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Numerical, Laboratory And Field Studiesof Gas Production FromNatural Hydrate Accumulations in Geologic Media

Description: We discuss the range of activities at Lawrence BerkeleyNational Laboratory in support of gas production from natural hydrates.Investigations of production from the various classes of hydrate depositsby numerical simulation indicate their significant promise as potentialenergy sources. Laboratory studies are coordinated with the numericalstudies and are designed to address knowledge gaps that are important tothe prediction of gas production. Our involvement in field tests is alsobriefly discussed.
Date: October 17, 2006
Creator: Moridis, George J.; Kneafsey, Timothy J.; Kowalsky, Michael & Reagan, Matthew
Partner: UNT Libraries Government Documents Department

Summary report on Phase 1 feasibility study of in-drift diffusion

Description: This report summarizes the work performed and findings obtained during the Phase 1 (feasibility study) of the Engineered Barrier System (EBS) in-drift diffusion evaluation. The objective of this work is to characterize and reduce uncertainties associated with measurements of diffusion coefficients and modeling of diffusion processes. Phase 1 of the study evaluates measurement and modeling uncertainties as well as scoping alternative measurement and modeling approaches. Phase 2 of the study consists of rigorous diffusion testing of invert materials (employing approaches developed from Phase 1) and the development of a calibrated invert diffusion model (with separate surface and internal water components if needed to interpret measured diffusion data) for use in total system performance assessment (TSPA).
Date: October 10, 2001
Creator: Hu, Qinhong; Kneafsey, Timothy & Wang, Joe
Partner: UNT Libraries Government Documents Department

Water injection as a means for reducing non-condensible andcorrosive gases in steam produced from vapor-dominated reservoirs

Description: Large-scale water injection at The Geysers, California, hasgenerated substantial benefits in terms of sustaining reservoir pressuresand production rates, as well as improving steam composition by reducingthe content of non-condensible gases (NCGs). Two effects have beenrecognized and discussed in the literature as contributing to improvedsteam composition, (1) boiling of injectate provides a source of "clean"steam to production wells, and (2) pressurization effects induced byboiling of injected water reduce upflow of native steam with large NCGconcentrations from depth. In this paper we focus on a possibleadditional effect that could reduce NCGs in produced steam by dissolutionin a condensed aqueous phase.Boiling of injectate causes pressurizationeffects that will fairly rapidly migrate outward, away from the injectionpoint. Pressure increases will cause an increase in the saturation ofcondensed phase due to vapor adsorption on mineral surfaces, andcapillary condensation in small pores. NCGs will dissolve in theadditional condensed phase which, depending upon their solubility, mayreduce NCG concentrations in residual steam.We have analyzed thepartitioning of HCl between vapor and aqueous phases, and have performednumerical simulations of injection into superheated vapor zones. Oursimulations provide evidence that dissolution in the condensed phase canindeed reduce NCG concentrations in produced steam.
Date: January 8, 2007
Creator: Pruess, Karsten; Spycher, Nicolas & Kneafsey, Timothy J.
Partner: UNT Libraries Government Documents Department

Laboratory measurements on core-scale sediment/hydrate samples topredice reservoir behavior

Description: Measurements on hydrate-bearing laboratory and field samplesare necessary in order to provide realistic bounds on parameters used innumerically modeling the production of natural gas from hydrate-bearingreservoirs. The needed parameters include thermal conductivity,permeability, relative permeability-saturation(s) relationships, andcapillary pressure-saturation(s) relationships. We have developed atechnique to make hydrate-bearing samples ranging in scale from coreplug-size to core-size in the laboratory to facilitate making thesemeasurements. In addition to pressure and temperature measurements, weuse x-ray computed tomography scanning to provide high-resolution dataproviding insights on processes occurring in our samples. Several methodsare available to make gas hydrates in the laboratory, and we expect thatthe method used to make the hydrate will impact the behavior of thehydrate sample, and the parameters measured.
Date: November 2, 2005
Creator: Kneafsey, Timothy J.; Seol, Yongkoo; Moridis, George J.; Tomutsa,Liviu & Freifeld, Barry M.
Partner: UNT Libraries Government Documents Department

An evaluation of the active fracture concept with modelingunsaturated flow and transport in a fractured meter-sized block ofrock

Description: Numerical simulation is an effective and economical tool for optimally designing laboratory experiments and deriving practical experimental conditions. We executed a detailed numerical simulation study to examine the active fracture concept (AFC, Liu et al., 1998) using a cubic meter-sized block model. The numerical simulations for this study were performed by applying various experimental conditions, including different bottom flow boundaries, varying injection rates, and different fracture-matrix interaction (by increasing absolute matrix permeability at the fracture matrix boundary) for a larger fracture interaction under transient or balanced-state flow regimes. Two conceptual block models were developed based on different numerical approaches: a two-dimensional discrete-fracture-network model (DFNM) and a one-dimensional dual continuum model (DCM). The DFNM was used as a surrogate for a natural block to produce synthetic breakthrough curves of water and tracer concentration under transient or balanced-state conditions. The DCM is the approach typically used for the Yucca Mountain Project because of its computational efficiency. The AFC was incorporated into the DCM to capture heterogeneous flow patterns that occur in unsaturated fractured rocks. The simulation results from the DCM were compared with the results from the DFNM to determine whether the DCM could predict the water flow and tracer transport observed in the DFNM at the scale of the experiment. It was found that implementing the AFC in the DCM improved the prediction of unsaturated flow and that the flow and transport experiments with low injection rates in the DFNM were compared better with the AFC implemented DCM at the meter scale. However, the estimated AFC parameter varied from 0.38 to 1.0 with different flow conditions, suggesting that the AFC parameter was not a sufficient to fully capture the complexity of the flow processes in a one meter sized discrete fracture network.
Date: May 30, 2003
Creator: Seol, Yongkoo; Kneafsey, Timothy J. & Ito, Kazumasa
Partner: UNT Libraries Government Documents Department

On-Site Geologic Core Analysis Using a Portable X-ray ComputedTomographic System

Description: X-ray computed tomography (CT) is an established techniquefor nondestructively characterizing geologic cores. CT providesinformation on sediment structure, diagenetic alteration, fractures, flowchannels and barriers, porosity, and fluid-phase saturation. A portableCT imaging system has been developed specifically for imaging whole-roundcores at the drilling site. The new system relies upon carefully designedradiological shielding to minimize the size and weight of the resultinginstrument. Specialized x-ray beam collimators and filters maximizesystem sensitivity and performance. The system has been successfullydeployed on the research vessel Joides Resolution for Ocean DrillingProgram's Leg 204 and 210, within the Ocean Drilling Program'srefrigerated Gulf Coast Core Repository, as well as on the Hot Ice #1drilling platform located near the Kuparuk Field, Alaska. A methodologyfor performingsimple densiometry measurements, as well as scanning forgross structural features, will be presented using radiographs from ODPLeg 204. Reconstructed CT images from Hot Ice #1 will demonstrate the useof CT for discerning core textural features. To demonstrate the use of CTto quantitatively interpret dynamic processes, we calculate 95 percentconfidence intervals for density changes occurring during a laboratorymethane hydrate dissociation experiment. The field deployment of a CTrepresents a paradigm shift in core characterization, opening up thepossibility for rapid systematic characterization of three-dimensionalstructural features and leading to improved subsampling andcore-processing procedures.
Date: March 1, 2004
Creator: Freifeld, Barry M.; Kneafsey, Timothy J. & Rack, Frank
Partner: UNT Libraries Government Documents Department

Core Scale and Pore Scale Studies of Carbon Dioxide Migration Insaline Formations

Description: Understanding core scale and pore scale migration of CO2 will improve our ability to predict storage capacity and determine the effectiveness of solubility and capillary (residual CO2) trapping. While the theoretical underpinnings of multi-phase flow are well developed for oil and gas production, there are few, if any measurements relevant to CO2 storage in saline formations. To fill this gap, core scale and porescale measurements of CO2 migration in sandstone are being conducted.
Date: October 3, 2005
Creator: Benson, Sally M.; Tomutsa, Liviu; Silin, Dmitriy; Kneafsey,Timothy & Miljkovic, Ljubinko
Partner: UNT Libraries Government Documents Department

Correspondence of the Gardner and van Genuchten/Mualem relativepermeability function parameters

Description: The Gardner and van Genuchten models of relativepermeability are widely used in analytical and numerical solutions toflow problems. However, the applicab ility of the Gardner model to realproblems is usually limited, because empirical relative permeability datato calibrate the model are not routinely available. In contrast, vanGenuchten parameters can be estimated using more routinely availablematric potential and saturation data. However, the van Genuchten model isnot amenable to analytical solutions. In this paper, we introducegeneralized conversion formulae that reconcile these two models. Ingeneral, we find that the Gardner parameter alpha G is related to the vanGenuchten parameters alpha vG and n by alpha G=alpha vG ~; 1:3 n. Thisconversion rule will allow direct recasting of Gardner-based analyticalsolutions in the van Genuchten parameter space. The validity of theproposed formulae was tested by comparing the predicted relativepermeability of various porous media with measured values.
Date: January 3, 2007
Creator: Ghezzehei, Teamrat A.; Kneafsey, Timothy J. & Su, Grace W.
Partner: UNT Libraries Government Documents Department

Development of a portable x-ray computed tomographic imaging system for drill-site investigation of recovered core

Description: A portable x-ray computed tomography (CT) system was constructed for imaging core at drill sites. Performing drill-site-based x-ray scanning and CT analysis permits rapid evaluation of core properties (such as density, lithologic structure, and macroporosity distribution) and allows for real-time decision making for additional core-handling procedures. Because of the speed with which scanning is performed, systematic imaging and electronic cataloging of all retrieved core is feasible. Innovations (such as a novel clamshell shielding arrangement integrated with system interlocks) permit safe operation of the x-ray system in a busy core handling area. The minimization of the volume encapsulated with shielding reduces the overall system weight and facilitates instrument portability. The x-ray system as originally fabricated had a 110 kV x-ray source with a fixed 300-micron focal spot size. A 15 cm image intensifier with a cesium iodide phosphor input screen was coupled to a CCD for image capture. The CT system has since been modified with a 130 kV micro-focal x-ray source. With the x-ray system's variable focal spot size, high-resolution studies (10-micron resolution) can be performed on core plugs and coarser (100-micron resolution) images can be acquired of whole drill cores. The development of an aluminum compensator has significantly improved the dynamic range and accuracy of the system. An x-ray filter has also been incorporated, permitting rapid acquisition of multi-energy scans for more quantitative analysis of sample mineralogy. The x-ray CT system has operated reliably under extreme field conditions, which have varied from shipboard to arctic.
Date: May 2003
Creator: Freifeld, Barry M.; Kneafsey, Timothy J.; Tomutsa, Liviu & Pruess, Jacob
Partner: UNT Libraries Government Documents Department

Estimation of composite thermal conductivity of a heterogeneousmethane hydrate sample using iTOUGH2

Description: We determined the composite thermal conductivity (ktheta) ofa porous methanehydrate sample (composedof hydrate, water, and methan egas) as a function of density using iTOUGH2. X-ray computed tomography(CT) was used to visualize and quantify the density changes that occurredduring hydrate formation from granular ice. The composite thermalconductivity was estimated and validated by minimizing the differencesbetween the observed and the predicted thermal response using historymatching. The estimated density-dependent composite thermal conductivityranged between 0.25 and 0.58 W/m/K.
Date: May 15, 2006
Creator: Gupta, Arvind; Kneafsey, Timothy J.; Moridis, George J.; Seol,Yongkoo; Kowalsky, Michael B. & Sloan Jr., E.D.
Partner: UNT Libraries Government Documents Department

Methane Hydrate Formation and Dissociation in a PartiallySaturated Core-Scale Sand Sample

Description: We performed a sequence of tests on a partiallywater-saturated sand sample contained in an x-ray transparent aluminumpressure vessel that is conducive to x-ray computed tomography (CT)observation. These tests were performed to gather data for estimation ofthermal properties of the sand/water/gas system and thesand/hydrate/water/gas systems, as well as data to evaluate the kineticnature of hydrate dissociation. The tests included mild thermalperturbations for the estimation of the thermal properties of thesand/water/gas system, hydrate formation, thermal perturbations withhydrate in the stability zone, hydrate dissociation through thermalstimulation, additional hydrate formation, and hydrate dissociationthrough depressurization with thermal stimulation. Density changesthroughout the sample were observed as a result of hydrate formation anddissociation, and these processes induced capillary pressure changes thataltered local water saturation.
Date: November 3, 2005
Creator: Kneafsey, Timothy J.; Tomutsa, Liviu; Moridis, George J.; Seol,Yongkoo; Freifeld, Barry M.; Taylor, Charles E. et al.
Partner: UNT Libraries Government Documents Department

Preliminary relative permeability estimates of methanehydrate-bearing sand

Description: The relative permeability to fluids in hydrate-bearing sediments is an important parameter for predicting natural gas production from gas hydrate reservoirs. We estimated the relative permeability parameters (van Genuchten alpha and m) in a hydrate-bearing sand by means of inverse modeling, which involved matching water saturation predictions with observations from a controlled waterflood experiment. We used x-ray computed tomography (CT) scanning to determine both the porosity and the hydrate and aqueous phase saturation distributions in the samples. X-ray CT images showed that hydrate and aqueous phase saturations are non-uniform, and that water flow focuses in regions of lower hydrate saturation. The relative permeability parameters were estimated at two locations in each sample. Differences between the estimated parameter sets at the two locations were attributed to heterogeneity in the hydrate saturation. Better estimates of the relative permeability parameters require further refinement of the experimental design, and better description of heterogeneity in the numerical inversions.
Date: May 8, 2006
Creator: Seol, Yongkoo; Kneafsey, Timothy J.; Tomutsa, Liviu & Moridis,George J.
Partner: UNT Libraries Government Documents Department

Mobility of Tritium in Engineered and Earth Materials at the NuMIFacility, Fermilab: Progress report for work performed between June 13and September 30, 2006

Description: This report details the work done between June 13 andSeptember 30, 2006 by Lawrence Berkeley National Laboratory (LBNL)scientists to assist Fermi National Accelerator Laboratory (Fermilab)staff in understanding tritium transport at the Neutrino at the MainInjector (NuMI) facility. As a byproduct of beamline operation, thefacility produces (among other components) tritium in engineeredmaterials and the surrounding rock formation. Once the tritium isgenerated, it may be contained at the source location, migrate to otherregions within the facility, or be released to theenvironment.
Date: October 25, 2006
Creator: Pruess, Karsten; Conrad, Mark; Finsterle, Stefan; Kennedy, Mack; Kneafsey, Timothy; Salve, Rohit et al.
Partner: UNT Libraries Government Documents Department

Physical property changes in hydrate-bearingsediment due to depressurization and subsequent repressurization

Description: Physical property measurements of sediment cores containing natural gas hydrate are typically performed on material exposed at least briefly to non-in situ conditions during recovery. To examine effects of a brief excursion from the gas-hydrate stability field, as can occur when pressure cores are transferred to pressurized storage vessels, we measured physical properties on laboratory-formed sand packs containing methane hydrate and methane pore gas. After depressurizing samples to atmospheric pressure, we repressurized them into the methane-hydrate stability field and remeasured their physical properties. Thermal conductivity, shear strength, acoustic compressional and shear wave amplitudes and speeds are compared between the original and depressurized/repressurized samples. X-ray computed tomography (CT) images track how the gas-hydrate distribution changes in the hydrate-cemented sands due to the depressurization/repressurization process. Because depressurization-induced property changes can be substantial and are not easily predicted, particularly in water-saturated, hydrate-bearing sediment, maintaining pressure and temperature conditions throughout the core recovery and measurement process is critical for using laboratory measurements to estimate in situ properties.
Date: June 1, 2008
Creator: Kneafsey, Timothy; Waite, W.F.; Kneafsey, T.J.; Winters, W.J. & Mason, D.H.
Partner: UNT Libraries Government Documents Department

Experimental and numerical simulation of dissolution andprecipitation: Implications for fracture sealing at Yucca Mountain,Nevada

Description: Plugging of flow paths caused by mineral precipitation in fractures above the potential repository at Yucca Mountain, Nevada, would reduce the probability of water seeping into the repository. As part of an ongoing effort to evaluate thermal-hydrologic-chemical (THC) effects on flow in fractured media, we performed a laboratory experiment and numerical simulations to investigate mineral dissolution and precipitation under anticipated temperature and pressure conditions in the repository. To replicate mineral dissolution by vapor condensate in fractured tuff, water was flowed through crushed Yucca Mountain tuff at 94 C. The resulting steady-state fluid composition had a total dissolved solids content of about 140 mg/L; silica was the dominant dissolved constituent. A portion of the steady-state mineralized water was flowed into a vertically oriented planar fracture in a block of welded Topopah Spring Tuff that was maintained at 80 C at the top and 130 C at the bottom. The fracture began to seal with amorphous silica within five days. A 1-D plug-flow numerical model was used to simulate mineral dissolution, and a similar model was developed to simulate the flow of mineralized water through a planar fracture, where boiling conditions led to mineral precipitation. Predicted concentrations of the major dissolved constituents for the tuff dissolution were within a factor of 2 of the measured average steady-state compositions. The mineral precipitation simulations predicted the precipitation of amorphous silica at the base of the boiling front, leading to a greater than fifty-fold decrease in fracture permeability in 5 days, consistent with the laboratory experiment. These results help validate the use of a numerical model to simulate THC processes at Yucca Mountain. The experiment and simulations indicated that boiling and concomitant precipitation of amorphous silica could cause significant reductions in fracture porosity and permeability on a local scale. However, differences in fluid flow rates ...
Date: August 31, 2001
Creator: Dobson, Patrick F.; Kneafsey, Timothy J.; Sonnenthal, Eric L.; Spycher, Nicolas & Apps, John A.
Partner: UNT Libraries Government Documents Department

Field investigation of the drift shadow

Description: A drift shadow is an area immediately beneath an undergroundvoidthat, in theory, will be relatively drier than the surrounding rockmass. Numerical and analytical models of water flow through unsaturatedrock predict the existence of a drift shadow, but field tests confirmingits existence have yet to be performed. Proving the existence of driftshadows and understanding their hydrologic and transport characteristicscould provide a better understanding of how contaminants move in thesubsurface if released from waste emplacement drifts such as the proposednuclear waste repository at Yucca Mountain, Nevada. We describe the fieldprogram that will be used to investigate the existence of a drift shadowand the corresponding hydrological process at the Hazel-Atlas silica-sandmine located at the Black Diamond Mines Regional Preserve in Antioch,California. The location and configuration of this mine makes it anexcellent site to observe and measure drift shadow characteristics. Themine is located in a porous sandstone unit of the Domengine Formation, anapproximately 230 meter thick series of interbedded Eocene-age shales,coals, and massive-bedded sandstones. The mining method used at the minerequired the development of two parallel drifts, one above the other,driven along the strike of the mined sandstone stratum. Thisconfiguration provides the opportunity to introduce water into the rockmass in the upper drift and to observe and measure its flow around theunderlying drift. The passive and active hydrologic tests to be performedare described. In the passive method, cores will be obtained in a radialpattern around a drift and will be sectioned and analyzed for in-situwater content and chemical constituents. With the active hydrologic test,water will be introduced into the upper drift of the two parallel driftsand the flow of the water will be tracked as it passes near the bottomdrift. Tensiometers, electrical resistance probes, neutron probes, andground penetrating radar may be used to monitor the change in moisturecontent and potential over time as water ...
Date: September 8, 2005
Creator: Su, Grace W.; Kneafsey, Timothy J.; Ghezzehei, Teamrat A.; Marshall, Brian D. & Cook, Paul J.
Partner: UNT Libraries Government Documents Department

Use of computed X-ray tomographic data for analyzing the thermodynamics of a dissociating porous sand/hydrate mixture

Description: X-ray computed tomography (CT) is a method that has been used extensively in laboratory experiments for measuring rock properties and fluid transport behavior. More recently, CT scanning has been applied successfully to detect the presence and study the behavior of naturally occurring hydrates. In this study, we used a modified medical CT scanner to image and analyze the progression of a dissociation front in a synthetic methane hydrate/sand mixture. The sample was initially scanned under conditions at which the hydrate is stable (atmospheric pressure and liquid nitrogen temperature, 77 K). The end of the sample holder was then exposed to the ambient air, and the core was continuously scanned as dissociation occurred in response to the rising temperature. CT imaging captured the advancing dissociation front clearly and accurately. The evolved gas volume was monitored as a function of time. Measured by CT, the advancing hydrate dissociation front was modeled as a thermal conduction problem explicitly incorporating the enthalpy of dissociation, using the Stefan moving-boundary-value approach. The assumptions needed to perform the analysis consisted of temperatures at the model boundaries. The estimated value for thermal conductivity of 2.6 W/m K for the remaining water ice/sand mixture is higher than expected based on conduction alone; this high value may represent a lumped parameter that incorporates the processes of heat conduction, methane gas convection, and any kinetic effects that occur during dissociation. The technique presented here has broad implications for future laboratory and field testing that incorporates geophysical techniques to monitor gas hydrate dissociation.
Date: February 2002
Creator: Freifeld, Barry M.; Kneafsey, Timothy J.; Tomutsa, Liviu; Stern, Laura A. & Kirby, Stephen H.
Partner: UNT Libraries Government Documents Department

Studies of Reaction Kinetics of Methane Hydrate Dissocation in Porous Media

Description: The objective of this study is the description of the kinetic dissociation of CH4-hydrates in porous media, and the determination of the corresponding kinetic parameters. Knowledge of the kinetic dissociation behavior of hydrates can play a critical role in the evaluation of gas production potential of gas hydrate accumulations in geologic media. We analyzed data from a sequence of tests of CH4-hydrate dissociation by means of thermal stimulation. These tests had been conducted on sand cores partially saturated with water, hydrate and CH4 gas, and contained in an x-ray-transparent aluminum pressure vessel. The pressure, volume of released gas, and temperature (at several locations within the cores) were measured. To avoid misinterpreting local changes as global processes, x-ray computed tomography scans provided accurate images of the location and movement of the reaction interface during the course of the experiments. Analysis of the data by means of inverse modeling (history matching ) provided estimates of the thermal properties and of the kinetic parameters of the hydration reaction in porous media. Comparison of the results from the hydrate-bearing porous media cores to those from pure CH4-hydrate samples provided a measure of the effect of the porous medium on the kinetic reaction. A tentative model of composite thermal conductivity of hydrate-bearing media was also developed.
Date: March 10, 2005
Creator: Moridis, George J.; Seol, Yongkoo & Kneafsey, Timothy J.
Partner: UNT Libraries Government Documents Department


Description: Natural-gas hydrates have been encountered beneath the permafrost and considered a nuisance by the oil and gas industry for years. Engineers working in Russia, Canada and the USA have documented numerous drilling problems, including kicks and uncontrolled gas releases, in arctic regions. Information has been generated in laboratory studies pertaining to the extent, volume, chemistry and phase behavior of gas hydrates. Scientists studying hydrate potential agree that the potential is great--on the North Slope of Alaska alone, it has been estimated at 590 TCF. However, little information has been obtained on physical samples taken from actual rock containing hydrates. The work scope drilled and cored a well The Hot Ice No. 1 on Anadarko leases beginning in FY 2003 and completed in 2004. An on-site core analysis laboratory was built and utilized for determining the physical characteristics of the hydrates and surrounding rock. The well was drilled from a new Anadarko Arctic Platform that has a minimal footprint and environmental impact. The final efforts of the project are to correlate geology, geophysics, logs, and drilling and production data and provide this information to scientists developing reservoir models. No gas hydrates were encountered in this well; however, a wealth of information was generated and is contained in this report. The Hot Ice No. 1 well was drilled from the surface to a measured depth of 2300 ft. There was almost 100% core recovery from the bottom of surface casing at 107 ft to total depth. Based on the best estimate of the bottom of the methane hydrate stability zone (which used new data obtained from Hot Ice No. 1 and new analysis of data from adjacent wells), core was recovered over its complete range. Approximately 580 ft of porous, mostly frozen, sandstone and 155 of conglomerate were recovered in the Ugnu ...
Date: February 1, 2005
Creator: Sigal, Richard; Newsham, Kent; Williams, Thomas; Freifeld, Barry; Kneafsey, Timothy; Sondergeld, Carl et al.
Partner: UNT Libraries Government Documents Department

Methane hydrate formation and dissociation in a partially saturated sand

Description: To predict the behavior of hydrate-bearing sediments and the economic extractability of natural gas from reservoirs containing gas hydrates, we need reservoir simulators that properly represent the processes that occur, as well as accurate parameters. Several codes are available that represent some or all of the expected processes, and values for some parameters are available. Where values are unavailable, modelers have used estimation techniques to help with their predictions. Although some of these techniques are well respected, measurements are needed in many cases to verify the parameters. We have performed a series of experiments in a partially water saturated silica sand sample. The series included methane hydrate formation, and dissociation by both thermal stimulation and depressurization. The sample was 7.6 cm in diameter and 25 cm in length. In addition to measuring the system pressure and temperatures at four locations in the sample, we measured local density within the sample using x-ray computed tomography. Our goals in performing the experiment were to gather information for estimating thermal properties of the medium and to examine nonequilibrium processes.
Date: November 24, 2004
Creator: Kneafsey, Timothy J.; Tomutsa, Liviu; Taylor, Charles E.; Gupta, Arvind; Moridis, George; Freifeld, Barry et al.
Partner: UNT Libraries Government Documents Department

Geothermal reservoir simulation to enhance confidence in predictions for nuclear waste disposal

Description: Numerical simulation of geothermal reservoirs is useful and necessary in understanding and evaluating reservoir structure and behavior, designing field development, and predicting performance. Models vary in complexity depending on processes considered, heterogeneity, data availability, and study objectives. They are evaluated using computer codes written and tested to study single and multiphase flow and transport under nonisothermal conditions. Many flow and heat transfer processes modeled in geothermal reservoirs are expected to occur in anthropogenic thermal (AT) systems created by geologic disposal of heat-generating nuclear waste. We examine and compare geothermal systems and the AT system expected at Yucca Mountain, Nevada, and their modeling. Time frames and spatial scales are similar in both systems, but increased precision is necessary for modeling the AT system, because flow through specific repository locations will affect long-term ability radionuclide retention. Geothermal modeling experience has generated a methodology, used in the AT modeling for Yucca Mountain, yielding good predictive results if sufficient reliable data are available and an experienced modeler is involved. Codes used in geothermal and AT modeling have been tested extensively and successfully on a variety of analytical and laboratory problems.
Date: June 15, 2002
Creator: Kneafsey, Timothy J.; Pruess, Karsten; O'Sullivan, Michael J. & Bodvarsson, Gudmundur S.
Partner: UNT Libraries Government Documents Department

Methane Hydrate Formation and Dissocation in a Partially Saturated Sand--Measurements and Observations

Description: We performed a sequence of tests on a partially water-saturated sand sample contained in an x-ray transparent aluminum pressure vessel that is conducive to x-ray computed tomography (CT) observation. These tests were performed to gather data for estimation of thermal properties of the sand/water/gas system and the sand/hydrate/water/gas systems, as well as data to evaluate the kinetic nature of hydrate dissociation. The tests included mild thermal perturbations for the estimation of the thermal properties of the sand/water/gas system, hydrate formation, thermal perturbations with hydrate in the stability zone, hydrate dissociation through thermal stimulation, additional hydrate formation, and hydrate dissociation through depressurization with thermal stimulation. Density changes throughout the sample were observed as a result of hydrate formation and dissociation, and these processes induced capillary pressure changes that altered local water saturation.
Date: March 1, 2005
Creator: Kneafsey, Timothy J.; Tomutsa, Liviu; Moridis, George J.; Seol, Yongkoo; Freifeld, Barry; Taylor, Charles E. et al.
Partner: UNT Libraries Government Documents Department

Characterizing unsaturated diffusion in porous tuff gravel

Description: Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (for example, the potential high-level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using x-ray computed tomography in partially saturated gravel, and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of potassium chloride in 2-4 mm granular tuff at multiple water contents were calculated from electrical conductivity measurements using the Nernst-Einstein equation. In the second, we used laser ablation with inductively coupled plasma-mass spectrometry to perform micro-scale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube-cube and cube-tetrahedron). The x-ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 x 10{sup -14} m{sup 2}/s for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.
Date: November 12, 2003
Creator: Hu, Qinhong; Kneafsey, Timothy J.; Roberts, Jeffery J.; Tomutsa, Liviu & Wang, Joseph, S.Y.
Partner: UNT Libraries Government Documents Department