Showing papers on "Overpressure published in 2001"

The effects of obstructions on overpressure resulting from premixed flame deflagration

Abstract: This paper introduces a new experimental set-up for investigating the effects of obstruction geometry, blockage ratio and venting pressure on overpressures resulting from premixed flame deflagration. Obstructions shaped as cylinders, triangles, squares, diamonds and plates or walls are studied here covering blockage ratios ranging from about 10% to more than 75%. It is found that the deflagration overpressure increases with increasing venting pressure. Also, the maximum overpressure increases, generally with increasing blockage ratio but the rate of increase depends on the obstruction geometry. The wall/plate type obstruction leads to the highest overpressures and the cylindrical obstruction yields the lowest overpressure. The time taken to reach the maximum overpressure decreases with increasing blockage ratio and changes with obstruction geometry implying that the flame accelerates faster due to changed local turbulence levels and length scales.

Coupled changes in pore pressure and stress in oil fields and sedimentary basins

Abstract: Repeated pressure measurements undertaken throughout the depletion of oil fields demonstrate that reduction in pore pressure is accompanied by a reduction in total minimum horizontal stress (σ h ), a phenomenon described herein as oil field-scale pore pressure/stress ( P p /σ h ) coupling. Virgin pressure measurements (i.e. those unaffected by depletion) through normally and overpressured sequences in sedimentary basins demonstrate that overpressure development is accompanied by an increase in σ h , described herein as sedimentary basin-scale P p /σ h coupling. With depletion of the Ekofisk Field, North Sea, minimum horizontal stress decreased at approximately 80% of the rate of reduction of reservoir pore pressure (i.e. Δσ h /Δ P p ≈0.8). Virgin pressures measured in exploration wells surrounding the Ekofisk Field (Norwegian quadrants 1 and 2) indicate that with overpressure development Δσ h /δ P p ≈0.73 (assuming shallow, normally pressured sequences are representative of overpressured sequences prior to overpressure development). Hence, despite the different temporal and spatial scales, the rate of decrease of minimum horizontal stress with pore pressure due to depletion of the Ekofisk Field is similar to the rate of increase of minimum horizontal stress with pore pressure due to overpressure development in the surrounding region. Basin-scale exploration pressure data in the Ekofisk region may thus provide an indication of the reservoir stress changes associated with depletion. Knowledge of such stress changes is critical because they can lead to the collapse of uncased wellbores, sand production and to faulting/fracturing and seismicity with field development.

Bubble growth during decompression of magma: Experimental and theoretical investigation

Abstract: A model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt. The model accounts for the interplay of three dynamic processes: decompression rate of the magma, deformation of the viscous melt around the growing bubble, and diffusion of volatiles into the bubble. Generally, these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically. For a better understanding of the physics of the processes, we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance, diffusion or linear decompression rate. We show that the solutions are a function of time and two dimensionless numbers, which are the ratios of either the diffusive or viscous time scales over the decompression time scale. The conditions for each growth regime are provided as a function of the two governing dimensionless parameters. Analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts. The model solutions, including the division to the growth regimes as function of the two parameters, provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure, supersaturation and gas volume fraction. The model results are in agreement with the conditions of Plinian explosive eruption (e.g. Mount St. Helens, 18 May 1980), where high gas overpressure is expected. The conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions, mostly in equilibrium degassing and partly in overpressure conditions. We show that in a situation of quasi-static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble, insensitive to the diffusivity profile.

Effects of explosion energy and depth to the formation of blast wave and crater : Field explosion experiment for the understanding of volcanic explosion

Abstract: We made field explosion experiments as an analogue of volcanic explosion to understand the relationship between the explosion condition and the resultant surface phenomena. The main parameters we employed were explosion depth and explosion energy. Through the experiments we confirmed that scaled depth, which is the depth divided by cube root of energy, is the main parameter determining the properties of explosive volcanism. The energy assigned to blast wave decreased exponentially against the scaled depth. The scaled crater diameter became maximum when the scaled depth was about 4 × 10−3 m/J¹/³. Scaled crater diameters by nuclear, chemical subsurface and some volcanic explosions were almost the same. From the scaling law, the overpressure at crater rim was estimated to be several MPa, which corresponded to typical rock strength. Probably the ground-forming materials were broken inside the area where overpressure exceeded their strength.

Mechanical haemolysis in shock wave lithotripsy (SWL): II. In vitro cell lysis due to shear

Abstract: In this work we report injury to isolated red blood cells (RBCs) due to focused shock waves in a cavitation-free environment. The lithotripter-generated shock wave was refocused by a parabolic reflector. This refocused wave field had a tighter focus (smaller beam width and a higher amplitude) than the lithotripter wave field, as characterized by a membrane hydrophone. Cavitation was eliminated by applying overpressure to the fluid. A novel passive cavitation detector (HP-PCD) operating at high overpressure (up to 7 MPa) was used to measure acoustic emission due to bubble activity. The typical 'double-bang' emission measured in the lithotripter free-field was replaced by a continuum of weak signals when the fluid was enclosed in a pressure chamber. No acoustic emissions were measured above an overpressure of 5.5 MPa. Aluminium foils were used to study shock wave damage and had distinct deformation features corresponding to exposure conditions, i.e. pitting and denting accompanied by wrinkling. Pitting was eliminated by high overpressure and so was due to cavitation bubble collapse, whereas denting and wrinkling were caused by the reflected shock wave refocused by the parabolic reflector. RBCs suspended in phosphate-buffered saline (PBS) were exposed to the reflected wave field from a parabolic reflector and also from a flat reflector. Exposure to the wave field from the parabolic reflector increased haemolysis four-fold compared with untreated controls and was twice that of cell lysis with the flat reflector. Recently we analysed deformation and rupture of RBCs when subjected to a flow field set up by a focused shock. The cell lysis results presented here are in qualitative agreement with our theoretical prediction that haemolysis is directly related to the gradient of shock strength and validates shearing as a cell lysis mechanism in SWL.

The seismic response to overpressure: a modelling study based on laboratory, well and seismic data

Abstract: We investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore-pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P-wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Overpressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore-pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs.

AAPG Memoir 76, Chapter 5: The Role of Shale Pore Structure on the Sensitivity of Wire-Line Logs to Overpressure

Abstract: Petrophysical characteristics of shales have been analyzed to improve our understanding of wire-line log response to overpressure. Bulk density and neutron porosity logs sometimes mask pore-pressure increases that are clearly evident on sonic and resistivity logs. This may be because sonic and resistivity logs respond to transport properties, whereas neutron and density logs reflect bulk properties.

Overpressure associated with a convergent plate margin: East Coast Basin, New Zealand

Abstract: The East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread low-permeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma−1) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma−1. Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.

Geological controls on overpressure in the Northern Carnarvon Basin

Abstract: A small, but significant fraction of wells drilled in the Northern Carnarvon Basin have encountered problems with overpressure: better pore pressure prediction would improve safety and economy for drilling operations. In the Northern Carnarvon Basin the occurrence of overpressure and likely mechanisms are under investigation as part of the Australian Petroleum Cooperative Research Centre (APCRC) Research Program on Pore Pressure Prediction. Previous workers have proposed a number of mechanisms as the main cause of overpressure including undercompaction, hydrocarbon generation, horizontal stress and clay reactions. A preliminary regional study was undertaken incorporating over 400 well completion reports which identified approximately 60 wells with mud weights greater than 1.25 S.G. A subset of these wells was investigated and more reliable but much scarcer pressure indicators such as kicks or direct pressure measurements were examined. Depth-pressure profiles of wells across the region are variable and commonly show pressure compartmentalisation. Using a range of indicators, it was observed that overpressured strata in the Barrow Subbasin: occur over a wide depth range (2,500 to 4,000+ mbsl); occur over a wide stratigraphic range (Late Triassic to Late Cretaceous); are not regionally limited by major structural boundaries; are associated with sequences dominated by finegrained sediments with variable clay mineralogy; and in depositionally, or structurally, isolated sandstones; and mainly to the west of the Barrow and Dampier Subbasins around the Alpha Arch and Rankin Trend, coinciding with thickest Tertiary deposition. Previous published work in the study area has tended to support hydrocarbon generation as the primary cause of overpressure, though more recent publications have emphasised compaction disequilibrium. The log response (DT, RHOB and NPHI) of overpressured clay-rich strata has been investigated to constrain the type of overpressure mechanism. A normal compaction trend has been derived for four stratigraphic groupings; Muderong Shale, Barrow Group, Jurassic and Triassic. All overpressure occurrences were accompanied by an increase in sonic transit time. Not all wells have suitable log data for evaluation, but all stratigraphic groups show some evidence of elevated porosity associated with overpressure consistent with disequillibrium compaction as a dominant mechanism. Overpressures in the Barrow Group in Minden-1 and the Jurassic section within Zeepaard–1 do not have accompanying porosity anomalies suggesting a different overpressure mechanism model is needed.

Overpressure phenomena in the Precaspian Basin

Abstract: Overpressured zones in the Precaspian Basin occur at depths below 4000 m. The most extensive zone is in the subsalt reservoirs throughout the basin. In spite of the extended literature database on overpressure phenomena, the Precaspian Basin and its Palaeozoic rock sequence is of a special interest because of its long geological history. The delineation of overpressure generation in the completely compacted sediments is very important in order to exclude disequilibrium compaction as the most significant overpressure mechanism for the Tertiary basins. Seal effectiveness and pressure compartment longevity have mainly resulted from other processes: origin of secondary minerals, hydrocarbon saturation and bitumen incorporation in mudrocks. Solid bitumen precipitation in clays forms a more perfect seal than the capillary pressure. The Astrakhan gas field is the best example, where the compositional evolution of formation fluids has changed the clay permeability.

Method for verifying the tightness of a tank system in a motor vehicle

Abstract: A method for verifying the tightness of a tank system, wherein an overpressure or a negative pressure is introduced into the tank system by means of a pressure source and the time curve of at least one operating parameter of the pressure source is detected when the overpressure/negative pressure is introduced up to a first point in time or until a first pressure level is reached, whereupon said operating parameter time curve is compared with an expected time curve for a tight tank. A lack of tightness is deduced if the detected time curve deviates from the diagnostic curve by at least one predefined value. The invention is characterized in that the overpressure/negative pressure in the tank system is raised until a second point in time or second pressure level is reached, and the pressure source is detected once again and compared with another expected time curve for a tight tank.

Numerical modelling of overpressure generation in the barrow sub-basin, northwest australia

Abstract: A numerical basin modelling approach was used to investigate the hydrodynamics and development of overpressures in a Jurassic Prospect in the Greater Gorgon area of the Barrow Sub-basin. Abnormally large fluid pressures have been encountered by numerous exploratory wells in the Carnarvon Basin; however, the mechanisms responsible for overpressuring were uncertain. To better evaluate the risk of encountering overpressures while drilling Jurassic targets in the area, quantitative basin modelling was conducted. Groundwater flow, heat transfer, and hydrocarbon generation were simulated along two geologic cross-sections. Model results for the fluid pressure history of the Jurassic Prospect were constrained by hydrodynamic data from specific wells in the region. A series of modelling experiments was used to determine the relative significance of compaction disequilibrium, tectonic uplift, organic maturation and permeability on overpressure generation. Results indicate that compaction disequilibrium and the permeability of shale layers are the dominant controls on overpressures, while organic maturation does not contribute a significant amount to the pressure anomaly. Quantitative basin modelling applied to pressure prediction provides critical insight needed prior to drilling and well construction.

Device for controlling a liquid flow

Abstract: A device for controlling a liquid flow by a gas pressure. The device includes a pressure chamber, in which an overpressure, an underpressure or atmospheric pressure may prevail. The pressure in the pressure chamber can be used for allowing the liquid flow to pass or for stopping the liquid flow. Furthermore the liquid flow can be limited to a predetermined rate of flow. The device may interalia be used in sewage systems, hydraulic engineering works and irrigation works.

Developments in the Congestion Assessment Method for the prediction of vapour-cloud explosions

Abstract: Publisher Summary The implementation of the Seveso 2 directive in Europe has emphasized the need for simple methods to predict the possible consequences of a gas explosion in industrial plants. Historically, the first approach used for such predictions was the analogy with high explosives such as TNT. However, the pressure decay from a detonation is markedly different from that produced by a vapor-cloud explosion (deflagration). Because there are other simple methods widely available, there is no reason to use TNT equivalence anymore. Other simple methods include the TNO multi-energy method and the Baker method. Congestion Assessment Method (CAM) is a clear method for deriving the source overpressure from the plant layout and fuel type. It was later on updated with new pressure decay curves that were calculated on the basis of a realistic explosion source. The obstacles in a real plant environment are typically much more complex than the simple arrays of cylinders used in most idealized experiments. The effect of this is to increase the “macroscopic” flame area generation above that pertaining to rows of uniform cylinders. There are several methods which have a similar approach to the prediction of overpressure in congested plant gas explosions. However, CAM takes a more realistic approach than the other methods in such cases.

AAPG Memoir 76, Chapter 1: Comparision of Overpressure Magnitude Resulting from the Main Generating Mechanisms

Abstract: Overpressure is created by two main processes: (1) stress applied to a compressible rock and (2) fluid expansion. Both processes are most effective in fine-grained lithologies, such as mudrocks and chalks. Both processes involve ineffective fluid expulsion to create pressures in excess of hydraulic equilibrium, emphasizing the importance of permeability (a poorly known rock property in fine-grained sedimentary rocks) in controlling pore pressure in the subsurface. Overpressure generation and fluid expulsion can be modeled assuming Darcy flow though a pore matrix. The basin conditions favoring high-magnitude overpressure from stress are a high sedimentation (loading) rate and/or strong lateral compressive forces. A high sedimentation rate, as a means to create rapid increase in temperature, also favors high- magnitude overpressure from fluid expansion mechanisms. An alternative method to achieve a rapid increase in temperature is a thermal pulse associated with tectonic or magmatic processes.

Porosity and effective stress relationships in mudrocks

Abstract: It has generally been assumed that porosity reduction during mechanical compaction of a sediment is controlled by the increase in vertical effective stress. But the theory of mechanical compaction shows that it is the mean effective stress which controls porosity reduction. According to published data, horizontal stresses increase with overpressure, as well as with depth, so mean stress and vertical stress profiles are poorly correlated in overpressured sections. In this study, a new methodology was developed whereby mudrock pore pressures were estimated principally by comparison of void ratios calculated from wireline log response with hydrostatic mean effective stress (the mean effective stress assuming the pore pressure is hydrostatic). These pressure estimates in the low permeability units were compared to the direct measurements in the aquifer units and an interpretation is made as to the origin of the excess pressure. The results of analysis of seven wells from SE Asia are presented including one study where seismic velocity analysis and basin modelling were performed to assess the pore pressure. The main conclusions of the study are: The proposed new methodology for estimating shale pore pressure from void ratio and mean effective stress analysis appears to be more consistent with the data and represents an improvement on previous methodologies using porosity and vertical effective stress or depth. Analysis of the mudrocks in this study indicates that the shales often appear to have significantly higher pressures than the adjacent aquifer units. The results of using mean (as opposed to vertical) effective stress analysis indicates that the pressure profiles in the wells studied, the profiles disequilibrium compaction can account for all or nearly all of the encountered overpressures. Evidence has been found for significant overpressure generated by fluid expansion in one of the seven wells studied.« Further work to refine the Breckels and Van Eekelen (1982) relationship between overpressure and horizontal stress is proposed to improve the accuracy of the methodology used in this study.

Rocket Launch-Induced Vibration and Ignition Overpressure Response

Abstract: Rocket-induced vibration and ignition overpressure response environments are predicted in the low-frequency (5 to 200 hertz) range. The predictions are necessary to evaluate their impact on critical components, structures, and facilities in the immediate vicinity of the rocket launch pad.

High-speed Visualization of Flame Propagation in Explosions

Abstract: Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions This area increase enhances the burning rate and increases the resultant explosion overpressure In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake The data quantifies the flame speed, shape and area for a range of obstacle shapes

Water Effects on Shock Wave Delay in Free Fields

Abstract: Water effects on shock wave propagation in a free field environment was studied numerically Numerical results are in good agreement with experimental measurementsThe results show that peak overpressure is reduced by 17%～46% when explosives are surround by water with the amount of 1～5 times of explosive mass

Differential pressure sensor device having over pressure protection

Abstract: The present invention provides overpressure protection for differential pressure detection elements to facilitate accurate pressure measurement using a minimal amount of fluid. The invention includes two volume adjustable cavities (34, 36), each of these cavities being located in fluid communication with opposite sides of a sensor (60).