TOUGH2 Code Research Articles

Thermo-Hydro-Mechanical Coupled Modeling of In-Situ Behavior of the Full-Scale Heating Test in the Callovo-Oxfordian Claystone

Within the context for deep geological disposal (DGD) of high-level radioactive waste (HLW), thermo-hydro-mechanical (THM) coupled numerical modeling has become significantly important for studying the safe disposal of HLW. In this work, a 3D mechanical module is incorporated into the thermal–hydraulic (TH) coupled code TOUGH2, thus forming an integrated THM coupled simulator referred to as TOUGH2Biot. The Galerkin finite element method is used to discretize the space for rock mechanical calculation. The mechanical process is sequentially coupled with the fluid and heat flow processes, which further gives feedback to the flow through stress-dependent hydraulic properties (e.g., porosity and permeability). Based on the available geological data at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL) in France, the improved simulator is used to analyze the coupled THM behaviors of the Callovo-Oxfordian claystone (COx) induced by thermal loading. The anisotropy of material parameters (e.g., permeability and thermal conductivity) caused by the bedding and of in-situ stresses are well considered in our model. The numerical simulation can reasonably reproduce the field observations, including changes in temperature and pore pressure at monitoring boreholes during the ALC1604 experiment. The modeling results indicate that the anisotropic effects are remarkable, and temperature, pore pressure, and effective stress along the bedding increase more rapidly than in the vertical direction. Insight into numerical results through the visual model is beneficial for helping us to interpret the field observations and to understand the complex THM problem in the COx claystone formation. The numerical method and the modeling results presented in this work can be effectively used in support of performance assessment studies of HLW disposal sites to build confidence in the safety of future applications of nuclear energy systems.

The implementation of genetic algorithm for the identification of DNAPL sources

Contamination caused by dense non-aqueous phase liquids (DNAPLs) is a serious peril to the groundwater supply quality. The DNAPLs are largely undetected and yet are likely to be a significant factor limiting site remediation. It is necessary to identify the sources of pollution and control pollution in the early stage. The primary objective of this research was to investigate the pollution source and intensity of migrating DNAPLs. This objective was accomplished through the combination of genetic algorithm (GA) and T2VOC numerical experimental simulation. This study considers two synthetic cases of numerical modeling based on the T2VOC from the TOUGH2 code family to assess and evaluate the efficacity of the algorithm. The first case identified concurrently the intensity as well as the location of the one unknown contaminated water source. The second case distinguished various contaminated water sources. For the single source pollution identification, the estimation of intensity error was 0.08%, and the error of GA at second and third true value calculation was 0.05% and 0.03%, respectively, achieving the goal of pollution identification accurately. While for the multiple sources, the deviation of intensity was ranging between 1.02 and 5.65%, the results suggested that source No.2 was mainly responsible for the leakage, and source No.3 adopted the secondary responsibility among others. These two cases suggested that GA established by optimal search strategy could be adopted for effective identification of DNAPL pollution sources. This study provides information that could facilitate the development and implementation of innovative remediation strategies.

Mapping the Vulnerability of Groundwater to Wastewater Spills for Source Water Protection in a Shale Gas Region

Source water protection in areas of shale gas development encompasses identifying areas that are the most vulnerable to groundwater quality deterioration due to spills of natural gas production wastewater. This study uses the density-dependent flow and transport code TOUGH2 to quantify the time and distance of travel of saline wastewater plumes for different hydrogeological settings in Northeast British Columbia. The models were designed to address three main factors identified from the DRASTIC method for vulnerability assessment: (1) depth to water, (2) impact of vadose zone, and (3) conductivity of the aquifer materials. The vadose zone permeability and depth to water table are dominant controls on the wastewater migration rate and footprint. Overall, the vulnerability in the region is relatively low, with exceptions near river valleys and areas with shallow water tables. The vulnerability maps can be used as a preliminary risk assessment tool, as they are based on the main factors influencing the potential of a wastewater spill to contaminate an aquifer.

Numerical and thermodynamic modeling for estimating production capacity of NW Sabalan geothermal field, Iran

In this paper, a new estimate of the power production capacity of the NW Sabalan reservoir is presented based on an evaluation of the recent field data. An improved three-dimensional numerical model (EOS1) was created utilizing the Tough2 simulator code. A rectangular prism with a length of 11.5 km and a width of 8 km (92.9 km) and a depth of 5.11 km is assumed for modeling. Also, 17 horizontal layers were considered for the model, with a layer thickness ranging from 100 to 1000 m, extending from the elevation of -1000 to 4110 masl. The model was validated using temperature and pressure profiles at ten deep exploration wells. The best-fit model of the natural state demonstrated the existence of an upflow of high-temperature fluid in the southern part of the area (with a recharge rate of 108 kg/s). For evaluating and predicting the power production, a thermodynamic model of the single-flash geothermal power plant was developed for the NW Sabalan field. The optimum separation pressure of approximately 5.1 bar was obtained, assuming the quality of the turbine outlet steam is equal to 0.86. At this pressure, the system yields about 31 MW of electricity as a best estimate. Then simulations of four scenarios of 25, 50, 100, and 150 MWe were carried out for 30 years using the TOUGH2 code. The results of the scenarios showed that the main factors controlling the production capacity in the NW Sabalan field are the expected pressure drop and the enthalpy of the produced fluid. This field cannot provide the required enthalpy of the produced fluid for more than 150 MWe. The field can provide enough high enthalpy fluid for a maximum capacity of 114 MWe for 70 years. For this capacity, a limit of 3.125 km2 was identified on the area of the main production zone.

Assessment of the Effective Variants Leading to Higher Efficiency for the Geothermal Doublet, Using Numerical Analysis‒Case Study from Poland (Szczecin Trough)

Numerical models of geothermal doublet allows us to reduce the high risk associated with the selection of the most effective location of a production well. Furthermore, modeling is a suitable tool to verify possible changes in operational geothermal parameters, which guarantees liveliness of the system. An appropriate selection of software as well as the methodology used to generate numerical models significantly affects the quality of the obtained results. In this paper, the authors discuss the influence of such parameters as grid density and distance between wells on the efficiency of geothermal heating plant. The last stage of the analysis was connected with estimation of geothermal power potential for a hypothetical geothermal doublet. Numerical simulations were carried out using the TOUGH2 code, which applies the finite-difference method. The research was conducted in the Szczecin Trough area (NW Poland), based on archival data from Choszczno IG-1 well. The results demonstrated that in the studied case of the Choszczno region, the changes in the distance of boreholes can have a visible influence on obtained results; however the grid density of the numerical model did not achieve a significant impact on it. The results show the significant importance of numerical modeling aimed at increasing the efficiency of a potential geothermal heating plant.

Combining natural gas recovery and CO2-based geothermal energy extraction for electric power generation

We investigate the potential for extracting heat from produced natural gas and utilizing supercritical carbon dioxide (CO2) as a working fluid for the dual purpose of enhancing gas recovery (EGR) and extracting geothermal energy (CO2-Plume Geothermal – CPG) from deep natural gas reservoirs for electric power generation, while ultimately storing all of the subsurface-injected CO2. Thus, the approach constitutes a CO2 capture double-utilization and storage (CCUUS) system. The synergies achieved by the above combinations include shared infrastructure and subsurface working fluid. We integrate the reservoir processes with the wellbore and surface power-generation systems such that the combined system’s power output can be optimized. Using the subsurface fluid flow and heat transport simulation code TOUGH2, coupled to a wellbore heat-transfer model, we set up an anticlinal natural gas reservoir model and assess the technical feasibility of the proposed system. The simulations show that the injection of CO2 for natural gas recovery and for the establishment of a CO2 plume (necessary for CPG) can be conveniently combined. During the CPG stage, following EGR, a CO2-circulation mass flowrate of 110 kg/s results in a maximum net power output of 2 MWe for this initial, conceptual, small system, which is scalable. After a decade, the net power decreases when thermal breakthrough occurs at the production wells. The results confirm that the combined system can improve the gas field’s overall energy production, enable CO2 sequestration, and extend the useful lifetime of the gas field. Hence, deep (partially depleted) natural gas reservoirs appear to constitute ideal sites for the deployment of not only geologic CO2 storage but also CPG.

Seismogenic potential of withdrawal-reinjection cycles: Numerical modelling and implication on induced seismicity

Induced seismicity can be associated to the activity of fluid withdrawal and injection from/into the shallow crust (fracking, wastewater disposal into the deep crust, Enhanced Geothermal Systems technology, fluid extraction in oil fields and geothermal power plants). Long-term injection of large volumes of fluids is normally associated with induced seismicity, but the effect of withdrawal-reinjection in the same reservoir is less known, at least regarding its relation to simple injection. However, it is common experience worldwide that small (i.e. 10 MW or less) geothermal plants with withdrawal and re-injection of fluids in the same reservoir are mostly not associated with significant induced/triggered seismicity. This paper aims at understanding how to discriminate, on a numerical modelling basis, the seismogenic potential of withdrawal-reinjection with respect to injection only. With this aim, we analysed the induced pressure changes, the perturbed volumes of rocks and the potential for induced seismicity due to these operations. A set of simulations of injection or withdrawal-reinjection cycles, obtained by using the numerical code TOUGH2®, is applied to simple models of geothermal reservoirs, with varying permeability and lateral boundary constraints. For each permeability model, we then compare the time growth of perturbed volumes obtained with withdrawal-reinjection cycles to those obtained during simple injection, using the same flow rates. The size of perturbed volumes is then related to the maximum magnitude of induced/triggered seismicity, using models accredited in recent literature. Our results show that, for all models, withdrawal-reinjection is by far less critical than simple injection, because the perturbed volumes are remarkably smaller and remain constant over the simulated time, so minimizing the likelihood of interference with seismogenic faults. These results have significant implications for geothermal projects, and in the assessment of the potential risk related to fluid stimulation and induced seismicity.

TOUGH-UDEC: A simulator for coupled multiphase fluid flows, heat transfers and discontinuous deformations in fractured porous media

A numerical simulator entitled TOUGH-UDEC is introduced for the analysis of coupled thermal-hydraulic-mechanical processes in fractured porous media. Two existing well-established codes, TOUGH2 and UDEC, are coupled to model multiphase fluid flows, heat transfers, and discontinuous deformations in fractured porous media by means of discrete fracture representation. TOUGH2 is widely used for the modeling of heat transfers and multiphase multicomponent fluid flows, and UDEC is a well-known distinct element code for rock mechanics. The two codes are solved sequentially, with coupling parameters passed to each equation at specific intervals. After solving thermal-hydraulic equations within the TOUGH2 code, pressure and temperature information is imported into the UDEC model. After solving the mechanical equation within the UDEC code the calculated fracture apertures are converted to the equivalent permeability and porosity values for a TOUGH2 flow analysis. The solution is calculated by iteratively following an explicit sequence for numerical efficiency. Verifications are presented to demonstrate the capabilities of the coupled TOUGH-UDEC simulator. Three application examples of (1) shear dilation due to increased pore pressure, (2) thermal stress and (3) CO2 injection, show that the new simulator can be an effective tool for geoengineering applications involving shear activation of fractures and faults.

Scenario analysis on operational productivity for target EGS reservoir in I-lan area, Taiwan

A basic production scenario analysis (BPSA) using the TOUGH2 code (coupled with a fit-for-purpose modified EGS module) as a relevant simulator was conducted to study a potential geothermal site in I-lan Triangle, Taiwan. The outcome of numerical simulation shows that the capability of the code is compliant with heat production model analysis under a pair of recharging and production wells. In the BPSA application, the model was constituted with an assumed size of 2 km × 2 km × 2.5 km, taking into consideration a relevant boundary condition and using Case-A, B, and C to represent varied depths of recharging points. Heat production histories of target reservoirs containing a fault zone were simulated using a deterministic approach, and a collection of 30-year heat production P/T histories was obtained for all 3 cases. The effects of a fault zone near the production point are evaluated and found to be significant only when distance between the fault’s boundary and production point is minimum. The study also shows that relative locations of the recharging point from production point are important to reach an adequate design of production well layout in a geothermal plant. Significant temperature decline can be observed in Case-C of BPSA, where the distance between R-1 and P-1 is the least among the 3 cases studied. Results of this study would provide a useful numerical tool for evaluating future EGS development projects in Taiwan. By adopting a suitable conceptual model and a group of carefully selected relevant parameters, site-specific reservoir engineering problems associated with the recharging and production operation in a representative EGS geothermal field can be foreseen and predicted in a quantitative way.

Modeling of Gas Migration Through Low-Permeability Clay Rock Using Information on Pressure and Deformation from Fast Air Injection Tests

The characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal, which is also of great importance for shale gas exploration, cap-rock behavior of hydrocarbon reservoirs, and hbox {CO}_{2} sequestration. Laboratory tests have been performed on Opalinus Clay, a Mesozoic claystone that is being investigated in Switzerland as a potential host rock for the storage of nuclear waste. The laboratory program included specific water and air injections tests, as well as oedometer and isotropic compression tests. Undisturbed core samples have been retrieved from a shallow borehole in the Mont Terri Underground Research Laboratory (URL) and from a deep borehole in northern Switzerland. For the shallow cores from Mont Terri URL, largely linear-elastic deformations associated with the gas injection test could be inferred and the change in void ratio was accounted for by the pore compressibility. The corresponding change in permeability was obtained from the results of the water tests, indicating a log-linear relation between permeability and porosity. The derived porosity change and the corresponding change in permeability were implemented in the standard TOUGH2 code, which reproduced the measured gas test results using fitted water retention data derived from laboratory measurements. Similar air injection tests performed on Opalinus Clay cores from the borehole at greater depth showed overall similar behavior, but at lower porosities, lower permeability values, and lower compressibility. These cases indicated nonlinear behavior which was implemented using an effective stress-dependent porosity change and the associated change in permeability. In addition, the anisotropy associated with the bedding planes of the clay formation was considered by assuming different properties for “soft” and “hard” layers to account for storage capacity for the injected gas prior to gas breakthrough. The computed change in the overall porosity could be compared to the measured axial deformation during the gas injection test and was used for calibration of the parameters describing the relationship between the effective stress and porosity, as well as the corresponding change in permeability and capillary pressure.

Numerical Simulation Study on Mitigation of the Pressure Build-up in the Geological Formation During Injection of CO2

The injection of supercritical CO2 into the deep underground increases the pore pressure in the geologic formation, first locally around the injection point, later spreads radially throughout the formation. The range of pressure increase depends on the injection rate, injectivity and reservoir volume. The increase of pressure in the reservoir may cause several problems, including fracturing of sealing layer, fault reactivation and changes in other hydrogeological conditions. In order to reduce the pressure buildup in the reservoir during CO2 injection, two methods were studied in this paper by numerical simulations using the TOUGH2 code. One describes production of formation water as proposed by Buscheck et al. in 2014 (dual-mode well), and the second method describes production of formation water via a second well during CO2 injection. We employed a simple reservoir model based on available data of the CCS demonstration project at the Tomakomai area in Hokkaido, Japan. The efficiency and influence of two reservoir volumes, and a hypothetical placed production well, on pressure build-up in the storage formation were tested. Two models with different volumes, and an injection rate of 1 Mt/yr were applied and three cases were simulated for each model. The first case only considered injection of CO2 for 100 years without production. The second case examined the dual-mode well, in which formation water was produced for 5 years prior to the start of CO2 injection for 100 years. The last (third) case assumed utilization of an observation well for producing water in parallel with CO2 injection. Judging from the results, the following conclusion can be drawn: 1) The dual-mode well with short duration of water production (5 years) was not so effective to maintain the reservoir pressure in the large reservoir volume considered here. The method would be better suited for smaller reservoir, otherwise a very long-term production would be necessary. 2) Water production in parallel with injection was very effective in order to maintain the reservoir pressure and to avoid harmful effects on the overlying seal layers and other hydrogeological conditions. The methods can be applied to make CCS technology much more sufficient through increasing the effective capacity of injectable CO2. It may also lead to more opportunities related to site selection. However, it has to be emphasized, that the effectiveness of the production strategies investigated here may highly depend on site conditions. Therefore, the results obtained in this study should be regarded as a preliminary evaluation for the Tomakomai site specifications. Further investigations would be necessary, when more data become available through the site investigation and even operations.

TOUGH–RBSN simulator for hydraulic fracture propagation within fractured media: Model validations against laboratory experiments

This paper presents coupled hydro-mechanical modeling of hydraulic fracturing processes in complex fractured media using a discrete fracture network (DFN) approach. The individual physical processes in the fracture propagation are represented by separate program modules: the TOUGH2 code for multiphase flow and mass transport based on the finite volume approach; and the rigid-body-spring network (RBSN) model for mechanical and fracture-damage behavior, which are coupled with each other. Fractures are modeled as discrete features, of which the hydrological properties are evaluated from the fracture deformation and aperture change. The verification of the TOUGH–RBSN code is performed against a 2D analytical model for single hydraulic fracture propagation. Subsequently, modeling capabilities for hydraulic fracturing are demonstrated through simulations of laboratory experiments conducted on rock-analogue (soda-lime glass) samples containing a designed network of pre-existing fractures. Sensitivity analyses are also conducted by changing the modeling parameters, such as viscosity of injected fluid, strength of pre-existing fractures, and confining stress conditions. The hydraulic fracturing characteristics attributed to the modeling parameters are investigated through comparisons of the simulation results.

TOUGH2 전산코드의 MINC 기법을 이용한 불포화 암반 내 단열 해석 방법론 개발

TOUGH2 전산코드의 MINC 기법을 이용한 불포화 암반 내 단열 해석 방법론 개발

The influence of variability of calculation grids on the results of numerical modeling of geothermal doublets - an example from the Choszczno area, north-western Poland

The numerical modeling enables us to reduce the risk related to the selection of best localization of wells. Moreover, at the stage of production, modeling is a suitable tool for optimization of well operational parameters, which guarantees the long life of doublets. The thorough selection of software together with relevant methodology applied to generation of numerical models significantly improve the quality of obtained results. In the following paper, we discuss the impact of density of calculation grid on the results of geothermal doublet simulation with the TOUGH2 code, which applies the finite-difference method. The study area is located between the Szczecin Trough and the Fore-sudetic Monocline, where the Choszczno IG-1 well has been completed. Our research was divided into the two stages. At the first stage, we examined the changes of density of polygon calculation grids used in computations of operational parameters of geothermal doublets. At the second stage, we analyzed the influence of distance between the production and the injection wells on variability in time of operational parameters. The results demonstrated that in both studied cases, the largest differences occurred in pressures measured in production and injection wells whereas the differences in temperatures were less pronounced.

Temperature and tortuosity effect on gas migration in a high-level waste disposal tunnel

AbstractCorrosion of steel canisters, disposed of in a repository for high-level waste (HLW), leads to generation of hydrogen gas for a long period after the repository's closure. The accumulation of hydrogen gas may lead to significant desaturation and unacceptable build-up of pressure in the backfilled disposal tunnels if the gas cannot escape through the low-permeability host rock. Consequently, the investigation of gas migration is of high relevance in the assessment of the repository's performance.In this paper, the results of numerical investigations on gas migration performed using the computer code TOUGH2 (USA) are presented. The objective was to investigate migration of gas generated in a single disposal tunnel of a conceptual geological repository in a clay formation, which was suggested for benchmark studies in the European Commission project FORGE (Fate Of Repository GasEs). The analysis was focused on evaluation of the impact of an initial temperature in the repository and of different tortuosity models on gas migration. It was revealed that gas migration results were dependent on tortuosity model, while temperature variation in the repository had minor impact.

Experimental and numerical modeling of CO2 leakage in the vadose zone

AbstractThis study presents the experimental and modeling results of CO2 injection and transport in the vadose zone performed in PISCO2 facilities at the ES.CO2 center in Ponferrada (North Spain). During 46 days of experiments, 62.10 kg of CO2 were injected through 16 micro‐injectors in a 35 m3 experimental unit filled with sandy material. Monitoring and mapping of surface CO2 flux were performed periodically to assess the evolution of CO2 migration through the soil and to the atmosphere. Numerical simulations were run using TOUGH2 code with EOS7CA research module considering two phases (gas and liquid) and three components (H2O, CO2, air). Two layers (sand, gravel) and atmosphere boundary were implemented taking into account heterogeneous soils, homogeneous soil, rainfall, temperature, and liquid saturation to allow a better understanding of CO2 behavior in the vadose zone. This combined experimental and modeling approach shows that CO2 leakage in the vadose zone quickly comes out through preferential migration pathways and spots with the ranges of fluxes in the ground/surface interface from 2.5 to 600 g·m−2·day−1. This gas channeling is mainly related to soil compaction and climatic perturbation. This has significant implications for design‐adapted detection and monitoring strategies of early leakage in commercial CO2 storage. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

Modelling geothermal and operating parameters of EGS installations in the lower triassic sedimentary formations of the central Poland area

The article presents the results of modelling geothermal conditions in the Lower Triassic sedimentary formations of the Polish Lowland area (central Poland) and an electricity production model for a prospective EGS (Enhanced Geothermal System) installation situated in that area. On the basis of comprehensive analyses, this area has been selected as optimal for EGS plants operating in sedimentary complexes in the Polish Lowland. Numerical modelling was conducted using TOUGH2 code and served to evaluate the energy performance of the prospective EGS plant operating in the area. Modelling results indicate that the energy performance of the EGS plant is strongly dependent on the volume and permeability of the artificially fractured zone and its net power is dependent on the power consumed by the circulating pumps that stimulate the flow. For the top layer of the Buntsandstein formation at a depth of ca. 5500 m and temperature of ca. 170 °C, the modelled net power of an EGS plant operating in the area ranged from 2 to 3 MW for a circulation of 200 m3/h, and at 100 m3/h it ranged from 1.3 to 1.6 MW depending on the permeability and volume of the fractured zone used for the circulation in question.

Laboratory and Numerical Studies of Heat Extraction from Hot Porous Media by Means of Supercritical $$\hbox {CO}_{2}$$ CO 2

The use of $$\hbox {CO}_{2}$$ as a heat transfer fluid has been proposed as an alternative to water in enhanced geothermal systems (EGS) and in $$\hbox {CO}_{2}$$ -plume geothermal systems (CPG). Numerical simulations have shown that under expected EGS operating conditions, $$\hbox {CO}_{2}$$ would achieve more efficient heat extraction performance compared to water, especially at sites with low geothermal temperatures and low subsurface heat flow rates. With increased interest in carbon capture and sequestration (CCS), the possibility of combining geothermal energy production with carbon sequestration is actively being explored. Simulations have shown that $$\hbox {CO}_{2}$$ -based geothermal energy production could substantially offset the cost of CCS. Since numerical models are critical for the planning and operation of geothermal systems that employ $$\hbox {CO}_{2}$$ as the working fluid, it is important to validate the results of the current numerical tools against real- world experimental data. In a set of laboratory experiments, we have investigated heat extraction by flowing dry supercritical $$\hbox {CO}_{2}$$ through a heated porous medium in a laboratory pressure vessel and have compared experimental results with a numerical model using TOUGH2 with the ECO2N module. In addition, experiments were performed using (1) $$\hbox {CO}_{2}$$ and (2) water as the working fluids under similar operating conditions in order to compare the heat transfer behavior and the overall heat extraction rates. Our laboratory apparatus is capable of operating at temperatures up to 200 $$^{\circ }\hbox {C}$$ , pressures up to 34.5 MPa, and flow rates up to 400 ml/min. The experimental system was designed such that measurements and controls at the boundaries could be readily modeled using TOUGH2. We have made estimates of the density and the effective thermal conductivity of our saturated porous media, and have found that both properties change significantly during the course of experiments. The large changes in $$\hbox {CO}_{2}$$ density, due to decreasing system temperatures, can result in fluid accumulation in the system that may have significant impacts on geothermal reservoir management. The large changes in thermal conductivity as a function of temperature are of concern because the TOUGH2 code does not update the thermal conductivity of the system during the course of a simulation. Our data can be used by geologic reservoir modelers to ensure that their models accurately capture the heat extraction behavior of $$\hbox {CO}_{2}$$ to aid in the further investigations of EGS, CPG, and CCS.

Fluid geochemistry and soil gas fluxes (CO2–CH4–H2S) at a promissory Hot Dry Rock Geothermal System: The Acoculco caldera, Mexico

The Acoculco caldera has been recognized by the Mexican Federal Electricity Company (CFE) as a Hot Dry Rock Geothermal System (HDR) and could be a potential candidate for developing an Enhanced Geothermal System (EGS). Apart from hydrothermally altered rocks, geothermal manifestations within the Acoculco caldera are scarce. Close to ambient temperature bubbling springs and soil degassing are reported inside the caldera while a few springs discharge warm water on the periphery of the caldera. In this study, we infer the origin of fluids and we characterize for the first time the soil degassing dynamic. Chemical and isotopic (δ18O–δD) analyses of spring waters indicate a meteoric origin and the dissolution of CO2 and H2S gases, while gas chemical and isotopic compositions (N2/He, 3He/4He, 13C, 15N) reveal a magmatic contribution with both MORB- and arc-type signatures which could be explained by an extension regime created by local and regional fault systems. Gas geothermometry results are in agreement with temperature measured during well drilling (260°C–300°C). Absence of well-developed water reservoir at depth impedes re-equilibration of gases upon surface. A multi-gas flux survey including CO2, CH4 and H2S measurements was performed within the caldera. Using the graphical statistical analysis (GSA) approach, CO2 flux measurements were classified in two populations. Population A, representing 95% of measured fluxes is characterized by low values (mean: 18gm−2day−1) while the remaining 5% fluxes belonging to Population B are much higher (mean: 5543gm−2day−1). This low degassing rate probably reflects the low permeability of the system, a consequence of the intense hydrothermal alteration observed in the upper 800m of volcanic rocks. An attempt to interpret the origin and transport mechanism of these fluxes is proposed by means of flux ratios as well as by numerical modeling. Measurements with CO2/CH4 and CO2/H2S flux ratios similar to mass ratios of sampled gases were considered as reflecting advective transport. A numerical model of CO2 migration in the subsoil system under fully water and gas saturated conditions was performed using the TOUGH2 code in order to reproduce semi-quantitatively field measurements. The main results show that high flux values produced by advective geothermal degassing can be very localized and that low and heterogeneous permeability conditions can induce low advective CO2 flux values. Therefore, in this case the populations discriminated by the GSA method should not be interpreted in terms of origin and/or transport mechanism but rather in terms of permeability conditions.

T2GGM: A Coupled Gas Generation Model for Deep Geologic Disposal of Radioactive Waste

Deep geologic disposal of radioactive waste is being planned in a number of international programs. Within a deep geologic repository (DGR), gases can be generated by corrosion of metals and degradation of organics. Reactions, and thus gas generation rates, are dependent upon pressures, temperature, and the availability of water or water vapor within the repository. Furthermore, many reactions consume water. Consumption rates and repository state are not known a priori and are in general coupled processes.A numeric model of coupled gas generation and transport has been developed and implemented in the T2GGM code. T2GGM consists of a gas generation model (GGM), which calculates rates of gas generation and water consumption within the DGR due to corrosion and microbial degradation of the waste packages, integrated with the widely used two-phase-flow code TOUGH2, which models the subsequent two-phase transport of the water and gas through the repository and into the DGR shafts and geosphere.T2GGM has been applied to assess gas transport from a proposed low- and intermediate-level radioactive waste DGR and to study the impact of container corrosion in a hypothetical used fuel DGR.