Showing papers on "Overpressure published in 2000"

Paleohydrology of the Delaware Basin, Western Texas: Overpressure Development, Hydrocarbon Migration, and Ore Genesis

Abstract: This study integrates quantitative modeling techniques with field observations to establish a paleohydrologic framework of the Delaware basin, western Texas. The reconstructed paleohydrologic models allow for a better understanding of the development and maintenance of anomalous overpressures, hydro carbon generation and migration, and ore genesis in the basin. Results of numerical modeling show that disequilibrium compaction and oil generation might generate excess fluid pressures during the Late Permian in response to the rapid deposition of evaporite beds. The preservation of this overpressure to the present, however, requires the presence of an extremely low-permeability (<10-11 d) top seal. Most shaly sediments, with permeability ranging from 10-4 to 10-8 d, thus may be too permeable, by several orders of magnitude, to preserve overpressure for more than 250 m.y. The predicted present-day gas window is located within the overpressure zone, suggesting that the volume increase associated with the oil-to-gas conversion may be attributed to present overpressures. The native sulfur deposits likely formed in a fluid mixing zone resulting from the Laramide uplift of the western basin during the Tertiary. In our model, meteoric water recharged along the basin's uplifted western margin and discharged basinward. Hydrocarbons migrated landward by pressure gradients and buoyancy and discharged upward along faults in the western basin, where they mixed with meteoric water. Many oil and mineral reservoirs may have formed in the fluid mixing zone, where extensive chemical reactions take place. In the Culberson sulfur ore district, for example, fluids including hydrocarbons and meteoric water migrated upward through faults from underlying carrier beds, into the Permian Salado limestone. There, the mixture of fluid drives biochemical reactions that precipitate native sulfur.

Prediction of vapour cloud explosions using the SCOPE model

Abstract: The SCOPE 3 model (Shell Code for Overpressure Prediction in gas Explosions) has been developed to predict the overpressures which could be generated by gas explosions in vented enclosures, such as offshore modules. SCOPE 3 attempts, wherever possible, to model the underlying physical processes in an explosion. This phenomenological approach gives greater confidence in predictions for full-scale events than methods based simply on correlations of experimental data.

Gas generation and overpressure: Effects on seismic attributes

Abstract: Drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones. Knowledge of pore pressure using seismic data, as for instance from seismic-while-drilling techniques, will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures. We assume a simple basin-evolution model with a constant sedimentation rate and a constant geothermal gradient. Oil/gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow. Assuming a first-order kinetic reaction, with a reaction rate satisfying the Arrhenius equation, the oil/gas conversion fraction is calculated. Balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations. This excess pore pressure determines the effective pressure, which in turn determines the skeleton bulk moduli. If the generated gas goes into solution in the oil, this effect does not greatly change the depth and oil/gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure. The seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas. Moreover, the velocities and attenuation factors versus frequency are obtained by including rock-frame/fluid viscoelastic effects to match ultrasonic experimental velocities. For the basin-evolution model used here, pore pressure is seismically visible when the effective pressure is less than about 15 MPa and the oil/gas conversion is about 2.5% percent.

A feasibility study for pore-pressure prediction using seismic velocities in the offshore Nile Delta, Egypt

Abstract: Overpressure in a formation, caused by abnormally high fluid pressures, is a concern during all phases of oil field operations—exploration, drilling, casing, completion, and reservoir evaluation. Accurate knowledge of formation pore pressure and fracture pressure is essential for drilling efficient and safe wells with optimum mud weights. Furthermore, knowledge of these pressures aids understanding of fluid migration pathways, sealing potential, and probability of fault leakage. Overpressure by definition occurs when pore pressure exceeds normal hydrostatic pressure and is related to certain environmental conditions in a given earth section. In the offshore Nile Delta, for example, low permeability shale in the Pliocene can trap fluids and cause overpressured shale as a result of undercompaction. Overpres-sured sediments also can be caused by fluid expansion mechanisms (e.g., heating, hydrocarbon maturation and expulsion of intergranular water during clay transformation). Local tectonic compression can also generate overpressured sediments. Given the young age and shallowness of overpressured sediments in the offshore Nile Delta, the observed pore pressure is largely attributed to undercompaction. Several methods for detecting/estimating overpressured formations are based on interpretation of drilling data, wireline logs, and geophysical data. Drilling and wireline log data are obtained while the well is drilled. They cannot, therefore, be used for predrill pore-pressure prediction. This paper describes a feasibility study to predict pore pressure before drilling and the subsequent calibration of pore pressure and seismic velocities in a key well. Port Fouad Field was discovered in 1982 when exploratory well Port Fouad-1 penetrated gas-bearing formations in the late Miocene. The field is approximately 35 km northeast of Port Said (Figure 1). Figure 1. Map of Port Fouad Field. Gas is present at shallow and deep depths. Shallow zones, less than 2200 m, belong to the Kafr El Sheikh Formation of Pliocene age which is characterized by high porosity and permeability. The …

Generation of overpressure by cementation of pore space in sedimentary rocks

Abstract: SUMMARY Overpressure build-up is studied when the main cause for porosity reduction is cementation of the pore space sourced locally. The average porosity reduction for siliclastic sediments is modelled with nth-order kinetics. It is shown that the overpressure in one layer at a constant depth will decrease exponentially with time in the case of first-order kinetics for the porosity reduction. The overpressure is studied in one layer subjected to cementation during constant burial along a thermal gradient. A small overpressure build-up is shown above the window for cementation, with a steep rise in overpressure in the upper part of the window. The overpressure build-up is then seen to decrease rapidly towards the end of the window for cementation. Overpressure build-up is also studied when cementation is the main cause for porosity reduction in the entire column of sediments during deposition and burial. The overpressure regime characterized by gravity numbers larger than one is studied. This regime corresponds to low or moderate overpressures in the case of mechanical compaction. Cementation is shown to imply a steep pressure build-up in the window of cementation, which will easily exceed the lithostatic pressure. The porosity loss due to cementation is seen to have a strong impact on the permeability, which leads to the formation of a pressure seal. Although most of the potential for fluid expulsion is exhausted below the seal, because most of the porosity is cemented up, the permeability of the seal is sufficiently low for hydrofracturing to take place. This scenario is consistent with overpressure observations in many wells.

Non‐equilibrium compaction and abnormal pore‐fluid pressures: effects on rock properties1

Abstract: Knowledge of pore pressure using seismic data will help in planning the drilling process to control potentially dangerous abnormal pressures. Various physical processes cause anomalous pressures on an underground fluid. Non-equilibrium compaction is a significant process of overpressure generation. This occurs when the sedimentation rate is so rapid that the pore fluids do not have a chance to 'escape' from the pore space. The model assumes a closed system and that the pore space is filled with water and hydrocarbon in a liquid state. Balancing mass and volume fractions yields the fluid pressure versus time of deposition and depth of burial. Thermal effects are taken into account. The pore pressure, together with the confining pressure, determines the effective pressure which, in turn, determines the bulk moduli of the rock matrix. We assume a sandstone saturated with hydrocarbons and water, for which calibration of the model with experimental data is possible. The seismic velocities and attenuation factors are computed by using Biot's theory of dynamic poroelasticity and the generalized linear solid. The example shows that the formation can be overpressured or underpressured depending on the properties of the saturating fluid. Wave velocities and quality factors decrease with decreasing differential pressure. The effect is important below approximately 20 MPa. The model is in good agreement with experimental data for Berea sandstone and provides a tool for predicting pore pressure from seismic attributes.

Fault controlled pressure modelling in sedimentary basins

Abstract: The starting point for this Dr. Ing. thesis was the requirement for models describing basin scale overpressure for use in basin modelling studies. It is necessary to identify, understand and describe both the generating mechanisms and the fluid flow in sedimentary basins in order to meet this requirement.All of the models developed are based upon a study area consisting of fault bounded pressure compartments. A reservoir simulator has applied a lateral cross fault transmissibility model and by defining the overpressure history in some of the pressure compartments it has been possible to model the overpressure through geological time. A simpler model estimating the present day overpressure distribution based upon the same fault transmissibility model is developed. This model is able to provide a snapshot of the main present day pressure trends. This model has further been developed to a full pseudo 3D-pressure simulator including generation and dissipation of pressure through time. Due to the quantification of generation and dissipation of overpressure it is possible to model hydraulic fracturing and leakage through the overlying seals. In addition to the models describing the overpressure, an algorithm that converts the pressure compartments into a quadrangle grid system is developed.The main part of the work in this thesis deals with quantifying basin scale generation and dissipation of overpressure. Different models are developed based upon the available literature and knowledge achieved during this work. All the models used are thoroughly described. The pressure simulator PRESSIM is developed in order to test, improve and reject different models. Several simulations of the overpressure history in the North Viking Graben and the Haltenbanken area offshore Norway are presented. The modelled overpressures are calibrated to the observed pressures revealed by exploration wells. In general, the results are very satisfactory due to relatively small deviations between the modelled and observed overpressures. The simulations provide an increased knowledge of the mechanisms generating and dissipating the overpressure and form a good basis for discussing the nature of the overpressure. In addition, the results suggest which mechanisms are the most important in the different parts of the basin. The pressure simulator PRESSIM can be used to test and verify alternative pressure generation models and flow descriptions in a basin. It is possible to model the position and timing of the hydraulic fracturing because the pressure simulations are based upon a water budget for each pressure compartment.

Overpressure system of hydrocarbon-bearing basins in china

Abstract: With the deepening of petroleum exploration and drilling, the overpressures are revealed in China's hydrocarbon-bearing basins and so far 29 overpressure provinces are found. The provinces have close ties with the petroleum concept. The overpres- sure systems are generally source rocks and sometimes also caprocks acting as pressure confining bed for the underlying petroleum and gas accumulation. The overpressure, which is the motive power for hydrocarbon migration, compels the fluids pass through various fractured faults or transporting strata and migrates from high potential zone to low potential zone and then accumulate. The hard overpressure reservoir(pressure coefficient2)is difficult to form, except the pressure coefficient in the adjoining mudstone(source rock)is bigger than that in the reservoir. Under some special conditions, the hard overpressure reser voir may also form. Therefore the first thing is to get a clear understanding about the origin and distribution of overpressures, especially the zones with abnormal pressure and temperature. Because both horizontally and vertically these abnormal zones are the main leak zones for subsurface fluid and favorable accumulation zones of hydrocarbon, as well as the target zones for drilling.

Method for overpressure detection from compressional-and- shear-wave data

Abstract: The present invention includes a method for overpressure detection and pore pressure change monitoring in subsurface gas, liquid hydrocarbon, or water reservoirs from compressional- and shear-wave measurement data. As part of this method, one or more Poisson's ratios are determined from field-based measurement data and are then compared against known Poisson's ratio values representative of the particular subsurface formation type. By applying a Poisson's ratio—pore pressure criterion that is appropriate for that type of formation, an overpressure in the formation is identified.

A simple model of flow patterns in overpressured sedimentary basins with heat transport and fracturing

Abstract: Overpressure zones in sedimentary basins are defined as sediments where the fluid pore pressure is substantially higher than hydrostatic. Chief among the causes for the formation of overpressure is the rapid deposition of low-permeablity sediments so that compaction and dewatering are inhibited. This results in the support of the overlying material in part by the fluid, rather than by grain to grain contact. Many overpressure zones also have associated geothermal anomalies whereby the geotherms are hotter than normal. In this paper, we present a one-dimensional model of overpressure generation and dissipation, with hydraulic fracture. The fluid flow is coupled to the fluid temperature field and to the motion of the solid sediment matrix. Compacted coordinates are introduced to decouple the fluid velocity and temperature from the solid matrix velocity. Using a maximum sedimentation rate of 1 km Myr−1 and a hydraulic conductivity at the top of the layer equal to 5×10−12 m s−1, 3 km of partially compacted sediment are deposited in 5 Myr, generating an overpressure of 0.81 of the lithostatic pressure at the bottom of the sediment layer. An undercompaction of 19% relative to normally compacted sediment is developed at a final depth of 3.66 km by the end of sedimentation. Dissipation of the overpressure takes place on a timescale of the order of 40 Myr. The equilibrium temperature generates thermal gradients in excess of 38°C km−1 at shallow depths. Fast sedimentation rate and high permeability in the fractured state favor oscillating behavior and propagation of fracture waves. Overpressure zones may constitute a proximal source of hydrothermal fluids for the genesis of ore deposits in sedimentary basins.

OTC 11912 Detecting Overpressure from Seismic Velocity Calibrated to Log and Core Measurements

Abstract: By analyzing experimental data we show that in many gas-filled rocks, the Poisson's ratio (PR) decreases with decreasing differential pressure (confining minus pore pressure). In many liquid-saturated rocks the opposite is true: PR increases with decreasing differential pressure. This means that in gassaturated rocks, PR decreases with increasing pore pressure and in liquid-saturated rocks it increases with increasing effective 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 seismic pore pressure and pore fluid monitoring during production as well as for overpressure detection from surface seismic, cross-well, sonic logs and measurements ahead of the drill bit.

Estimating pore pressure in the Cooper Basin, South Australia: sonic log method in an uplifted basin

Abstract: The use of sonic log data to analyse overpressure in the Cooper Basin, South Australia is complicated by the occurrence of Tertiary uplift in the basin. Uplift and overpressure are both associated with anomalous porosity/depth relationships: the former being witnessed by overcompaction and the later by undercompaction. Hence uplift may mask overpressure effects on log data. A normal compaction trend was determined for the Cooper Basin by averaging sonic log data from 29 wells. The Maree Subgroup was omitted from the normal compaction trend because it has a markedly different sonic log signature. Shale sequences were isolated for study by applying a gamma ray filter (API > 100). In order to remove the effects of uplift, the normal compaction trend was adjusted to fit the trend of the upper, normally pressured part of the sequence in each well. Quantitative pore pressure analysis was undertaken on shale sequences from 8 wells using the Eaton (1972) method once the effect of uplift had been removed. The predictions of the Eaton (1972) method are consistent with pressure measurements (DSTs and mud weights) in sandstones in 7 of the 8 wells, suggesting that both uplift and overpressure have been successfully determined. Assuming that the shales at Moomba 55 are overpressured, as suggested by the Eaton (1972) method, these shales must be isolated from adjacent near normally pressured sandstones. Furthermore mud weights, which are only at best an approximation of formation pressure, are especially unreliable in overpressured shales adjacent to near-normally pressured sandstones.

CFD Analysis of Unsteady Ignition Overpressure Effects on Delta II and Delta III Launch Vehicles

Abstract: This paper describes a computational fluid dynamic (CFD) simulation procedure for prediction of transient engine exhaust plume flowfield effects acting on Delta II and Delta III launch vehicles before liftoff. Transient vehicle forces, pressures, temperatures, and plume concentrations are predicted for use in analyses of main engine nozzle and launch-leg clearance, base pressure environments, and plume contamination. Ignition overpressure (IOP) effects produced by unsteady exhaust plume flowfields are simulated for Delta II and Delta III launch vehicles on Pad 17B at Cape Canaveral Air Force Station (CCAFS). CFD results predict that the Pad 17B J-deflector, horizontal exhaust duct, and louver exhaust essentially induce either an uprangeor downrange-directed lateral force. This result is in agreement with observed Delta II 7295-9.5 Skynet main engine gimbal angles at liftoff.

Confined explosions of high explosives

Abstract: In the present paper, effects of turbulent combustion of the detonation products of high explosives in confined explosions are studied. Explosions of TNT, TNT/RDX and the mixtures of these explosives with Al are investigated in a steel chamber of 150-liter volume. The chamber is filled with nitrogen or air under normal conditions. Overpressure histories are recorded at the chamber wall. The dependence of the mean overpressure on the loading density of TNT is determined. This dependence is used to determine the TNT equivalent of explosives tested for confined explosions. On the basis of the measured overpressure histories and the results of thermochemical calculations, the rate of energy release and final heat effects of combustion of the detonation products in air are also estimated. Finally, the degree of after-burning of the detonation products is calculated for explosive mixtures tested.

Fuel injection for an internal combustion engine, with a multistage high-pressure pump and two pressure reservoirs

Abstract: In a fuel injection system for an internal combustion engine, having at least two different, high system pressures, in which a first pressure reservoir, supplied by a high-pressure pump, is provided for the higher system pressure, and a second pressure reservoir is provided for the lower system pressure, and at least the first pressure reservoir can be made to communicate by line for a fuel injection with the injectors of the individual cylinders of the engine, the pressure level of the high-pressure pump is greater than the higher system pressure, and the first pressure reservoir has an overpressure valve leading to the second pressure reservoir, and the second pressure reservoir has an overpressure valve leading to the fuel tank.

Prediction of Successive Radial Shock-on-Shock Interactions Using Thin Shell Model

Abstract: Introduction The susceptibility of certain types of systems to specialized and unique acoustic waveforms continues to be of interest for some aeroacoustic commercial and military applications. Detrimental effects on systems can occur when significant amounts of unwanted acoustic energy get absorbed at the systems’ resonant frequencies. In contrast to high dynamic pressure waves that cause immediate and observable stress and strain, damage caused by, for example, a continuous sub-audible pressure wave tuned to a system’s resonance is subtle. A key technical challenge is to create an overpressure that lasts for a prescribed amount of time. This study shows that an elongated pressure pulse with a large positive phase can be created in the far field by combining multiple successive shock waves in the high overpressure region. The key physical idea is that successive shocks will travel in changing ambient backgrounds. When the successive ambient densities are lower, the successive shock velocities will be larger than the previous ones. This is analytically demonstrated for two successive shock waves. Recent experiments [l] suggest that a radial geometry shock-on-shock interaction created in the strong-to-intermediate shock wave region near the source can produce a resultant radial acoustic wave in the far-field that has an elongated positive pressure phase. The ability to create such a pulse may have useful applications for selected aeroacoustic applications. Figure 1 is a schematic of the experimental configuration used in [l]. The conjecture that an expanded positive pulse could be created by allowing a second pulse to catch up to the first one is explored semi-quantitatively by examining the behavior of two successive radial blast waves in the strong shock wave regime (neglecting the ambient pressure). For a uniform background density, prA, the shock radius, R,. as a function of time is given by the familiar expression [2,3]