Showing papers on "Overpressure published in 1999"

Overpressure detection from compressional- and shear-wave data

Abstract: By analyzing experimental data we show that in many room-dry rocks, the Poisson's ratio (PR) decreases with decreasing differential pressure (confining minus pore pressure). This means that in gas-saturated rocks, PR decreases with increasing pore pressure. We confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling. This effect can be used as a new tool for overpressure detection from surface seismic, cross-well, sonic logs and measurements ahead of the drill bit.

Fluid overpressure and stress drop in fault zones

Abstract: Using the average aspect (length/maximum thickness) ratios of 379 mineral-filled extension (mode I) veins from an active fault zone, the fluid overpressure, during their development, with reference to the minimum compressive principal stress, σ3, is estimated at 20 MPa. Emplacement of such veins increases σ3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures. On meeting a subhorizontal stress barrier, vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic. In this model, stress barriers, and thus water sills, can form at any depth in, and in any type of, fault zones. For such a high fluid pressure, the product of the coefficient of sliding friction and the normal stress in the Modified Griffith Criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock. For typical in situ tensile strengths of 2–3 MPa, the driving stresses for slip on overpressured fault planes is 4–6 MPa. These results are in good agreement with the commonly measured average static stress drops of 3–6 MPa during earthquakes.

Gas Generation and Accumulation in the West Siberian Basin

Abstract: We appreciate the discussion by Murris (2001) and take the opportunity to clarify some of his objections and discuss some of the still open questions in more detail. Murris's main objections to the conclusions of Littke et al. (1999) concern, first, the influence of overpressure in the Lower Cretaceous and Upper Jurassic on hydrocarbon distribution and, second, the model of gas release by exsolution (degassing) from deep ground water as the main mechanism for accumulating the huge gas fields of northern West Siberia. The first objective of Murris's discussion is how the overpressure in Jurassic strata might have influenced the distribution of hydrocarbons in the overlying Cretaceous sediments. Two crucial points regarding this question are the reason and timing of overpressuring and its influence on hydrocarbon movement. We agree with Murris that the effect of compaction disequilibrium is not sufficient to explain the extent of overpressure in the Urengoy area and that hydrocarbon generation in the Bazhenov Formation has contributed to the buildup of overpressure. Two-dimensional basin modeling along a seismic line through the Urengoy field, however, showed that hydrocarbon generation in Jurassic source rocks (Bazhenov and the Tyumen formations) did only generate sufficient overpressure to explain the recent situation if the models were laterally completely sealed. This sealing appears to be highly unlikely. Furthermore, hydrocarbon generation ceased during the Miocene, when the tectonic movement started to lift the source rocks into shallower depths, where lower temperatures were reached. Thus, any overpressure caused by petroleum generation must have been preserved over a period of about 5-20 m.y. In addition, the overpressure is not restricted to the Jurassic Bazhenov Formation but extends into the Cretaceous, including the Achimov sands. Uplift might have contributed to the overpressure, however. Maximum pore pressure from the stage of deepest burial might have …

Overpressures: Causal Mechanisms, Conventional and Hydromechanical Approaches

Abstract: Abnormal fluid pressure regimes are commonly encountered at depth in most sedimentary basins. Relationships between effective vertical stress and porosity have been applied, since 1970 to the Gulf Coast area, to assess the magnitude of overpressures. Positive results have been obtained from seismic and basin-modeling techniques in sand-shale, vertical-stress-dominated tertiary basins, whenever compaction disequilibrium conditions apply. However, overpressures resulting from other and/or additional causes (tectonic stress, hydrocarbon generation, thermal stress, fault-related transfer, hydrofracturing. . . ) cannot be quantitatively assessed using this approach. A hydromechanical approach is then proposed in addition to conventional methods. At any depth, the upper bound fluid pressure is controlled by in situ conditions related to hydrofracturing or fault reactivation. Fluid-driven fracturing implies an episodically open system, under a close to zerominimum effective stress regime. Sound knowledge of present-day tectonic stress regimes allows a direct estimation of minimum stress evolution. A quantitative fluid pressure assessment at depth is therefore possible, as in undrained or/and compartmented geological systems, pressure regimes, whatever their origin, tend to rapidly reach a value close to the minimum principal stress. Therefore, overpressure assessment will be improved, as this methodology can be applied to various geological settings and situations where present-day overpressures originated from other causal mechanisms, very often combined. However, pressure trends in transition zones are more difficult to assess correctly. Additional research on cap rocks and fault seals is therefore required to improve their predictability. In addition to overpressure assessment, the minimum principal stress concept allows a better understanding of petroleum system, as fault-related hydrocarbon dynamic transfers, hydrofractured domains and cap-rock sealing efficiency depend on the subtle interaction, through time, between overpressure and minimum principal stress regimes.

Clearing of blast waves from building facades.

Abstract: In the blast loading of buildings by high-explosive charges, the phenomenon of clearing has been overlooked recently-this study offers a revisit. Clearing occurs when a blast wave interacts with the facade of a building, generating a reflected overpressure. The top and sides of the building will be loaded by the lower-magnitude 'side-on' pressure and there will be a pressure difference between these surfaces. Pressure equalization occurs via clearing, with the pressure on the facade decaying rapidly. Contrastingly, for an infinitely large reflecting surface, the reduction will be slower because there will be no region of lower pressure accessible. In this experimental study, small spherical charges of plastic explosive were detonated above instrumented, rigid, circular steel plates carrying flush-mounted pressure transducers. The results are presented as graphs of peak overpressure and peak scaled impulse plotted against scaled distance and are compared with predictions from the established ConWep database. The study demonstrates how the effects of clearing are more pronounced towards the edges of the plates, particularly for the scaled impulse values. As the scaled distance increases, there is relatively little difference between the experimental and database predictions. The results reinforce the use of ConWep as a conservative design tool because it overestimates the scaled impulse.

Charge and overpressure modelling in the North Sea: multi-dimensional modelling and uncertainty analysis

Abstract: Over the past one and a half decades developments in our understanding of earth physics and the increase in computer power have enabled quantitative modelling of subsurface temperature, hydrocarbon charge, migration, overpressures and palinspastics. The North Sea has provided the test bed for many of these efforts. Initial developments focused on one dimensional models of temperature and charge as these were computationally possible at that time, and by the late 80s such models were widely available. Despite the current availability of 2D charge modelling tools, most modelling carried out at present is still ID. Hydrocarbon generation is driven by the temperature field which depends on the heat flow, which is only truly vertical in exceptional circumstances. In principle, a whole lithosphere ID model will only agree with a 3D model for a horizontally layered section. Thermal conductivity anisotropy (lateral/vertical conductivity) of mudstones can reach three. Lateral differences in geology also promote changes in geothermal gradients and therefore lateral heat movement. At the lithosphere scale, differences in the depth to the convecting asthenosphere, and the distribution of hot spots may also cause lateral heat flow. In the North Sea, the complex geology particularly within the syn-rift section makes heat flow strongly 3D. In the Inner Moray Firth and Halten Terrace there are considerable differences between the predictions of fully transient 1D and 3D models for the temperature and hydrocarbon expulsion flux histories of the kitchen areas. Differences can exceed 15°C close to major faults: kitchen areas in the hanging walls are typically cooler than would be expected from ID models, whereas basement cored highs are typically hotter than might be expected. The differences between the predictions of ID and 3D models are driven by structural effects and are thus temporally variant. The importance of 3D effects in charge modelling is compounded in hydrocarbon migration models. Temporal changes in carrier bed interconnectivity, fault seal properties and basin structure influence migration, re-migration and even refilling of breached traps. For instance, hydrocarbons found in the Tertiary of the Central North Sea are derived from underlying Jurassic reservoirs where high overpressures have lead to hydrofracturing and hence leakage. Numerical models of overpressure generation, including rapid sedimentation (under-compaction), thermal expansion (aquathermal pressuring), smectite to illite transition (water generation), etc., tell only part of the story: in order for pressures to rise the fluids have to be contained by rocks with sufficiently low permeability. Furthermore, overpressures are transient and gradually diminish when generation ceases. These processes can be described by a relatively simple differential equation, which can be solved to predict overpressures in one or more dimensions. However, in the North Sea, widespread aquifers (e.g. the Fulmar Formation sands) allow pressure transmission out of synclines. Thus, simple ID models are of limited use, and pressure modelling requires prediction of the stratigraphy and temporal variations in aquifer connectivity resulting from structural movement. Palinspastic restoration is thus a prerequisite. In areas such as the Central North Sea, where there is good pressure calibration and relatively simple stratigraphy, modelling enables prediction of overpressure, trap integrity and reservoir continuity. One problem with physical models discussed above has been their inability to incorporate uncertainties. New models are capable of taking into account uncertainties in both model parameters (e.g. compaction coefficients) and input data (e.g. stratigraphic age) to calculate the uncertainty in the modelled variable (e.g. hydrocarbon flux history). Such models provide a much more realistic evaluation of wildcat prospects. The potential of basin models for prospect evaluation and field development is still in its infancy. From a commercial view point, in areas such as the North Sea where data are readily available to most competitors there is little advantage in having the same models as everybody else. The advantage lies in being able to evaluate acreage faster and with greater accuracy, both in the prediction and the uncertainties.

The origin and influence of overpressure with reference to the north west shelf, australia

Abstract: The dominant cause of overpressure in basins is rapid loading of fine-grained sediments in which incomplete dewatering leads to additional overburden load being supported partly by the pore fluids. The principal controls on the magnitude of overpressure created are permeability and compressibility of the fine-grained rocks, coupled with the loading or sedimentation rate. High magnitude overpressure requires rapid sedimentation and/or evolution of sediment permeability to nanoDarcy values at shallow depth. By contrast, most fluid expansion mechanisms can be shown to be ineffective at generating large magnitude overpressure at realistic basin conditions. Only gas generation (either directly from kerogen or by oil to gas cracking) has the potential to create large magnitude overpressure, and only if the connected reservoir volume is very restricted. The origin of overpressure in the North West Shelf, especially the Northern Carnarvon Basin has previously been suggested to be due to petroleum generation, principally because the top of overpressure is coincident with, or lies below, the hydrocarbon generation window. To achieve high magnitude overpressure by this mechanism requires large volumes of gas generative source rocks connected to reservoirs of extremely limited extent. The volume of reservoir rocks in the basins is relatively high, and gas generation appears to be only a secondary mechanism. The most likely origin of overpressure is burial of the Jurassic and Lower Cretaceous group sediments (including the Muderong Shale) with early development of the Muderong Shale as a pressure seal. Lateral stress cannot be discounted as an additional mechanism of overpressure generation. However, lateral strain appears to be significantly less than vertical strain. Overpressure has the potential to influence the petroleum system in the North West Shelf if there has been high magnitude overpressure for prolonged periods of geological time. Normally pressured units today may have had a history of overpressure in the geological past. Reservoir quality can be enhanced by overpressure, but trap seal integrity either strengthened or weakened by overpressure. Timing of maturation and migration of hydrocarbon can also be affected.

Analysis of Overpressured Reservoirs with A New Material Balance Method

Abstract: This work presents application of a new material balance method to detect aquifer influence and calculate water influx and original gas in place for four over-pressured reservoirs. Calculation of water influx needs to satisfy a set of three equations as opposed to the existing method of one equation of unit slope. In each application, the presence of aquifer influence was identified first, and then material balance was used to determine original gas in place and water influx. The overpressure effect was handled by integrating rock compressibility over operating reservoir pressure. Compositional effects were modeled with R v (volatile oil/gas ratio) by matching PVT data using Walsh-Towler algorithm or an Equation-of-State. This new method is internally consistent, which avoids potential pitfalls of existing methods. Comparison with other methods in analyzing overpressure reservoirs shows this new method is more robust and comprehensive.

Airblast Effects Research: Small-Scale Experiments and Calculations

Abstract: : A series of 1:15 scale model experiments were conducted to investigate the effects of loading density on the dynamic airblast flow parameters produced by spherical charge detonations in an underground ammunition storage magazine (shotgun design) Chamber loading densities ranged from 04 to 50 kg/cubic meter (TNT Equivalent) Total pressure measurements were made on the center line of the model access tunnel using a miniaturized, probe type, pressure mount Side-on and total pressure measurements provide the data required to quantify the pressure regime at these locations within the model The data also provide an estimate of the dynamic flow regime Calculations for a full scale magazine were performed using the CTH hydrocode Comparisons of the CTH calculated full scale pressure waveforms with the scaled-up recorded waveforms are excellent Detonations in an underground magazine are confined so that the blast wave follows a restricted flow path to the portal Comparisons of the measured and calculated pressures show that this confinement drastically increases the pressures of the blast wave by an order of magnitude or more above that produced in the free field by the same explosion source

Method for checking the operability of a tank-venting system of a vehicle

Abstract: The invention is directed to a method for checking the operability of a tank-venting system of a vehicle. A pressure source is operatively connected to the tank-venting system and a first fill level of the tank is determined and stored at the start of a standstill of the vehicle. An overpressure relative to atmospheric pressure is introduced into the tank-venting system during the standstill of the vehicle and an operating characteristic variable of this pressure source is detected as the overpressure is introduced into the tank-venting system. A conclusion as to the presence of a leak in the tank-venting system is drawn from the operating characteristic variable. A second fill level of the tank at the end of the standstill is determined and stored. The difference of the first and second fill levels is formed and a determination is made as to whether the difference exceeds a pregiven threshold value; and, only if the threshold value is exceeded, a fault announcement is outputted.

10. fluid overpressures in western mediterranean sediments, sites 974-979 1

Abstract: A compaction model was developed and applied to five sites drilled as part as Ocean Drilling Program Leg 161, during which overpressured sediments were cored. The long-term compaction coefficient for the porosity variation is very high (1.3‐ 3.4 × 10‐7 Pa‐1). Assuming that the fluid overpressures result from disequilibrium compaction, the fluid overpressures can be estimated from the difference between the hydrostatic porosity (i.e., the porosity distribution that would have resulted at equ ilibrium compaction with hydrostatic pore-fluid pressures) and the porosity deduced from downhole measurement analysis. Fluid overpressuring starts at very shallow depths (120‐150 m below seafloor) and, in some cases, very quickly reaches the reduced lithostatic pressure. The reduced lithostatic pressure corresponds to the upper limit of pore-fluid overpressure before which natural fracturing occurs in unconsolidated sediments. Fluid overpressure is commonly correlated with the presence of gas (mostly methane). The ingredients for capillary sealing, two fluid phases in a layered sequence of fine and coarse sediments, exist in all the sedimentary sections described in this paper, and layers filled with free gas are clearly revealed as s pikes in the porosity derived from the density log. Capillary sealing is shown to be quantitatively capable of retaining the overpres sures observed.

Effect of increased ambient pressure on lithotripsy‐induced cavitation in bulk fluid and at solid surfaces

Abstract: By reducing cell damage but maintaining stone comminution, overpressure (OP)—increased hydrostatic pressure—offers the promise of safer more effective shock wave lithotripsy (SWL) [Delius, UMB 23, 611–617]. A current hypothesis is that stones offer cavitation sites—sites of violent bubble activity—when the cavitation nuclei in a free field have been dissolved into the fluid by overpressure. Cavitation around solid bodies and in a free field with and without overpressure is investigated. High‐speed photography and dual passive cavitation detection (PCD) were used to measure the temporal and spatial extent of cavitation in a 10‐cm3 plastic chamber that minimally altered the shock wave. In a free field at OP=0 atm, cavitation bubbles formed and collapsed in 280±20 μs. The time of collapse tc was halved at OP=1 atm, and cavitation activity was not detectable at OP=3 atm. With a stone present, a bubble ∼10 times larger than free‐field bubbles grew on the surface of the stone and collapsed after 440±50 μs. With...

Wind-Tunnel Overpressure Signatures From a Low-Boom HSCT Concept With Aft-Fuselage-Mounted Engines

Abstract: A 1:300 scale wind-tunnel model of a conceptual High-Speed Civil Transport (HSCT) designed to generate a shaped, low-boom pressure signature on the ground was tested to obtain sonic-boom pressure signatures in the Langley Research Center Unitary Plan Wind Tunnel at a Mach number of 1.8 and a separation distance of about two body lengths or four wing-spans from the model. Two sets of engine nacelles representing two levels of engine technology were used on the model to determine the effects of increased nacelle volume. Pressure signatures were measured for (model lift)/(design lift) ratios of 0.5, 0.63, 0.75, and 1.0 so that the effect of lift on the pressure signature could be determined. The results of these tests were analyzed and used to discuss the agreement between experimental data and design expectations.

Could tectonic overpressure cause ultrahigh-pressure metamorphism?

Abstract: Could ultrahigh-pressure metamorphic rocks be caused, at depth of 32 km, by tectonic overpressure resulting from differential stress? The differential stress is limited by rock strength, which depends on strain rate and temperature. Therefore, tectonic overpressure could not go beyond 1 GPa, and could not cause the uitrahigh-pressure metamorphism. The stress-strain in plastic deformation, or in plastic stage of elasto-plastic deformation, has no linear relation, and should not be described by using linear equation of elastic deformation.

Overpressure prevention and/or reduction method in closed room

Abstract: The method involves using a device which produces an aperture in a wall of the closed room at an overpressure and/or at an escape of gases in the closed room at a filling of a balloon-like and elastic body, especially an airbag. The method involves preventing and/or reducing an overpressure of gaseous media in a closed room, especially a vehicle interior, due to an increase of the interior pressure, and for adjusting the interior pressure with an environmental pressure and/or for a ventilation of a closed room, after an escape of gases at a filling of a balloon-like and elastic body, especially an airbag, in the closed room, whereby a device produces an aperture in a wall of the closed room at overpressure and/or at an escape of gases in the closed room. An Independent claim is included for an arrangement for the prevention and/or reduction of overpressure.

Rod-like article container having overpressure absorbing mechanism

Abstract: PROBLEM TO BE SOLVED: To simplify structure at a low-cost and to improve the reliability in the airtightness by axially moving a cup part inversely to a takeout part in generating overpressure in a sleeve, by at least partially absorbing the overpressure, and by providing an elastic means under a member for sealing. SOLUTION: A sealing structure formed in the bottom and the top of a cup part compresses air captured in a part of a sleeve 2 partitioned by two seal areas. This compression generates overpressure in the sleeve 2 inside, and the overpressure axially compresses a foam body block 25 and slightly returns the cup part 11 back to the sleeve 2 inside. The axial height of a housing 18 formed in a screw rod member 8 is sufficient enough to respond thereto and axially move the lock end 17 across the approximately equal distance to the compression of the foam body block 25. Responding to the compression, the compressed air under the foam body block 25 is released via an unsealed lock area between an operation member 4 and the sleeve 2. COPYRIGHT: (C)2000,JPO

Experimental study of the restrain method of shock wave effect on loading equipment

Abstract: Harmful deformation on loading equipment is caused when it is lashed by initial shock wave of jet flow field It has harmful effect on the compatibility of rocket and loading equipment This fact should be paid attention when rocket is loaded on new equipment The initial shock wave can be studied by using of electrical measurement technology and optical display method of flow field On the base of the experimental study of initial shock wave, the restrain technology is studied to reduce the harmful effect of the wave The restrain device is put at the rear of the jet By using of this technology, the peak overpressure of shock wave is reduced by a relatively large scale The compatibility of rocket and its loading equipment could be improved The restrain result is also discussed in this paper

Analytical Blast Model Formulation With Computer Code

Abstract: : Overpressure time history data from warhead blast experiments yield peak overpressure P as a function of spatial position. Dr. Owen Litt has proposed a model for P based on the peak overpressure characteristics of a bare spherical charge. The direction independent peak overpressure function for a bare spherical charge is modified to have nonspherical level surface structure by specifying surfaces of constant peak overpressure. This introduces a directional component into the model. In this report, the original formulation is refined and generalized and a mathematical model and computer code are presented to evaluate the function. Such a computational device is required for model parameter estimation and experiment design.

Baking oven devaporation

Abstract: A baking oven comprises at least one hearth with a baking chamber, a heater, a vaporization device for the supply of vapor and/or fresh air, a devaporization device for the discharge of vapor and/or used air and an overpressure reduction device for overpressure in the baking chamber to be reduced. The devaporization device and the overpressure reduction device are connected to the hearth via a joint discharge duct.