The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Ahmad Ghassemi - One of the best experts on this subject based on the ideXlab platform.
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three dimensional finite element simulations of hydraulic Fracture height growth in layered formations using a coupled hydro mechanical model
International Journal of Rock Mechanics and Mining Sciences, 2020Co-Authors: Qian Gao, Ahmad GhassemiAbstract:Abstract In this paper, we treat the important problem of hydraulic fracturing in the presence of elastic modulus and stress contrast in layered rock systems encountered in petroleum resources development using a fully coupled 3D hydro-mechanical model. First, the model is validated by simulating a laboratory hydraulic fracturing experiment dealing with the influence of stress contrast. Good agreements in the distribution of Fracture Aperture, injection pressure and Fracture footprint have been achieved. Then, numerical analyses are performed to investigate the influence of in-situ stress and formation layer properties, such as Young's modulus and Fracture energy release rate on the height growth and containment of hydraulic Fractures. Comparing the results of simulations using conventional thickness-weighted Young's modulus to those from explicit modeling of layers' Young's moduli, it is found given that for the same amount of injection volume, the thickness-weighted modulus generates a higher injection pressure. Explicit modeling of the layers influences the hydraulic Fracture Aperture distribution in the pay zone as well as in the surrounding layers. A relatively large Fracture Aperture is observed in the layer with the lower Young's modulus. Also, the hydraulic Fracture tends to propagates mainly in the lower Young's modulus layers which could facilitate containment of the hydraulic Fracture by limiting height growth in the stiffer layers. When considering the influence of stress contrast on height growth, the conventional equilibrium height model produces a relatively large Aperture and overestimates the Fracture height. Simulations using typical injection rate, fluid viscosity, and in-situ stress, show stress contrast larger than a certain value, for example 30% of the in-situ minimum horizontal stress, can effectively inhibit height growth. When the payzone is bounded by ductile layers, the injection pressure is higher and the corresponding Aperture at the injection point is larger than those obtained using uniform rock properties.
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a reactive thermo poroelastic analysis of water injection into an enhanced geothermal reservoir
Geothermics, 2014Co-Authors: Chakra Rawal, Ahmad GhassemiAbstract:Abstract Coupled thermo-poro-chemo-mechanical processes in geothermal systems impact the reservoir response during injection and production procedures by affecting Fracture permeability. A three-dimensional numerical model is presented to analyze these processes during fluid injection into geothermal reservoirs. The solid mechanics aspect of the problem is computed using the displacement discontinuity boundary element method (BEM) while transport processes within the facture are modeled using the finite element method (FEM). The FEM and BEM formulations are integrated to set up a system of equations for unknown temperature, pressure, concentration, and Fracture Aperture. The fluid diffusion, heat conduction and solute diffusion in the reservoir are treated using BEM so that the need of infinite reservoir domain discretization is eliminated. The numerical model is used to analyze the Fracture response to non-isothermal reactive flow in EGS. Numerical examples of SiO 2 undersaturated-cold water injection into the geothermal reservoir show that silica dissolves from the rock matrix, increasing the Fracture Aperture. The zone of silica dissolution spreads into the Fracture with continuous fluid injection. At large injection times, thermoelastic stress has a greater impact on Fracture Aperture compared to poroelastic stress. Simulations that consider natural Fracture stiffness heterogeneity show the development of a non-uniform flow path within the crack, with lower rock matrix cooling and thus enhanced silica reactivity in the high stiffness regions. As a result, areas of higher joint normal stiffness show lower Aperture increases in response to the thermo-poroelastic processes, but a higher Aperture expansion due to silica dissolution. Depending on the injectate saturation state with respect to quartz, silica is added or removed from the rock matrix. This process is likely to impact the rock matrix properties and its mechanical response to stress perturbations associated with fluid circulation.
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a three dimensional thermo poroelastic model for Fracture response to injection extraction in enhanced geothermal systems
Geothermics, 2011Co-Authors: Ahmad Ghassemi, X ZhouAbstract:Water injection in enhanced geothermal systems sets in motion coupled poro-thermo-chemo-mechanical processes that impact the reservoir dynamics and productivity. The variation of injectivity with time and the phenomenon of induced seismicity can be attributed to the interactions between these processes. In this paper, a three-dimensional transient numerical model is developed and used to simulate fluid injection into geothermal reservoirs. The approach couples Fracture flow and heat transport to thermo-poroelastic deformation of the rock matrix via the displacement discontinuity (DD) method. The use of the boundary integral equations, for the pressure diffusion and heat conduction in the rock matrix, eliminates the need to discretize the infinite reservoir domain. The system of linear algebraic equations for the unknown displacement discontinuities, and fluid and heat sources are used in a finite element formulation for the fluid flow and heat transport in the Fracture. This yields a system of equations which are solved to obtain the temperature, pressure, and Aperture distributions within the Fracture at every time step. In this way, the temporal variation of the Fracture Aperture and fluid pressure, caused by pressurization and thermo-poroelastic stresses, are determined. Numerical experiments using the model illustrate the feed-back between matrix dilation, shrinkage, and pressure in the Fracture. It is observed that whereas the poroelastic effects dominate the early stage of injection pressure profile and the Fracture Aperture evolution, thermoelastic effects become dominant for large injection times.
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a three dimensional integral equation model for calculating poro and thermoelastic stresses induced by cold water injection into a geothermal reservoir
International Journal for Numerical and Analytical Methods in Geomechanics, 2009Co-Authors: X X Zhou, Ahmad Ghassemi, A H D ChengAbstract:Poro-mechanical and thermo-mechanical processes change the Fracture Aperture and thus affect the water flow pattern in the Fracture during the cold water injection into enhanced geothermal systems (EGS). In addition, the stresses generated by these processes contribute to the phenomenon of reservoir seismicity. In this paper, we present a three-dimensional (3D) partially coupled poro-thermoelastic model to investigate the poroelastic and thermoelastic effects of cold water injection in EGS. In the model, the lubrication fluid flow and the convective heat transfer in the Fracture are modeled by the finite element method, while the pore fluid diffusion and heat conductive transfer in the reservoir matrix are assumed to be 3D and modeled by the boundary integral equation method without the need to discretize the reservoir. The stresses at the Fracture surface and in the reservoir matrix are obtained from the numerical model and can be used to assess the variation of in situ stress and induced seismicty with injection/extraction. Application of the model shows that rock cooling induces large tensile stresses and increases Fracture conductivity, whereas the rock dilation caused by fluid leakoff decreases Fracture Aperture and increases compressive total stresses around the injection zone. However, increases in pore pressure reduce the effective stresses and can contribute to rock failure, Fracture slip, and microseismic activity. Copyright © 2009 John Wiley & Sons, Ltd.
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effects of heat extraction on Fracture Aperture a poro thermoelastic analysis
Geothermics, 2008Co-Authors: Ahmad Ghassemi, Andrew Nygren, Alexander H D ChengAbstract:Poroelastic and thermoelastic effects of cold-water injection in an enhanced (or engineered) geothermal system (EGS) are investigated by considering flow in a pre-existing Fracture in a hot, rock matrix that could be permeable or impermeable. Assuming plane Fracture geometry, expressions are derived for changes in Fracture Aperture caused by cooling and fluid leak-off into the matrix. The corresponding induced pressure profile is also calculated. The problem is analytically solved for the cases pertaining to a constant fluid injection rate with a constant leak-off rate. Results show that although fluid loss from the Fracture into the matrix reduces the pressure in the crack, the poroelastic stress associated with fluid leak-off tends to reduce the Aperture and increase the pressure in the Fracture. High rock stiffness and low fluid diffusivity cause the poroelastic contraction of the Fracture opening to slowly develop in time. The maximum reduction of Aperture occurs at the injection point and become negligible near the extraction point. The solution also shows that thermally induced stress increases the Fracture Aperture near the injection point and, as a result, the fluid pressure at this point is greatly reduced. The thermoelastic effects are particularly dominant near the inlet compared to those of poroelasticity, but are pronounced everywhere along the Fracture for large times. Although poroelasticity associated with leak-off does not change the Fracture Aperture significantly for low permeability rocks, it can lead to pore pressure increase and cause nearby Fractures to slip.
Suresh G Kumar - One of the best experts on this subject based on the ideXlab platform.
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Influence of Fracture Heterogeneity Using Linear Congruential Generator (LCG) on the Thermal Front Propagation in a Single Geothermal Fracture-Rock Matrix System
MDPI AG, 2018Co-Authors: Nikhil Bagalkot, Alireza Zare, Suresh G KumarAbstract:An implicit finite difference numerical model has been developed to investigate the influence of Fracture heterogeneity on the propagation of thermal front in a single horizontal Fracture-matrix system. Instead of depending on a complex and data-demanding geostatistical method for a precise representation of Fracture Aperture, a statistical linear congruential generator (LCG) method was applied in the present study to replicate the unpredictable nature of Fracture Aperture morphology. The results have been compared with the parallel plate model and simple sinusoidal model. Finally, sensitivity analysis of Fracture Aperture size and fluid flow rate has been carried out to identify the conditions at which Fracture heterogeneity is critical. The results indicate that LCG-Aperture enhances the heat transfer between Fracture and hot rock matrix compared to the parallel and sinusoidal Fractures. Further, the temperature profiles in hot rock indicate that there was a greater loss of heat for the case of LCG-Aperture (25% loss) compared to sinusoidal (16%) and parallel plate (8%) Apertures. It was found that heterogeneity does not play a major role at small Fracture Aperture size (≤50 μm) and at low flow rates. However, as Fracture Aperture size increases, the heterogeneity plays a vital part even at low flow rates
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thermal front propagation in variable Aperture Fracture matrix system a numerical study
Sadhana-academy Proceedings in Engineering Sciences, 2015Co-Authors: Nikhil Bagalkot, Suresh G KumarAbstract:A numerical study on the effect of complex Fracture Aperture geometry on propagation of thermal front in a coupled single Fracture–matrix system has been carried out. Sinusoidal and logarithmic functions have been used to capture the variation in Fracture Aperture. Modifications have been made to existing coupled partial differential governing equations to consider the variation of Fracture Aperture. Effect of temperature on the thermal and physical properties of rock have been incorporated. A fully implicit finite difference scheme has been used to discretize the coupled governing equations. Thermal convection, dispersion and conduction are the major transport processes within Fracture, while conduction is the major transport process within rock matrix. The results suggest that variation of Fracture Aperture increases the heat transfer rate at the Fracture–matrix interface. Sensitivity analysis on rock thermal conductivity and Fracture Aperture have been carried out. The results suggest that the heat transfer from rock matrix to Fracture for the case of the parallel plate model is greatly dependent on the rock thermal conductivity (λ m) as compared to variable Aperture model. Further, the thermal front propagation for both parallel plate model and variable Aperture model is sensitive to changes in Fracture Aperture. The heat transfer rate at the interface is greater at smaller Fracture Apertures and decreases with increase in Aperture.
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changes in Fracture Aperture and fluid pressure due to thermal stress and silica dissolution precipitation induced by heat extraction from subsurface rocks
Geothermics, 2007Co-Authors: Ahmad Ghassemi, Suresh G KumarAbstract:Abstract The numerical model developed by Suresh Kumar and Ghassemi [Suresh Kumar, G., Ghassemi, A., 2005. Numerical modeling of non-isothermal quartz dissolution in a coupled Fracture–matrix system. Geothermics 34, 411–439] is used to study fluid pressure and permeability changes in a Fracture in a rock mass by taking into account the effects of thermal stresses and silica precipitation/dissolution, which is computed using linear reaction kinetics. Fluid flow in the Fracture is calculated based on the cubic law. Solute transport mechanisms by advection and dispersion are included in the model. Mass exchange between the horizontal Fracture and the rock matrix is accounted for by assuming diffusion-limited solute transport. Heat transfer between the Fracture and the rock matrix is modeled considering only conduction, while heat transport within the Fracture includes thermal advection, conduction, and dispersion in the horizontal plane. Pressures of the circulating fluid through the Fracture are allowed to vary with time, while the flow rate is assumed to remain constant. A series of numerical experiments are carried out to simulate a fluid injection/production operation. The temporal variation of Fracture Aperture in response to the individual and combined effects of thermal stress and silica dissolution/precipitation is examined. The results show that, for lower initial Fracture Apertures, the significant increase in Fracture permeability and the associated pressure drop at the injection point are mostly attributable to thermoelastic effects, while the increase in Fracture Aperture near the production well is mainly the result of silica dissolution. On the other hand, for larger initial Apertures, the effects of silica precipitation/dissolution are minimal and thermoelastic effects are prevalent, as the intensity of coupling between the high-permeability Fractures and the low-permeability rock matrix becomes weaker.
X Zhou - One of the best experts on this subject based on the ideXlab platform.
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a three dimensional thermo poroelastic model for Fracture response to injection extraction in enhanced geothermal systems
Geothermics, 2011Co-Authors: Ahmad Ghassemi, X ZhouAbstract:Water injection in enhanced geothermal systems sets in motion coupled poro-thermo-chemo-mechanical processes that impact the reservoir dynamics and productivity. The variation of injectivity with time and the phenomenon of induced seismicity can be attributed to the interactions between these processes. In this paper, a three-dimensional transient numerical model is developed and used to simulate fluid injection into geothermal reservoirs. The approach couples Fracture flow and heat transport to thermo-poroelastic deformation of the rock matrix via the displacement discontinuity (DD) method. The use of the boundary integral equations, for the pressure diffusion and heat conduction in the rock matrix, eliminates the need to discretize the infinite reservoir domain. The system of linear algebraic equations for the unknown displacement discontinuities, and fluid and heat sources are used in a finite element formulation for the fluid flow and heat transport in the Fracture. This yields a system of equations which are solved to obtain the temperature, pressure, and Aperture distributions within the Fracture at every time step. In this way, the temporal variation of the Fracture Aperture and fluid pressure, caused by pressurization and thermo-poroelastic stresses, are determined. Numerical experiments using the model illustrate the feed-back between matrix dilation, shrinkage, and pressure in the Fracture. It is observed that whereas the poroelastic effects dominate the early stage of injection pressure profile and the Fracture Aperture evolution, thermoelastic effects become dominant for large injection times.
Nikhil Bagalkot - One of the best experts on this subject based on the ideXlab platform.
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Influence of Fracture Heterogeneity Using Linear Congruential Generator (LCG) on the Thermal Front Propagation in a Single Geothermal Fracture-Rock Matrix System
Energies, 2018Co-Authors: Nikhil Bagalkot, Alireza Zare, Govindarajan Suresh KumarAbstract:An implicit finite difference numerical model has been developed to investigate the influence of Fracture heterogeneity on the propagation of thermal front in a single horizontal Fracture-matrix system. Instead of depending on a complex and data-demanding geostatistical method for a precise representation of Fracture Aperture, a statistical linear congruential generator (LCG) method was applied in the present study to replicate the unpredictable nature of Fracture Aperture morphology. The results have been compared with the parallel plate model and simple sinusoidal model. Finally, sensitivity analysis of Fracture Aperture size and fluid flow rate has been carried out to identify the conditions at which Fracture heterogeneity is critical. The results indicate that LCG-Aperture enhances the heat transfer between Fracture and hot rock matrix compared to the parallel and sinusoidal Fractures. Further, the temperature profiles in hot rock indicate that there was a greater loss of heat for the case of LCG-Aperture (25% loss) compared to sinusoidal (16%) and parallel plate (8%) Apertures. It was found that heterogeneity does not play a major role at small Fracture Aperture size (≤50 μm) and at low flow rates. However, as Fracture Aperture size increases, the heterogeneity plays a vital part even at low flow rates.
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Influence of Fracture Heterogeneity Using Linear Congruential Generator (LCG) on the Thermal Front Propagation in a Single Geothermal Fracture-Rock Matrix System
MDPI AG, 2018Co-Authors: Nikhil Bagalkot, Alireza Zare, Suresh G KumarAbstract:An implicit finite difference numerical model has been developed to investigate the influence of Fracture heterogeneity on the propagation of thermal front in a single horizontal Fracture-matrix system. Instead of depending on a complex and data-demanding geostatistical method for a precise representation of Fracture Aperture, a statistical linear congruential generator (LCG) method was applied in the present study to replicate the unpredictable nature of Fracture Aperture morphology. The results have been compared with the parallel plate model and simple sinusoidal model. Finally, sensitivity analysis of Fracture Aperture size and fluid flow rate has been carried out to identify the conditions at which Fracture heterogeneity is critical. The results indicate that LCG-Aperture enhances the heat transfer between Fracture and hot rock matrix compared to the parallel and sinusoidal Fractures. Further, the temperature profiles in hot rock indicate that there was a greater loss of heat for the case of LCG-Aperture (25% loss) compared to sinusoidal (16%) and parallel plate (8%) Apertures. It was found that heterogeneity does not play a major role at small Fracture Aperture size (≤50 μm) and at low flow rates. However, as Fracture Aperture size increases, the heterogeneity plays a vital part even at low flow rates
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Effect of random Fracture Aperture on the transport of colloids in a coupled Fracture-matrix system
Geosciences Journal, 2017Co-Authors: Nikhil Bagalkot, Govindarajan Suresh KumarAbstract:A variable Aperture model, including the random variation of Fracture Aperture as against the conventional parallel plate model, has been developed to adequately examine the transport of colloids/suspended particles in a single coupled Fracturematrix system. Rather than relying on a complex geostatistical method for an accurate representation of Fracture Aperture, which requires an enormous field data and resource for its validation, a simple statistical method (linear congruential generator) is implemented in the present article. The random variation of Fracture Aperture is an honest representation of the unpredictable geometry/ morphology of Fracture Aperture in comparison with widely applied the conventional parallel plate model or the simple mathematical functions based on fractal theory (self-affine structures). A considerable number of parameters are involved in investigating the extent of penetration of colloids into the rock matrix, which creates complexity and ambiguity in the analysis. To overcome this problem, a single parameter “Maximum Penetration Factor” has been introduced for simple and reliable assessment of diffusion of colloids within the rock matrix. Additionally, a non-dimensional parameter ‘Matrix Mitigation Factor’ has been introduced in the present study, which can provide a means of evaluating the diffusion of suspended particles within the rock matrix when it comes to real time applications like microbial enhanced recovery (MEOR) and chemical enhanced recovery (CEOR) in the petroleum industry (nanoparticles and nanofluids). A semi-implicit finite difference model has been adopted for solving the coupled partial differential equations in the present numerical study. Finally, Neumann and Robinson boundary conditions as a function of time have been applied at the Fracture inlet to better represent the field scenario as against the conventional constant source condition (Dirichlet). The model results indicate that there is a difference in concentration between the parallel plate model and random Fracture model when it comes to colloidal concentration in the Fracture and rock matrix. The variance in concentration is due to the inclusion of variation of the Aperture in the variable Aperture model, which is absent in the parallel plate model. Additionally, the results suggest that the variable source boundary condition has a significant influence on the transport of colloids in Fracture-matrix system. Finally, from the evaluation of the extent of diffusion of colloids into rock matrix, it was concluded that that variable Aperture model is associated with more mitigation of colloids compared to the parallel plate model, especially in the case of random Fracture.
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thermal front propagation in variable Aperture Fracture matrix system a numerical study
Sadhana-academy Proceedings in Engineering Sciences, 2015Co-Authors: Nikhil Bagalkot, Suresh G KumarAbstract:A numerical study on the effect of complex Fracture Aperture geometry on propagation of thermal front in a coupled single Fracture–matrix system has been carried out. Sinusoidal and logarithmic functions have been used to capture the variation in Fracture Aperture. Modifications have been made to existing coupled partial differential governing equations to consider the variation of Fracture Aperture. Effect of temperature on the thermal and physical properties of rock have been incorporated. A fully implicit finite difference scheme has been used to discretize the coupled governing equations. Thermal convection, dispersion and conduction are the major transport processes within Fracture, while conduction is the major transport process within rock matrix. The results suggest that variation of Fracture Aperture increases the heat transfer rate at the Fracture–matrix interface. Sensitivity analysis on rock thermal conductivity and Fracture Aperture have been carried out. The results suggest that the heat transfer from rock matrix to Fracture for the case of the parallel plate model is greatly dependent on the rock thermal conductivity (λ m) as compared to variable Aperture model. Further, the thermal front propagation for both parallel plate model and variable Aperture model is sensitive to changes in Fracture Aperture. The heat transfer rate at the interface is greater at smaller Fracture Apertures and decreases with increase in Aperture.
Philip H Stauffer - One of the best experts on this subject based on the ideXlab platform.
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discriminating underground nuclear explosions leading to late time radionuclide gas seeps
Geophysical Research Letters, 2020Co-Authors: Dylan R Harp, Philip H Stauffer, Michelle S Bourret, Edward M KwicklisAbstract:Utilizing historical data from the U.S. nuclear test program and freely available barometric pressure data, we performed an analytical barometric-pumping efficiency analysis to determine factors resulting in late-time radionuclide gas seeps from underground nuclear explosions. We considered sixteen underground nuclear explosions with similar geology and test setup, of which five resulted in the measurement of late-time radionuclide gas concentrations at the ground surface. The factors we considered include barometric frequency and amplitude, depth of burial, air-filled porosity, intact-rock permeability, Fracture Aperture, and Fracture spacing. The analysis indicates that the best discriminators of late-time radionuclide gas seeps for these explosions are barometric frequency and amplitude and air-filled porosity. While geologic information on Fracture Aperture and spacing is not available for these explosions, the sensitivity of barometric-pumping efficiency to Fracture Aperture indicates that Fracture a...
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radionuclide gas transport through nuclear explosion generated Fracture networks
Scientific Reports, 2015Co-Authors: Amy B Jordan, Philip H Stauffer, Earl E Knight, Esteban Rougier, Dale N AndersonAbstract:Underground nuclear weapon testing produces radionuclide gases which may seep to the surface. Barometric pumping of gas through explosion-Fractured rock is investigated using a new sequentially-coupled hydrodynamic rock damage/gas transport model. Fracture networks are produced for two rock types (granite and tuff) and three depths of burial. The Fracture networks are integrated into a flow and transport numerical model driven by surface pressure signals of differing amplitude and variability. There are major differences between predictions using a realistic Fracture network and prior results that used a simplified geometry. Matrix porosity and maximum Fracture Aperture have the greatest impact on gas breakthrough time and window of opportunity for detection, with different effects between granite and tuff simulations highlighting the importance of accurately simulating the Fracture network. In particular, maximum Fracture Aperture has an opposite effect on tuff and granite, due to different damage patterns and their effect on the barometric pumping process. From stochastic simulations using randomly generated hydrogeologic parameters, normalized detection curves are presented to show differences in optimal sampling time for granite and tuff simulations. Seasonal and location-based effects on breakthrough, which occur due to differences in barometric forcing, are stronger where the barometric signal is highly variable.
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uncertainty in prediction of radionuclide gas migration from underground nuclear explosions
Vadose Zone Journal, 2014Co-Authors: Amy B Jordan, George Zyvoloski, Mark Person, J K Maccarthy, Philip H Stauffer, Dale N AndersonAbstract:Underground nuclear explosions (UNEs) produce radionuclide gases that may seep to the surface over weeks to months. The objective of this research was to quantify the impact of uncertainties in hydrologic parameters (Fracture Aperture, matrix permeability, porosity, and saturation) and season of detonation on the timing of gas breakthrough. Numerical sensitivity analyses were performed, with barometric pumping providing the primary driving force for gas migration, for the case of a 1 kt UNE at 400-m depth of burial. Gas arrival time was most affected by matrix permeability and Fracture Aperture. Gases having higher diffusivity were more sensitive to uncertainty in the rock properties. The effect of seasonality in the barometric pressure forcing was found to be important, with detonations in March the least likely to be detectable based on barometric data for Rainier Mesa, Nevada. Monte Carlo realizations were performed with all four parameters varying simultaneously to determine their interrelated effects. The Monte Carlo method was also used to predict the window of opportunity for 133 Xe detection from a 1 kt UNE at Rainier Mesa, with and without matching the model to SF 6 and 3 He data from the 1993 Non-Proliferation Experiment. Results from the data-blind Monte Carlo simulations were similar but were biased toward earlier arrival time and less likely to show detectable 133 Xe. The estimated timing of gas arrival may be used to deploy personnel and equipment to the site of a suspected UNE, if allowed under the terms of the Comprehensive Nuclear Test-Ban Treaty.
