The Experts below are selected from a list of 234 Experts worldwide ranked by ideXlab platform
Olivier Mougin - One of the best experts on this subject based on the ideXlab platform.
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Comparison of pulsed three-dimensional CEST acquisition schemes at 7 tesla: steady State versus Pseudosteady State.
Magnetic resonance in medicine, 2016Co-Authors: Vitaliy Khlebnikov, Nicolas Geades, Dennis W.j. Klomp, Hans Hoogduin, Penny A. Gowland, Olivier MouginAbstract:Purpose: To compare two pulsed, volumetric chemical exchange saturation transfer (CEST) acquisition schemes: steady State (SS) and Pseudosteady State (PS) for the same brain coverage, spatial/spectral resolution and scan time. Methods: Both schemes were optimized for maximum sensitivity to amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects through Bloch McConnell simulations, and compared in terms of sensitivity to APT and NOE effects, and to transmit field inhomogeneity. Five consented healthy volunteers were scanned on a 7 Tesla Philips MRsystem using the optimized protocols at three nominal B1 amplitudes: 1 mT, 2 mT, and 3 mT. Results: Region of interest based analysis revealed that PS is more sensitive (P < 0.05) to APT and NOE effects compared with SS at low B1 amplitudes (0.7–1.0 mT). Also, both sequences have similar dependence on the transmit field inhomogeneity. For the optimum CEST presaturation parameters (1 mT and 2 mT for APT and NOE, respectively), NOE is less sensitive to the inhomogeneity effects (15% signal to noise ratio [SNR] change for a B1 dropout of 40%) compared with APT (35% SNR change for a B1 dropout of 40%). Conclusion: For the same brain coverage, spatial/spectral resolution and scan time, at low power levels PS is more sensitive to the slow chemical exchange-mediated processes compared with SS.
Longchen Duan - One of the best experts on this subject based on the ideXlab platform.
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An Analytical Solution of the Pseudosteady State Productivity Index for the Fracture Geometry Optimization of Fractured Wells
Energies, 2019Co-Authors: Hui Gao, Longchen DuanAbstract:The Pseudosteady State productivity index is very important for evaluating the production from oil and gas wells. It is usually used as an objective function for the optimization of fractured wells. However, there is no analytical solution for it, especially when the proppant number of the fractured well is greater than 0.1. This paper extends the established fitting solution for proppant numbers less than 0.1 by introducing an explicit expression of the shape factor. It also proposes a new asymptotic solution based on the trilinear-flow model for proppant numbers greater than 0.1. The two solutions are combined to evaluate the Pseudosteady State productivity index. The evaluation results are verified by the numerical method. The new solution can be directly used for fracture geometry optimization. The optimization results are consistent with those given by the unified fracture design (UFD) method. Using the analytical solution for the Pseudosteady State productivity index, optimization results can be obtained for rectangular drainage areas with arbitrary aspect ratios without requiring any interpolation or extrapolation. Moreover, the new solution provides more rigorous optimization results than the UFD method, especially for fractured horizontal wells.
Jay M. Khodadadi - One of the best experts on this subject based on the ideXlab platform.
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Effects of Insulated and Isothermal Baffles on Pseudosteady-State Natural Convection Inside Spherical Containers
Journal of Heat Transfer, 2010Co-Authors: Yuping Duan, S. F. Hosseinizadeh, Jay M. KhodadadiAbstract:The effects of insulated and isothermal thin baffles on Pseudosteady-State natural convection within spherical containers were studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106, and 107, baffles with three lengths positioned at five different locations were investigated (120 cases). The fluid that is heated adjacent to the sphere rises replacing the colder fluid, which sinks downward through the stratified stable thermal layer. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid, thus causing oscillations in the temperature and flow fields. Due to flow obstruction (blockage or confinement) effect of baffles and also because of the extra heating afforded by the isothermal baffle, multi-cell recirculating vortices are observed. This additional heat is directly linked to creation of another recirculating vortex next to the baffle. In effect, hot fluid is directed into the center of the sphere disrupting thermal stratified layers. For the majority of the baffles investigated, the Nusselt numbers were generally lower than the reference cases with no baffle. The extent of heat transfer modification depends on Ra, length, and location of the extended surface. With an insulated baffle, the lowest amount of absorbed heat corresponds to a baffle positioned horizontally. Placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement that is linked to disturbance of the thermal boundary layer. With isothermal baffles, heat transfer enhancement is achieved for a baffle placed near the bottom of the sphere due to interaction of the counterclockwise rotating vortex and the stratified layer. For some high Ra cases, strong fluctuations of the flow and thermal fields indicating departure from the Pseudosteady-State were observed.
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Effect of an Insulated Baffle on Pseudosteady-State Natural Convection Inside Spherical Containers
ASME JSME 2007 Thermal Engineering Heat Transfer Summer Conference Volume 1, 2007Co-Authors: Yuping Duan, S. F. Hosseinizadeh, Jay M. KhodadadiAbstract:The effect of an insulated thin baffle on Pseudosteady-State natural convection within spherical containers is studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent, two-dimensional axisymmetric form of the governing continuity, momentum and energy equations are solved. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104 , 105 , 106 and 107 , baffles with 3 lengths positioned at 5 different locations were investigated. In effect, a parametric study involving 60 cases were performed. The computational results were benchmarked against previous data available in the literature by comparing the heat transfer correlations, temperature distribution and streamline patterns for cases with no baffle. In general, regardless of the presence of an insulated baffle, fluid that is heated adjacent to the surface of the sphere rises replacing the colder fluid which sinks downward. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid. This behavior can lead to onset of oscillations in the temperature and flow fields. Due to blockage effect of an insulated thin baffle, multi-cell recirculating vortex structures are observed. The number and strength of these vortices depend on the position and length of the baffle. In the absence of heat transfer path through the insulated baffle, flow obstruction is the major feature of this problem. For the majority of the length and location combinations investigated, less heat is brought into the fluid thus lowering the time rate of rise of the bulk temperature. The extent of heat transfer modification depends on the Rayleigh number, length and location of the extended surface.Copyright © 2007 by ASME
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Effect of an Isothermal Baffle on Pseudosteady-State Natural Convection Inside Spherical Containers
ASME JSME 2007 Thermal Engineering Heat Transfer Summer Conference Volume 1, 2007Co-Authors: S. F. Hosseinizadeh, Yuping Duan, Jay M. KhodadadiAbstract:The influence of an isothermal thin baffle on Pseudosteady-State natural convection within spherical containers is studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent, two-dimensional axisymmetric form of the governing continuity, momentum and energy equations are solved. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104 , 105 , 106 and 107 , baffles with 3 lengths positioned at 5 different locations were investigated. In effect, a parametric study involving 60 cases were performed. The computational results were benchmarked against previous data available in the literature by comparing the heat transfer correlations, temperature distribution and streamline patterns for cases with no baffle. In general, regardless of the presence of an isothermal baffle, fluid that is heated adjacent to the surface of the sphere rises replacing the colder fluid which sinks downward. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid. This behavior can lead to onset of oscillations in the temperature and flow fields. Partly due to the blockage effect of an isothermal thin baffle and also the extra heating afforded by the baffle, multi-cell recirculating vortex structures are observed. The number and strength of these vortices depend on the position and length of the baffle. The additional heat that is brought into the baffle through the isothermal baffle is directly linked to creation of a counter clockwise rotating vortex next to the baffle. This baffle, in turn, directs hot fluid into the center of the sphere and disrupts thermal stratified layers. For the majority of the length and location combinations investigated, the Nusselt number is lower than the case with no baffle, however the time rate of rise of the bulk temperature can be greater for some combinations. The extent of heat transfer modifications depends on the Rayleigh number, length and location of the baffle.Copyright © 2007 by ASME
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Pseudosteady-State mixed convection inside rotating spherical containers
1999Co-Authors: Jay M. Khodadadi, X. ShiAbstract:A computational study of the Pseudosteady-State two-dimensional mixed convection within rotating spherical containers is presented. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent continuity, momentum and energy equations in the spherical coordinate system are solved. Natural convection effect is modeled via the Boussinesq approximation. For a fixed Prandtl number of 4.62, parametric studies were performed by varying the Rayleigh number in order to cover the laminar regime adequately. For a given Rayleigh number, the ratio of Gr/Re{sup 2} was varied between 0.1 and 10. Given a Rayleigh number, the streamline patterns maintain their general shape with a dominant rotating vortex. As the forced convection effect becomes less marked, the streamlines exhibit less pronounced gradients near the surface of the sphere. As the rotational effect become more marked, the extent of the deviation from the limiting case of non-rotating spheres becomes more noticed. However, the bottom of the sphere still remains to be the region with enhanced heat transfer. Given a rotational Reynolds number, the streamline patterns are not affected greatly as the natural convection is promoted, however the temperature gradients near the surface are markedly enhanced. It is noticed that asmore » natural convection effects are promoted, the greater portion of the sphere's surface experiences enhanced heat transfer rates. Given a Rayleigh number, the contours of the azimuthal velocity exhibit a nearly vertical equally-spaced pattern suggesting that solid-body rotation for high rotational Reynolds numbers. However, as the natural convection effects are enhanced, the contours become more slanted. The variation of the mean Nusselt number with the Reynolds and Rayleigh numbers is also quantified.« less
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Pseudosteady-State natural convection inside spherical containers partially filled with a porous medium
International Journal of Heat and Mass Transfer, 1999Co-Authors: Y. Zhang, Jay M. Khodadadi, F. ShenAbstract:Abstract A computational study of the Pseudosteady-State two-dimensional natural convection within spherical containers partially filled with a porous medium is presented. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent continuity, momentum and energy equations in the spherical coordinate system are solved within the composite system. The natural convection effect is modeled via the Boussinesq approximation, whereas the Darcy Law is utilized to treat the porous medium. For a reference case, flow and temperature field details during the transient evolution to the Pseudosteady-State are presented. It is shown that the dominant transport mechanism at the early stages is due to heat conduction and natural convection plays no role. A parametric study was performed with the values of the Rayleigh number ( Ra ) , Darcy number ( Da ) and the thermal conductivity ratio varying one at a time. The dependence of the flow and thermal fields on these parameters was elucidated. For low Ra and Da numbers, the flow field is restricted within the central fluid core. Only for high Ra and Da numbers, one can observe comparable fluid motion in both the porous medium and central fluid core regions. The local Nusselt number on the surface and interface temperature exhibit nearly uniform variations for low Ra and Da numbers, signifying little deviation from the limiting pure conduction case. For high Ra and Da numbers, marked heat transfer is observed on the bottom of the sphere. The interface temperature is also seen to deviate from uniform variation for high Ra and Da numbers. Only the intensity of the recirculating flow in the central fluid core region was seen to depend on the thermal conductivity ratio. The thermal conductivity ratio modifies the time scale of the thermal transport and only the relative magnitudes of the monitored quantities are affected.
Vitaliy Khlebnikov - One of the best experts on this subject based on the ideXlab platform.
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Comparison of pulsed three-dimensional CEST acquisition schemes at 7 tesla: steady State versus Pseudosteady State.
Magnetic resonance in medicine, 2016Co-Authors: Vitaliy Khlebnikov, Nicolas Geades, Dennis W.j. Klomp, Hans Hoogduin, Penny A. Gowland, Olivier MouginAbstract:Purpose: To compare two pulsed, volumetric chemical exchange saturation transfer (CEST) acquisition schemes: steady State (SS) and Pseudosteady State (PS) for the same brain coverage, spatial/spectral resolution and scan time. Methods: Both schemes were optimized for maximum sensitivity to amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects through Bloch McConnell simulations, and compared in terms of sensitivity to APT and NOE effects, and to transmit field inhomogeneity. Five consented healthy volunteers were scanned on a 7 Tesla Philips MRsystem using the optimized protocols at three nominal B1 amplitudes: 1 mT, 2 mT, and 3 mT. Results: Region of interest based analysis revealed that PS is more sensitive (P < 0.05) to APT and NOE effects compared with SS at low B1 amplitudes (0.7–1.0 mT). Also, both sequences have similar dependence on the transmit field inhomogeneity. For the optimum CEST presaturation parameters (1 mT and 2 mT for APT and NOE, respectively), NOE is less sensitive to the inhomogeneity effects (15% signal to noise ratio [SNR] change for a B1 dropout of 40%) compared with APT (35% SNR change for a B1 dropout of 40%). Conclusion: For the same brain coverage, spatial/spectral resolution and scan time, at low power levels PS is more sensitive to the slow chemical exchange-mediated processes compared with SS.
Kun Sang Lee - One of the best experts on this subject based on the ideXlab platform.
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Use of Pseudosteady-State Pressure Distribution to Represent Wells in the Finite-Element Mesh
All Days, 1999Co-Authors: Kun Sang LeeAbstract:In calculating the long-time performance of depletion reservoirs using a numerical method, an accurate and efficient simulation highly depends on the proper grid selection. The treatment of wells can particularly have a strong influence on computed results. In our model, we use a very fine hybrid grid in near-well regions to obtain good resolution in the vicinity of wellbores. For the efficient simulation of field-scale problems with multiple wells of differing production rates, a well model was introduced to reduce the concentration of elements near wells. The model was based on a near-wellbore approximation of the pressure distribution during Pseudosteady-State. Well performance calculated in the well model is correlated to the conditions in the appropriate location from the actual well. This approximation overcomes an inaccuracy for fluid flow calculation in the vicinity of wells and can be implemented easily in existing grids. A numerical model based on the approach was developed for a simulation study. Numerical examples are presented to show the accuracy and computational efficiency resulting from the use of a well model. With the approximation, it was possible to reduce the number of element grid nodes up to 1/3. The level of error due to the approximation was in an acceptable range (< 4%) for all cases considered. The key element to the accurate calculation is the ratio of the effective radius of the near-well region compared to the distance to the nearest boundary.
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A Well Model Based on the Pseudosteady-State Pressure Distribution in the Finite-Element Simulation of Depletion Reservoirs
Geosystem Engineering, 1999Co-Authors: Kun Sang LeeAbstract:ABSTRACT In calculating the long-time performance of closed reservoirs using a finite-element method, an accurate and efficient simulation highly depends on a proper grid selection. The treatment of wells can particularly have a strong influence on computed results. For the efficient simulation of field-scale problems with multiple wells of differing production rates, a well model was introduced to reduce the concentration of elements near wells. The model was based on a near-wellbore pressure distribution during Pseudosteady State, leading to a logarithmic type of approximation. Well performance calculated in the well model is correlated to the conditions in the appropriate location from the actual well. This approximation overcomes an inaccuracy for fluid flow calculation in the vicinity of wells and can be implemented easily in existing grids. A numerical model based on the approach was developed for a simulation study. Numerical examples are presented to show the accuracy and computational efficiency ...
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modified Pseudosteady State approach to calculate long time performance of closed gas reservoirs
Journal of the Korean Institute of Gas, 1998Co-Authors: Kun Sang LeeAbstract:This paper considers the applicability of a Pseudosteady-State approach to the long-time behavior of real gas flow in a closed reservoir. The method involves a combination of a linearized gas diffusivity equation using a normalized pseudotime and a material balance equation. For the simulation of field-scale problems with multiple wells of differing production rates over extended production periods, the Pseudosteady-State equation was solved successively for each flow period. Results from this study show that the approach provides a fast and accurate method for modeling the long-time behavior of gas reservoirs under depletion conditions.
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Succession-of-States Model for Calculating Long-Time Performance of Depletion Reservoirs
SPE Journal, 1998Co-Authors: Kun Sang Lee, Mark A. Miller, Kamy SepehrnooriAbstract:This paper (SPE 51023) was revised for publication from paper SPE 37030, first presented at the 1996 SPE Asia Pacific Oil & Gas Conference held in Adelaide, Australia, 28-31 October. Original manuscript received for review 16 September 1996. Revised manuscript received 20 May 1998. Revised manuscript approved 9 June 1998. Summary This study presents a new finite-element approach for directly calculating Pseudosteady-State flow behavior for wells in depletion systems. The approach allows for spatially dependent reservoir properties, complex reservoir geometries, and multiple wells. Results are verified against long-time transient solutions reported in the literature for several regularly shaped systems. The paper also demonstrates application of the approach to field-scale problems. Results show that this approach provides a fast and accurate method for modeling the long-time behavior of depletion reservoirs. The approach is particularly applicable to single-phase volumetric gas reservoirs. A bound reservoir with wells producing at constant rate will exhibit Pseudosteady-State behavior after the end of typically short-lived infinite-acting and transition flow periods. This study develops a new approach for directly calculating Pseudosteady-State flow behavior without solving the full time-dependent form of the diffusivity equation. This approach can be applied to the linearized forms of the diffusivity equation for either single-phase liquid or gas flow. A finite-element method is used that allows for spatially dependent reservoir properties, complex reservoir geometries, and multiple wells. The first part of this paper presents a verification of the approach by comparing results for some regularly shaped systems against full-transient solutions reported in the literature. For the simulation of field-scale problems with multiple wells of differing production rates, a well model based on a near-wellbore approximation of the pseudo pressure distribution during Pseudosteady-State is introduced to reduce the concentration of elements near wells. The second part of the paper demonstrates application of the direct Pseudosteady-State concept to actual reservoir problems. To account for rate changes during extended production periods, the Pseudosteady-State equation was solved successively for each flow period and combined with an overall reservoir material balance analysis. Results from this study show that this approach provides a fast and accurate method for modeling the long-time behavior of various types of reservoirs under depletion conditions. The approach is particularly applicable to single-phase volumetric gas reservoirs. P. 279
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Use of a Pseudosteady-State Solution to Predict Productivity of a Fractured Well with Spatially Varying Fracture Geometry
All Days, 1997Co-Authors: Kun Sang LeeAbstract:Abstract An infinite- or finite-conductivity vertical fracture intersecting a well produced at a constant rate within a closed system is considered. Fracture is modeled as a vertical slab with lengthwise variation in the fracture geometric properties. The long-time inflow performance is obtained from a finite-element solutions of Pseudosteady-State diffusivity equation, which considers fixed flux over all reservoir region. The first part of this paper examines the behavior of an infinite-conductivity fracture. Comparisons of well productivity calculated with transient and Pseudosteady-State solutions show a good agreement. Computations for anisotropic reservoirs were made to examine the effects of reservoir permeability anisotropy and fracture length on the late-time pressure response. It is demonstrated that well productivity is heavily dependent on the fracture orientation and main flow pattern. The second part investigates the effects of fracture permeability on the pressure response of a finite-conductivity fracture. The fracture conductivity is a decreasing function of distance from wellbore. The new fracture model and solution technique provides more realistic description for fracture geometry and long-time pressure response with a minimum computational effort. Introduction Many oil and gas wells are hydraulically fractured as a routine part of their completion. Hydraulic fracturing is an effective technique for increasing the productivity of damaged wells or wells producing from low flow capacity formations. This paper presents a technique to calculate production from hydraulically fractured wells. A closed system containing a well intercepted by a vertical fracture extending over the entire thickness of the formation is modeled. Various models to represent the fracture are compared. Also, a method for predicting inflow performance relationship of fractured wells is outlined. Numerical solution of long-time pressure behavior obtained directly from the Pseudosteady-State approach aids forecasting the well performance with minimum computational effort. The performance of wells penetrated by artificially induced hydraulic fractures has been discussed extensively by various authors in the literature. Most of them are used for determining fracture characteristics and formation properties using pressure transient analysis. Gringarten et al. (1974) and Gringarten and Ramey (1974) studied the transient behavior of three widely-used models: infinite-conductivity vertical fracture, uniform flux vertical fracture, and uniform flux horizontal fracture. They introduced the use of the boundary element method for the development of pressure transient solutions by an adaptation of the analogous solutions of heat conduction in solids. In a major development in the study of hydraulically fractured wells with finite conductivity, Cinco-Ley et al. (1978) published their bilinear flow paper, which was followed by a more definitive paper by Cinco-Ley and Samaniego (1981). They demonstrated that Gringarten's model for infinite-conductivity fractures was not valid for dimensionless fracture conductivities less than 300. An exhaustive review of well testing theory as applied to vertically fractured system was published by Cinco-Ley (1982). Using a finite-difference model, Bennett et al. (1983) examined the influence of variable fracture conductivity on well response and the effect of unequal wing lengths on pressure behavior. The influence of the settling of propping agents and the effect of fracture height on the well response were investigated by Bennett et al. (1986). Using a technique to forecast the production from horizontal wells, Mutalik et al. (1988) calculated shape factors for fractured wells located either centrally or off-centrally in the rectangular drainage area. P. 637^
