Showing papers on "Overpressure published in 2007"

Failure patterns caused by localized rise in pore-fluid overpressure and effective strength of rocks

Abstract: [1] In order to better understand the interaction between pore-fluid overpressure and failure patterns in rocks we consider a porous elasto-plastic medium in which a laterally localized overpressure line source is imposed at depth below the free surface. We solve numerically the fluid filtration equation coupled to the gravitational force balance and poro-elasto-plastic rheology equations. Systematic numerical simulations, varying initial stress, intrinsic material properties and geometry, show the existence of five distinct failure patterns caused by either shear banding or tensile fracturing. The value of the critical pore-fluid overpressure pc at the onset of failure is derived from an analytical solution that is in excellent agreement with numerical simulations. Finally, we construct a phase-diagram that predicts the domains of the different failure patterns and pc at the onset of failure.

Hierarchies of overpressure retardation of organic matter maturation: Case studies from petroleum basins in China

Abstract: The effects of overpressure on different aspects of organic matter maturation have been examined in three basins using multiple parameters. Overpressure in the Yinggehai Basin has retarded kerogen maturation, hydrocarbon generation, and thermal cracking of long-chain normal hydrocarbons, as well as evolution of isoprenoid hydrocarbons. Overpressure in the Dongpu depression, Bohai Bay Basin, seems to have retarded the maturation of bulk kerogens reflected by Rock-Eval Tmax, the thermal cracking of long-chain normal hydrocarbons, and the evolution of isoprenoid hydrocarbons, but seems to have had no detectable effects on vitrinite reflectance. Overpressure in the Qiongdongnan Basin has had no detectable retardation effects on all aspects of organic matter maturation. The observed phenomena suggest differential retardation of organic matter maturation (that is, different organic matter maturation reactions and different maturity parameters have been retarded by overpressure to different degrees in the same overpressured system) and four hierarchies of overpressure retardation. Hierarchy I retardation is defined as the state where the overpressure has retarded all aspects of organic matter maturation and is proposed to have been caused by the combination of high formation pressure and excess pressure, a high-water/organic matter ratio, and strong retention of reaction products in an early-developed, strong, persistent overpressured system. Hierarchy II retardation refers to the case in which all aspects of organic matter maturation except the maturation of hydrogen-poor kerogens, especially vitrinite, have been retarded and seem to have been caused by early-developed, moderate to strong, persistent overpressure. Hierarchy III retardation refers to the situation where overpressure has retarded only the thermal cracking of liquid hydrocarbons, but has had no detectable effect on the maturation of both hydrogen-rich and hydrogen-poor kerogens, and is proposed to have been caused by moderate to strong, persistent overpressures that began to develop around peak oil generation. Hierarchy IV retardation is defined as the situation where overpressure has had no detectable retardation on all aspects of organic matter maturation and may have been caused by cases wherein the overpressure developed too late, the intensity of the overpressure was too low, the duration of the overpressure was too short, or overpressured fluids had been expelled frequently. Resulting from both the diversities of organic matter maturation reactions and the complexities of overpressure generation and development, the hierarchies of overpressure retardation in different basins may be quite different, and it is necessary to use multiple parameters to understand the evolution of the petroleum systems in overpressured basins.

Blast overpressures from medium scale BLEVE tests

Abstract: The measured blast overpressures from recent tests involving boiling liquid expanding vapour explosions (BLEVE) has been studied The blast data came from tests where 04 and 2 m 3 ASME code propane tanks were exposed to torch and pool fires In total almost 60 tanks were tested, and of these nearly 20 resulted in catastrophic failures and BLEVEs Both single and two-step BLEVEs were observed in these tests This paper presents an analysis of the blast overpressures created by these BLEVEs In addition, the blast overpressures from a recent full scale fire test of a rail tank car is included in the analysis The results suggest that the liquid energy content did not contribute to the shock overpressures in the near or far field The liquid flashing and expansion does produce a local overpressure by dynamic pressure effects but it does not appear to produce a shock wave The shock overpressures could be estimated from the vapour energy alone for all the tests considered This was true for liquid temperatures at failure that were below, at and above the atmospheric superheat limit for propane Data suggests that the two step type BLEVE produces the strongest overpressure The authors give their ideas for this observation The results shown here add some limited evidence to support previous researchers claims that the liquid flashing process is too slow to generate a shock It suggests that liquid temperatures at or above the Tsl do not change this The expansion of the flashing liquid contributes to other hazards such as projectiles, and close in dynamic pressure effects Of course BLEVE releases in enclosed spaces such as tunnels or buildings have different hazards

Optical measurement and scaling of blasts from gram-range explosive charges

Abstract: Laboratory-scale experiments with gram-range explosive charges are presented. Optical shadowgraphy and high-speed digital imaging are used to measure the explosive-driven shock-wave position as a function of time. From this, shock Mach number-versus-distance from the explosion center can be found. These data then yield the peak overpressure and duration, which are the key parameters in determining the potential damage from an explosion as well as the TNT equivalent of the explosive. Piezoelectric pressure gage measurements of overpressure duration at various distances from the explosive charges compare well with theoretical calculations. A scaling analysis yields an approach to relate the gram-range blast to a large-scale blast from the same or different explosives. This approach is particularly suited to determining the properties and behavior of exotic explosives like triacetone triperoxide (TATP). Results agree with previous observations that the concept of a single TNT equivalence value is inadequate to fully describe an explosive yield, rather TNT equivalence factor and overpressure duration should be presented as functions of radius.

A volcanic degassing event at the explosive-effusive transition

Abstract: [1] A sequence of five Vulcanian explosions followed a lava dome collapse in July 2003 at Soufriere Hills Volcano. Each explosion occurred at ∼t = 190 n4.3 where n = 1–5 and t is the time (s) since the decompression rate peak during the collapse. Instead of a sixth explosion at the predicted time, a rapid emission of 97 × 103 kg SO2 was observed by a spectrometer network. This event represents the transition from explosive to effusive activity. After the last explosion, high magma ascent rates were maintained, but the critical overpressure explosion criterion was not reached. Instead, degassing and crystallisation in the upper conduit caused horizontal gradients in viscosity and flow rate, and brittle failure at the walls when the rate of shear strain exceeded a critical value. Development of a permeable shear zone allowed gas release, relief of overpressure and a return to effusive lava-dome building.

Overpressure generation and evolution in a compressional tectonic setting, the southern margin of Junggar Basin, northwestern China

Abstract: Overpressure is widespread in the southern margin of the Junggar Basin, northwestern China. Pressure measurements in drillstem tests and repeated formation tests and estimates from wire-line logs indicate contrasting overpressure values between permeable sandstones and adjacent low-permeability mudrocks. In addition, excess pressure differs among anticlines with similar depth, lithologies, and geologic age, indicating significant lateral changes of overpressure. Major factors controlling overpressure generation and distribution include rapid sediment deposition, pressure compartmentalization by thick mudrocks, tectonic stress, faulting, and folding. Clay transformation and hydrocarbon generation are believed to be insignificant in overpressure generation in the southern Junggar Basin. Numerical modeling of pressure generation and evolution suggests that faulting and stratal tilting associated with folding are the most significant factors in the overpressure generation of a permeable sandstone. The extremely high overpressure (pressure coefficient up to 2.43) may have been caused by hydraulic adjustment within permeable sandstones associated with structural deformation caused by post-Miocene intense tectonic activities.

Constraints on velocity-depth trends from rock physics models

Abstract: Estimates of depth, overpressure and amount of exhumation based on sonic data for a sedimentary formation rely on identification of a normal velocity–depth trend for the formation. Such trends describe how sonic velocity increases with depth in relatively homogeneous, brine-saturated sedimentary formations as porosity is reduced during normal compaction (mechanical and chemical). Compaction is ‘normal’ when the fluid pressure is hydrostatic and the thickness of the overburden has not been reduced by exhumation. We suggest that normal porosity at the surface for a given lithology should be constrained by its critical porosity, i.e. the porosity limit above which a particular sediment exists only as a suspension. Consequently, normal velocity at the surface of unconsolidated sediments saturated with brine approaches the velocity of the sediment in suspension. Furthermore, porosity must approach zero at infinite depth, so the velocity approaches the matrix velocity of the rock and the velocity–depth gradient approaches zero. For sediments with initially good grain contact (when porosity is just below the critical porosity), the velocity gradient decreases with depth. By contrast, initially compliant sediments may have a maximum velocity gradient at some depth if we assume that porosity decreases exponentially with depth. We have used published velocity–porosity–depth relationships to formulate normal velocity–depth trends for consolidated sandstone with varying clay content and for marine shale dominated by smectite/illite. The first relationship is based on a modified Voigt trend (porosity scaled by critical porosity) and the second is based on a modified time-average equation. Baselines for sandstone and shale in the North Sea agree with the established constraints and the shale trend can be applied to predict overpressure. A normal velocity–depth trend for a formation cannot be expressed from an arbitrary choice of mathematical functions and regression parameters, but should be considered as a physical model linked to the velocity–porosity transforms developed in rock physics.

Blast induced air overpressure and its prediction using artificial neural network

Abstract: Air blast is considered to be one of the most hazardous environmental disturbances created by blasting operation. Prediction of air overpressure (AOP) generated owing to blasting is difficult due to the influence of several factors in the air wave transmission. Blast design parameters, wind direction and speed, atmospheric temperature, humidity and topography, etc. are all affecting AOP. In this paper, an attempt has been made to predict AOP using artificial neural network (ANN) by incorporating the most influential parameters like maximum charge weight per delay, depth of burial of charge, total charge fired in a round and distance of measurement. To investigate the effectiveness of this approach, the predicted values of AOP by ANN were compared with those predicted by generalised equation incorporating maximum charge weight per delay and distance of measurement. Air overpressure data sets obtained from four different mines in India were used for the neural network as well as to form generalised...

Hazard evaluation of hydrogen–air deflagration with flame propagation velocity measurement by image velocimetry using brightness subtraction

Abstract: Deflagration phenomena in hydrogen–air mixtures initially filled in 1.4 m 3 spherical latex balloons were measured using a high-speed digital video camera and pressure transducers. The image velocimetry using brightness subtraction was introduced to eliminate the background effects for obtaining accurate time evolution records of flame propagation velocity. The maximum flame propagation velocity of about 100 m/s was observed with maximum overpressure 15 kPa at 1 m from ignition point. According to the detailed flame propagation velocity records, there were long deceleration durations. The observed maximum overpressure was smaller than the overpressure estimated by the basis of the observed maximum flame propagation velocity and the pressure wave theories of spherical flames. A new blast curve plot of scaled overpressure vs. distance was tentatively proposed.

Overpressure and slope stability in prograding clinoforms: Implications for marine morphodynamics

Abstract: [1] Rapid deposition of fine-grained sediments can lead to overpressure buildup along the fronts of prograding sedimentary bodies (clinoforms) ranging from deltas to continental margins. Overpressure reduces shear strength, which can lead to failure of submarine slopes, so that overpressure prediction is potentially of fundamental importance to marine morphodynamics. We generalize one-dimensional overpressure theory to migrating clinoforms where deposition is highly localized in space and time, and show that overpressure is characterized by a dimensionless loading intensity, the Gibson number (given by the product of sediment supply and slope, divided by sediment hydraulic diffusivity). Our results show that compared to terrestrial slopes, slope stability in overpressured clinoforms is especially sensitive to foreset steepness, which directly affects loading intensity. The utility of our theory is demonstrated by application to modern and ancient clinoforms with documented slope failures. This analysis suggests that (1) most muddy clinoforms will have significant overpressure, (2) many delta fronts and continental margins prograde at a limiting slope threshold for deep-seated landsliding, and (3) shallow overpressure and near-surface liquefaction may contribute to mobilization of fluid muds which actively prograde subaqueous deltas. We derive quantitative relationships between sediment supply and slope for clinoforms prograding at limiting equilibrium. Importantly, our results suggest that slope is an inverse function of sediment supply in these systems, implying that commonly used diffusive morphodynamics approaches may be inappropriate.

Overpressure development in rift basins: an example from the Malay Basin, offshore Peninsular Malaysia

Abstract: The Malay Basin is a Tertiary transtensional rift basin located in offshore Peninsular Malaysia. A study of the subsurface pressure data has revealed at least two major overpressure compartments that are sealed by regional shale units. The main, basin-centre overpressure compartment has a domal shape and, in profile, shows a convex-up top-of-overpressure surface. In the basin centre, the top of overpressure is generally between 1900 m and 2000 m depth, and is confined mainly to a particular stratigraphic unit, the Middle Miocene unit E. It appears that the top of overpressure is influenced by the underlying regional shale seal in unit F, and that the mild overpressure encountered in unit E represents the overpressure transition zone. Due to the convex-upward top-of-overpressure surface, overpressure is generally encountered at shallower depths in the basin centre compared to the basin flanks. A smaller overpressure compartment is also identified on the northeastern flank of the basin. This compartment is sealed by the onlapping, transgressive shale of unit L (Lower Miocene) and occurs at a depth of 2600–3000 m. Hence, regional shale seals have a strong influence on overpressure distribution. Disequilibrium compaction is believed to be the primary causal mechanism for the overpressure in the basin centre. Overpressure development is the consequence of high subsidence and sedimentation rates during basin extension. Low subsidence and sedimentation rates on the basin flanks do not generate overpressure. The domal shape of the top-of-overpressure surface is thus a result of different rates of subsidence and sedimentation across the basin. The basin9s overpressure history is simulated by means of a simple model. Modelling results indicate that the basin overpressure developed very early, during the syn-rift phase ( c. 30–21 Ma), when sediment burial rates were very high (>1000 m Ma −1 ). Overpressure build-up occurred rapidly during the syn-rift phase but has started to dissipate gradually since the post-rift phase began 21 Ma ago, when sedimentation rates were well below 1000 m Ma −1 . This suggests that disequilibrium compaction as an overpressure-generating mechanism was effective during only the syn-rift phase of basin development. Sedimentation rates during the post-rift phase (generally less than 500 m Ma −1 ) were not high enough to generate overpressure. The pre-existing overpressure is thus able to dissipate through the sedimentary column, causing the build-up of pore pressure in the post-rift section. Thus, the overpressure in the post-rift strata is probably of secondary origin, derived from excess pressure dissipated from the underlying syn-rift strata. Overpressure generated by disequilibrium compaction during the syn-rift phase has been modified and re-distributed as the basin evolved through the post-rift phase.

Origin of early overpressure in the Upper Devonian Catskill Delta Complex, western New York state

Abstract: The Upper Devonian Rhinestreet black shale of the western NewYork state region of the Appalachian Basin has experienced multiple episodes of overpressure generation manifested by at least two sets of natural hydraulic fractures. These overpressure events were thermal in origin and induced by the generation of hydrocarbons during the Alleghanian orogeny close to or at the Rhinestreet’s
3.1km maximum burial depth. Analysis of differential gravitational compaction strain of the organic-rich shale around embedded carbonate concretions that formed within a metre or so of the seafloor indicates that the Rhinestreet shale was compacted
58%. Compaction strain was recalculated to a palaeoporosity of 37.8%, in excess of that expected for burial 43 km. The palaeoporosity of the Rhinestreet shale suggests that porosity reduction caused by normal gravitational compaction of the low-permeability carbonaceous sedimentwas arrested at some depth shy of its maximumburial depth by pore pressure in excess of hydrostatic. The depth at which the Rhinestreet shale became overpressured, the palaeo- fluid retention depth, was estimated by use of published normal compaction curves and empirical porosity-depth algorithms to fall between 850 and 1380m. Early and relatively shallow overpressuring of the Rhinestreet shale likely originated by disequilibrium compaction induced by a marked increase in sedimentation rate in the latter half of the Famennian stage (LateDevonian) as the Catskill Delta Complex prograded westward across the Appalachian Basin in response to Acadian tectonics. The regional Upper Devonian stratigraphy of western New York state indicates that the onset of overpressure occurred at a depth of
1100m, well in advance of the Rhinestreet shale’s entry into the oil window during the Alleghanian orogeny.

Consequence analysis by means of characteristic curves to determine the damage to buildings from the detonation of explosive substances as a function of TNT equivalence

Abstract: In a previous paper, the characteristic overpressure–impulse–distance curves for the detonation of explosive substances were presented. They allow the overpressure and impulse to be determined at each distance from the detonation. When combined with damage criteria (such as those shown by the Probit equations), the characteristic curves allow consequence analysis for this kind of explosion to be carried out in only one step, as the damage is shown in the same diagram as the overpressure, impulse and distance. In this paper, diagrams and equations are presented to determine the damage to humans (eardrum rupture, death due to displacement and skull fracture, death due to displacement and whole body impact, and death due to lung damage or lung haemorrhage).

Impulsive loading on a concrete structure

Abstract: The current paper deals with the experimental determination of relationships of simultaneous ground shock and airblast parameters against impulsive loading The empirical relationships of peak air pressure, peak reflected airblast pressure along the height of the structure, peak ground acceleration, arriving shock of ground shock, duration of ground shock and the time lag between ground shock and airblast pressure reaching the concrete structure and so forth on a reinforced cement concrete containment scaled model owing to a surface explosion at a certain distance have been established Equations have been developed and utilised in software for a structural response analysis of a reactor containment subjected to surface explosions The generation and the effects of blast wave on the shell structure in the plastic range are discussed Critical distances have been determined for different amounts of blast charges for a typical shell structure The developed methodology of analysis may be adopted in order to

Control method for an overpressure valve in a common-rail fuel supply system

Abstract: Control method for an overpressure valve in a common-rail fuel supply system, the method including the phases of: delivering fuel under pressure, via a high-pressure pump, to a common rail equipped with the overpressure valve that is set to discharge the fuel present in the common rail into a discharge line when the fuel pressure inside the common rail exceeds a safety value; piloting, during a diagnostic test, the high-pressure pump to increase the fuel pressure inside the common rail beyond the safety value in order to trigger operation of the overpressure valve; determining, during the diagnostic test, the flow of the high-pressure pump and/or the fuel pressure inside the common rail; and comparing the flow of the high-pressure pump and/or the fuel pressure inside the common rail during the diagnostic test with respective threshold values.

The generation and continued existence of overpressure in the Delaware Basin, Texas

Abstract: The Delaware Basin, part of the larger Permian Basin, contains important hydrocarbon plays. Permian strata contain 71% of in-place oil and 53% of in-place gas, with the remainder hosted in the Lower Palaeozoic. Excessive pore fluid pressures (up to 〜 8000 psi above hydrostatic) are found within Early Permian and Pennsylvanian strata, which account for 30-35% of the hydrocarbon producing zones. This study has utilised an array of basin analysis techniques to analyse the pressure history of the Delaware basin. To fully appreciate the geopressure history in the Delaware Basin, a rigorous quantitative approach has been applied using advanced thermochronology techniques. This has enabled for the first time an accurate burial history curve to be established for the basin. The results show that maximum burial occurred in the basin at 55 Ma as a consequence of an additional 6890 ft of Mesozoic and Cenozoic sediment. The basin then underwent two major tectonic uplift events during the Cenozoic. The Laramide orogeny (55-50 Ma) uplifted and eroded off 3890 ft of sediment, then during the Eocene and Oligocene the basin subsided and accumulated a further 600 ft of sediment. The Basin and Range event (25-10 Ma) then uplifted and tilted the basin further, eroding off 3600 ft of sediment from the centre of the basin. The new burial history curve has been integrated with wireline logs and basin modelling software to evaluate the mechanism of overpressure generation and its maintenance through geological time. This study has shown that the main mechanism for overpressure generation in the Delaware Basin was disequilibrium compaction. Analysis of the sonic log using the Equivalent Depth Method and the Eaton Ratio Method, combined with velocity I density cross-plots, indicate that compaction is driven by vertical loading, and undercompaction seen in the basin is a consequence of the sediments' inability to dewater. Basin modelling shows that it was the rapid deposition of the Permian sediments that enabled disequilibrium compaction to generate overpressure. These techniques have also shown that a secondary cause of overpressure due to unloading mechanisms (e.g. gas generation or expansion with uplift, lateral transfer and hydrocarbon buoyancy) may be occurring within localised horizons below the Wolfcampian Series. Overpressure has been maintained within the basin for more than 250 Myr. Basin modelling and wireline logs have shown that numerous intercalated tight limestones (Mississippian to Late Permian) acted as pressure seals to maintain overpressure. In addition, low permeability mudstones (l0 (^-6) mD) have contributed to the inability of the Delaware Basin to reach pressure equilibrium.

System and Method for Generating and Controlling Conducted Acoustic Waves for Geophysical Exploration

Abstract: An improved system and method for generating and controlling conducted acoustic waves for geophysical exploration are provided. A plurality of overpressure waves are generated by at least one overpressure wave generator comprising at least one detonation tube having an open end. The at least one overpressure wave generator is oriented so the plurality of overpressure waves are not directed directly towards a target media. The recoil force of the at least one overpressure wave generator occurring during generation of the plurality of overpressure waves is coupled to the target media to generate conducted acoustic waves. The timing of the generation of the plurality of overpressure waves can be in accordance with a timing code and can be used to steer the conducted acoustic waves to a location of interest in the target media.

Full opening and reclosable explosion vent apparatus

Abstract: Explosion vent apparatus is provided, which includes a vent support unit defining a vent opening adapted to be aligned with an area to be protected from an overpressure condition, a vent panel movably mounted on the support unit in a normal closed position blocking the outlet and movable through a first displacement away from its closed position to a first overpressure- relieving position upon application of an overpressure of one magnitude of pressure while allowing movement of the vent panel through a second and greater displacement to a second overpressure-relieving position upon application of a higher overpressure. The vent panel does not deform or rupture upon opening, and recloses after pressure relief, either under spring pressure when opened under a lower first pressure, or by a fluid actuated reclosing mechanism upon full opening of the vent panel under a higher second pressure.

Evolution of Geopressure Field in Niuzhuang Sag in Dongying Depression and Its Effect on Petroleum Accumulation

Abstract: The present geopressure field in Niuzhuang sag in Dongying depression can be divided into dissimilar zones vertically and different areas horizontally. Overpressure is closely related to both the upper part of the fourth member and the lower part of the third member of Shahejie Formation whose source rocks matured. Large-scale faults surrounding Niuzhuang sag limit the horizontal overpressure distribution. Inside the sag it is strongly overpressured while outside it is much less overpressured. The forward modeling indicates that the overpressure evolution in the sag is divided into three stages: original-forming, meta-declining and latest-raising stages. And the main mechanism for generating overpressures evolved from undercompaction into combination of both undercompaction and hydrocarbon generation. Meanwhile, both lateral seal by large-scale faults surrounding Niuzhuang sag and vertical seal by the thick and shale-enriched bottom layer of the middle part of the third member of Shahejie Formation determined the special distribution of overpressure. The overpressure in Niuzhuang sag provided the petroleum migration with original forces in those two important accumulation periods. Coupling of overpressure evolution and tectonic activities played an active role in petroleum accumulation in the bordering faulted zone near Niuzhuang sag.

Puffer circuit breaker with an overpressure valve

Abstract: A circuit breaker comprises a puffer volume ( 9 ) and at least one overpressure valve ( 16 ) for discharging gas from the puffer volume ( 9 ), if the pressure therein exceeds a given threshold. The overpressure valve ( 16 ) is formed by a piston ( 17 ) and a spring ( 19 ) as well as by a cavity ( 18 ) in the stationary support body ( 10 ) of the moveable contact assembly ( 2 ). The overpressure valve ( 16 ) is of compact and simple design, has low hysteresis and large cross-section.

Influence of creep on water pressure measured from borehole tests in the Meuse/Haute-Marne Callovo-Oxfordian argillites

Abstract: Porewater pressure is an important parameter for use in safety assessment of underground waste disposal. In order to measure porewater pressure in the Callovo-Oxfordian argillites, ANDRA has performed several in-situ tests, which consist in measuring time evolution of water pressure in an almost closed chamber. This was accomplished using an in-place pressure sensor coupled with an electromagnetic transmission device (called an EPG probe). The measured values show a small hydraulic overpressure (0.1 MPa) compared with the estimated value at the corresponding depth. In the framework of a scientific cooperation agreement between Andra and Ineris, a study was undertaken to examine whether all or a part of this overpressure could be attributed to the hydro-mechanical coupled processes linked with the creep of argillites and the stress relaxation in the experimental measuring chamber-nearby. In the whole, the numerical results were in good agreement with the measured results. Poroviscoplasticity can explain the increasing pressure in the borehole. The measured overpressure can be reached with adequate viscoplastic model parameters.

Methods for enhancing pressure accuracy in a compression pump

Abstract: A method of measuring pressure for a cell in a pneumatic compression device may include performing an inflate/exhaust cycle including, for one or more cells of a plurality of cells in a pneumatic compression device, inflating a cell to a target pressure, wherein the target pressure is based on at least a desired pressure, measuring a pressure of the cell, determining an overpressure value associated with the cell based on at least the measured pressure, determining a subsequent target pressure associated with the cell based on at least the measured pressure and the overpressure value, deflating the cell, and inflating the cell until the subsequent target pressure is achieved.

The distribution and evolution of fluid pressure and its influence on natural gas accumulation in the Upper Paleozoic of Shenmu-Yulin area, Ordos Basin

Abstract: On the basis of measuring the pressure distribution and analyzing its origin in the Carboniferous and Permian of Shenmu-Yulin area, the evolution history of ancient pressure is restored mainly by means of the basin numerical simulation technique, in which the paleo-pressure has been constrained by the compaction restoration and the examination of fluid inclusion temperature and pressure. Then the development and evolution history of abnormal pressure and its effect on gas migration and accumulation are investigated. Studies show that the pressure in southeastern and northwestern parts of studied area is near to hydrostatic pressure, whereas in the remainder vast area the pressure is lower than the hydrostatic pressure, which is caused by difficulty to measure pressure accurately in tight reservoir bed, the calculating error caused by in-coordinate between topography relief and surface of water potential, pressure lessening due to formation arising and erosion. There are geological factors beneficial to forming abnormal high pressure in the Upper Palaeozoic. On the distraction of measured pressure, paleo-pressure data from compaction restoration and fluid inclusion temperature and pressure examining, the evolution history of ancient pressure is restored by the basin numerical simulation technique. It is pointed out that there are at least two high peaks of overpressure in which the highest value of excess pressure could be 5 to 25 MPa. Major gas accumulated in main producing bed of Shanxi Fm (P1s) and lower Shihezi Fm (P2x), because of two-fold control from capillary barrier and overpressure seal in upper Shihezi Fm (P2s). In the middle and southern districts, the two periods of Later Jurassic to the middle of Early Cretaceous, and middle of Later Cretaceous to Palaeocene are main periods of gas migration and accumulation, while they belong to readjustment period of gas reservoirs after middle of Neocene.

Numerical simulation of an underground structure under a hypothetic terrorist bombing

Abstract: An explosion inside an underground structure may affect the structures and personnel on ground surface in an area of several hundreds metre radius.In order to study the blast wave propagation generated from an underground explosion and its effect on structures and personnel in the vicinity,numerical simulation was carried out to estimate the blast pressure and ground shock.A three dimensional(3D) model was established to estimate the blast wave propagation inside and outside the underground structure.The blast wave intensities inside the structure at every exit of the structure and in the surrounding were outside the structure above ground are estimated.To estimate ground shock,a two dimensional(2D) model was developed to analyze stress wave propagation.The numerical results of overpressure and ground shock intensities were compared with the respective allowable criteria for structure and personnel protection given in the USA DoD manual and the NATO standard.The safe distance for protection of above-ground structures and personnel in a hypothetic underground terrorist bombing scenario was determined.

Overpressure prediction by mean total stress estimate using well logs for compressional environments with strike-slip or reverse faulting stress state

Abstract: Overpressure Prediction by Mean Total Stress Estimate Using Well Logs for Compressional Environments with Strike-Slip or Reverse Faulting Stress State. (December 2006) Aslihan Ozkale, B.S., Middle East Technical University,Turkey Chair of Advisory Committee: Dr. Jerome Schubert Predicting correct pore-pressure is important for drilling applications. Wellbore stability problems, kicks, or even blow-outs can be avoided with a good estimate of porepressure. Conventional pore-pressure estimation methods are based on one-dimensional compaction theory and depend on a relationship between porosity and vertical effective stress. Strike-slip or reverse faulting environments especially require a different way to determine pore-pressure, since the overburden is not the maximum stress. This study proposes a method which better accounts for the three-dimensional nature of the stress field and provides improved estimates of pore-pressure. We apply the mean total stress estimate to estimate pore-pressure. Pore pressure is then obtained by modifying Eaton’s pore-pressure equations, which require either resistivity or sonic log data. The method was tested in the Snorre Field in the Norwegian North Sea, where the field changes from strike-slip to reverse stress state. Eaton’s resistivity and sonic equations

Geological constraints of pore pressure detection in shales from seismic data

Abstract: Methods for detection of pore fluid overpressures in shales from seismic data have become widespread in the oil industry. Such methods are largely based on the identification of anomalous seismic velocities, and on subsequent determination of pore pressures through relationships between seismic velocities and the vertical effective stress (VES). Although it is well known that lithology variations and compaction mechanisms should be accounted for in pore pressure evaluation, a systematic approach to evaluation of these factors in seismic pore pressure prediction seems to be absent. We have investigated the influence of lithology variations and compaction mechanism on shale velocities from acoustic logs. This was performed by analyses of 80 wells from the northern North Sea and 24 wells from the Haltenbanken area. The analyses involved identification of large-scale density and velocity variations that were unrelated to overpressure variations, which served as a basis for the analyses of the resolution of overpressure variations from well log data. The analyses demonstrated that the overpressures in neither area were associated with compaction disequilibrium. A significant correlation between acoustic velocity and fluid overpressure nevertheless exists in the Haltenbanken data, whereas the correlation between these two parameters is weak to non-existing in the North Sea shales. We do not presently know why acoustic velocities in the two areas respond differently to fluid overpressuring. Smectitic rocks often have low permeabilities, and define the top of overpressures in the northern North Sea when they are buried below 2 km. As smectitic rocks are characterized by low densities and low acoustic velocities, their presence may be identified from seismic data. Smectite identification from seismic data may thus serve as an indirect overpressure indicator in some areas. Our investigations demonstrate the importance of including geological work and process understanding in pore pressure evaluation work. As a response to the lack of documented practice within this area, we suggest a workflow for geological analyses that should be performed and integrated with seismic pore pressure prediction.