with proposed earlier theory (Molkov et al., 1981-1995). Correlation covers the most wide range of explosion conditions at initial atmospheric pressure: enclosure volumes up to 8087 m3; vent ratios F/@I3 0,09-1,23; initially uncovered and covered vents with release overpressure 0-32 kPa and cover inertia 0-23 kgim2; maximum explosion overpressure down to 0,s kPa and up to 380 kPa; most dangerous near stoichiometric air mixtures of natural gas, methane and propane; various shapes of enclosures with ratio of sizes up to 4 : l ; point, plane and jet ignition; with and without complex obstacles and/or external explosions. It has been proved that the universal correlation is a reliable tool for fire and explosion safety engineering. It has been shown that best-fit method usually used by researchers for comparison of theoretical and experimental pressure-time profiles should exploit two adjustable parameters turbulence factor x and discharge coefficient p for satisfactory results.

Modeling of Ignition Overpressure in Minuteman Silos

Abstract: The measured ignition overpressure (IOP) generated in Minuteman silos during launches from "deep" 90-ft silos is as much as 50% larger than that from "shallow" 80-ft silos. To explain this phenomenon, five analyses have been conducted and are described in this paper: (1) a one-dimensional linear wave analysis exposes the physical cause for the wave and correctly approximates its timing and magnitude and near-independence of motor chamber pressure rise rate, (2) a one-dimensional Euler analysis predicts more accurately the timing and magnitude of the wave due to its allowance for nonquiescent nonlinear flow, and (3) two-dimensional analyses using three different Computational Fluid Dynamics (CFD) codes reveal the details of the vortical mixing and reacting flow in the bottom of the silo. All the analyses predict that in the absence of exhaust afterburning, deepening the silo delays the time at which the peak IOP occurs, but does not change the magnitude of the predicted peak IOP; hence, no enhancement is predicted for deep silos. However, when afterburning of the fuel-rich rocket exhaust with the ambient silo air is accounted for, the predicted peak IOP increases significantly; this increase is nearly twice as large in the deep silo as in the shallow silo due to the nearly doubled amount of available air and extent for mixing of this air with the propellant gas. The CFD simulations describe for the first time the physical details of this afterburning phenomenon.

Spark plug for venting excessive pressure

Abstract: An internal combustion engine spark plug provides an overpressure release mechanism to avert engine component damage as a result of hydrostatic lock caused by liquids (typically water) entering the combustion chamber under operating conditions. This spark plug incorporates predictably and adjustably weakened structural zones such that, upon encountering overpressure situations, the central portion of the plug is ejected, generating sufficient flow area to expel gasses and liquids from the combustion chamber and venting the cylinder to the atmosphere. The invention is also useful in the detection and avoidance of damage under conditions of detonation.

Impact Ignition of New and Aged Solid Explosives

Abstract: The critical impact velocities of 60.1 mm diameter steel projectiles required to produce ignition are measured for new and aged confined charges of the HMX-based solid explosives LX-10, LX-04, PBX-9404, and PBX-9501. External blast overpressure gauges are employed to determine the relative violence of the explosive reactions. The experiment is modeled in DYNA2D using recently developed material strength models, and thermal energy deposition thresholds for impact ignition are found.

How Overpressure and Diagenesis Interact in Sedimentary Basins – Consequences for Porosity Preservation in HPHT Reservoir Sandstones

Abstract: This paper critically examines the link between diagenesis and overpressure, how diagenetic reactions may produce overpressure, and conversely, how overpressure may influence diagenetic reactions. These questions are important, because they may hold a key to predicting overpressure and reservoir porosity in sedimentary basins. Our results are summarised below: Effect of diagenetic reactions on overpressure Smectite dehydration is unlikely to be a primary cause of overpressuring in sedimentary basins, because the volume of fluid released is small, and dehydration is inhibited by the build up of pressure. In addition, the exact reaction involved in the smectite to illite transition is not presently known. Thus it is not certain that there is an actual volume increase during the transformation. Quartz cementation is also unlikely to be a direct cause of overpressuring or underpressuring, because extensive cementation/dissolution requires an open system in which fluid is free to move and dissipate abnormal pressures. By contrast, the transformation of gypsum to anhydrite can potentially generate a fluid pressure significantly in excess of overburden pressure, depending on the rock permeability. The existence of basin-wide diagenetic pressure seals remains unsupported by direct evidence. Effect of overpressure on diagenetic reactions Overpressure inhibits pressure solution, and retards the development of late diagenetic quartz overgrowths, helping to preserve high porosities in reservoir sandstones during deep burial. The rate of quartz growth is fastest under conditions of high effective stress.

高速列車トンネル突入による圧縮波の衝撃波への遷移〔流体工学, 流体機械〕

Abstract: A guantitative investigation is carried out on the generation of a compression wave in highspeed train tunnels and on its transition to a shock wave. Discussions are based on results of experiments which were conducted in a scaled train tunnel simulator. The overpressure level of the compression wave is formulated using a modified Hara-Maeda formula, in which an effective blockage factor is taken into account. The length of the shock wave transition region is estimated from the pressure gradient of the compression wave at the tunnel entrance. The installation of a hood at the entrance is found to disperse the pressure rise, thereby the length of the shock wave transition region is increased. However, the length decreases significantly with an increase in the train speed. Conditions under which the entrance hood works effectively are formulated. Consequently, it is concluded that a critical train speed exists beyond which the entrance hood becomes no longer effective for retarding the transition.

Behavior and evaluation of an existing underground structure subjected to impulsive loads from an internal explosion

Abstract: An explosion is the result of a rapid chemical reaction which generates transient air pressure waves called blast waves. There has been much research on the processes of blast wave formation, propagation of blast waves, and quantification of the incident and reflected blast overpressures. The magnitude of blast overpressure, in a partially vented environment, is mainly a function of the type and quantity of detonating material, the amount of available venting, and the orientation and configuration of the reflecting surfaces. In addition to blast overpressure, an explosion can also generate high energy missiles (such as fragments), shock loads, and rapid rise of temperature in the confined space. This study concentrates on the effects of blast overpressure on a 40 feet diameter reinforced concrete cylinder with a hemispherical dome roof, supported on a 3 feet thick reinforced concrete pad, and buried under a minimum of 15 feet of soil used for radiation shielding at the top of the dome. The scope of this study is to determine whether the structure can withstand the blast overpressure generated by the postulated explosion without exceeding allowable design criteria.