Eric L. Sonnenthal

Experimental and numerical simulation of dissolution and precipitation: implications for fracture sealing at Yucca Mountain, Nevada

Plugging of flow paths caused by mineral precipitation in fractures above the potential repository at Yucca Mountain, Nevada could reduce the probability of water seeping into the repository. As part of an ongoing effort to evaluate thermal-hydrological-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 degrees 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 degrees C at the top and 130 degrees C at the bottom. The fracture began to seal with amorphous silica within 5 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 50-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 and thermal gradients between the experimental setup and anticipated conditions at Yucca Mountain need to be factored into scaling the results of the dissolution/precipitation experiments and associated simulations to THC models for the potential Yucca Mountain repository.

Constraints on the hydrology of the unsaturated zone at Yucca Mountain, NV from three-dimensional models of chloride and strontium geochemistry

Abstract Three-dimensional (3-D) simulations of the spatial and temporal variations in chloride and strontium concentrations in porewaters were performed to constrain infiltration rates, flow paths, and mixing processes in the unsaturated zone (UZ) at Yucca Mountain, NV. Chloride concentrations in infiltrating water were calculated from aerial distributions of precipitation and infiltration rates for the current climatic conditions and for the last glacial maximum, combined with effective chloride concentrations in precipitation. Modeled concentrations are roughly similar to measured porewater chloride concentrations from the Paintbrush nonwelded tuffs in the Exploratory Studies Facility (ESF) tunnel and in boreholes suggesting that the mean infiltration rate over the site is unlikely to be higher than the calculated mean infiltration rate for the modern climate (∼5 mm/year; [Flint, A.L., Hevesi, J.A., Flint, L.E., 1996. Conceptual and Numerical Model of Infiltration for the Yucca Mountain Area, Nevada. Milestone 3GU1623M. U.S. Geol. Surv. Water Res. Invest. Rep. U.S. Geological Survey, Denver, CO]). Porewaters from the late Pleistocene (>10 ka) could be present in the Paintbrush bedded tuffs and in the underlying Topopah Spring welded tuffs (TSw), predominately under regions of thick alluvium having little infiltration. However, porewaters at the potential repository level may have a higher proportion of Holocene recharge due to the higher calculated infiltration rate in this region. Dual-permeability simulations show that in low infiltration regions chemical disequilibrium can exist between fracture and matrix porewaters, as a result of the climate change 10,000 years ago. Below the potential repository level, simulations show significant mixing due to lateral flow on top of the low permeability basal vitrophyre in the Topopah Spring unit and on zeolitized tuffs in the Calico Hills unit. Perched water chloride concentrations are closely matched using the calculated conditions for the last glacial maximum climate, with some component of Holocene recharge. Measured strontium concentrations in the UZ and in the perched water bodies can be roughly matched by assuming conservative behavior in nonzeolitic units and strong ion exchange in zeolitic units, and indicate that the perched water bodies are poorly mixed. Differences in the Cl contents of samples having a bomb-pulse Cl-36 signature and those with a modern ratio indicate that waters of intermediate 36 Cl / Cl ratios may be mixtures, that without other isotopic data could be inferred as either Pleistocene or Holocene age waters.

Multiphase Reactive Transport Modeling of Seasonal Infiltration Events and Stable Isotope Fractionation in Unsaturated Zone Pore Water and Vapor at the Hanford Site

and diffusive transport). Developing tractable analytical equations for these processes requires simplifying asNumerical simulations of transport and isotope fractionation prosumptions, which lead to analytical methods that are not vide a method to quantitatively interpret vadose zone pore water stable isotope depth profiles based on soil properties, climatic condieasily adapted to field conditions. Previous numerical tions, and infiltration. We incorporate the temperature-dependent models have relied on assumptions such as neglecting equilibration of stable isotopic species between water and water vapor, the temperature dependence of isotope fractionation and their differing diffusive transport properties into the thermodyand treating the isotopic species as nonreactive tracers namic database of the reactive transport code TOUGHREACT. with concentrations defined by fixed partition coeffiThese simulations are used to illustrate the evolution of stable isotope cients. profiles in semiarid regions where recharge during wet seasons disPrior approaches to predicting the impact of infiltraturbs the drying profile traditionally associated with vadose zone pore tion water on stable isotope profiles include a semiemwaters. Alternating wet and dry seasons lead to annual fluctuations pirical model (Barnes and Allison, 1988), a mixing in moisture content, capillary pressure, and stable isotope composischeme (Mathieu and Bariac, 1996b), and an analytical tions in the vadose zone. Periodic infiltration models capture the effects of seasonal increases in precipitation and predict stable isotope model to predict overall average pore water isotope profiles that are distinct from those observed under drying (zero compositions (DePaolo et al., 2004). However, a more infiltration) conditions. After infiltration, evaporation causes a shift general approach is needed to link observed isotope to higher 18O and D values, which are preserved in the deeper pore compositions with dynamic hydrological processes, waters. The magnitude of the isotopic composition shift preserved in where precipitation events or temperature changes afdeep vadose zone pore waters varies inversely with the rate of infilfect the isotopic profile with depth. tration. We use the thermodynamic framework of the TOUGHREACT transport code (Xu and Pruess, 2001; Xu et al., 2003) to develop a general transport model for stable T fraction of precipitation that reaches the deep isotopes in vadose zone soil water and consider the vadose zone, or the net infiltration, is difficult to impact of infiltration processes on measured stable isopredict in arid regions, but important for understanding tope profiles from the Hanford Site. These reactive groundwater recharge and contaminant transport. At transport models of stable isotope transport provide a the USDOE’s Hanford Site in south-central Washingquantitative method to link the observed isotopic proton State, where a large amount of radionuclide contamfiles to soil properties, climatic conditions, and net infilination is present in the vadose zone, it is critical to tration into the vadose zone. know the net water infiltration flux, as this determines how rapidly radionuclides or other contaminants may Background: Stable Isotope Measurements reach groundwater. The vadose zone hydrological proThe isotopic compositions discussed here are meacesses that control net infiltration rate also affect the sured relative to a well-defined standard material (Stanratios of stable isotopes (i.e., 18O/16O and 2H/1H) in water dard Mean Ocean Water [SMOW]). Stable isotope comand water vapor. positions (‰) are calculated as delta values from the The transport of stable O and H isotopes in water isotopic ratio (R 18O/16O or 2H/1H), where within drying soil columns has been studied extensively (e.g., Barnes and Allison, 1983, 1984; Allison et al., 1994; RSample RStandard 1 1000 [1] Shurbaji et al., 1995; Mathieu and Bariac, 1996a; Melayah et al., 1996). Approaches used to predict stable isotope profiles in drying soils must consider the comBased on this system, typical ocean waters have D plex interaction of multiple processes (e.g., drainage, and 18O values near 0‰ relative to SMOW. Meteoric temperature effects on flow and isotope fractionation, precipitation over land varies as a function of temperature, latitude, and altitude, but generally has D and 18O values that are shifted to values less than zero M.J. Singleton, E.L. Sonnenthal, M.E. Conrad, D.J. DePaolo, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, because of the fractionation of lighter isotopes into the CA 94720; and G.W. Gee, Hydrology Group, Environmental Techvapor phase during the change from liquid to vapor. nology Division, Pacific Northwest National Laboratory, Richland, Craig (1961) documented a linear relationship, known WA. Received 30 Aug. 2003. Special Section: Research Advances in as the global meteoric water line (GMWL), between Vadose Zone Hydrology through Simulations with the TOUGH Codes. *Corresponding author (MJSingleton@lbl.gov). Abbreviations: GMWL, global meteoric water line; LBNL, Lawrence Berkeley National Laboratory; LMWL, local meteoric water line; Published in Vadose Zone Journal 3:775–785 (2004).  Soil Science Society of America PNNL, Pacific Northwest National Laboratory; SMOW, standard mean ocean water. 677 S. Segoe Rd., Madison, WI 53711 USA

Advances in subsurface modeling using the TOUGH suite of simulators

The TOUGH suite of nonisothermal multiphase flow and transport simulators is continually updated to improve the analysis of complex subsurface processes through numerical modeling. Driven by research questions in the Earth sciences and by application needs in industry and government organizations, the codes are extended to include the coupling of relevant processes and subsystems, to improve computational performance, to support model development and analysis tasks, and to provide more convenient pre- and post-processing capabilities. This review paper briefly describes the history of the simulator, discusses recent advances, and comments on potential future developments and applications.

Coupled THM Modeling of Hydroshearing Stimulation in Tight Fractured Volcanic Rock

In this study, we use the TOUGH-FLAC simulator for coupled thermo–hydro-mechanical modeling of well stimulation for an Enhanced Geothermal System (EGS) project. We analyze the potential for injection-induced fracturing and reactivation of natural fractures in a porous medium with associated permeability enhancement. Our analysis aims to understand how far the EGS reservoir may grow and how the hydroshearing process relates to system conditions. We analyze the enhanced reservoir, or hydrosheared zone, by studying the extent of the failure zone using an elasto-plastic model, and accounting for permeability changes as a function of the induced stresses. For both fully saturated and unsaturated medium cases, the results demonstrate how EGS reservoir growth depends on the initial fluid phase, and how the reservoir extent changes as a function of two critical parameters: (1) the coefficient of friction, and (2) the permeability-enhancement factor. Moreover, while well stimulation is driven by pressure exceeding the hydroshearing threshold, the modeling also demonstrates how injection-induced cooling further extends the effects of stimulation.

A reaction-transport model for calcite precipitation and evaluation of infiltration fluxes in unsaturated fractured rock.

The percolation flux in the unsaturated zone (UZ) is an important parameter addressed in site characterization and flow and transport modeling of the potential nuclear-waste repository at Yucca Mountain, NV, USA. The US Geological Survey (USGS) has documented hydrogenic calcite abundances in fractures and lithophysal cavities at Yucca Mountain to provide constraints on percolation fluxes in the UZ. The purpose of this study was to investigate the relationship between percolation flux and measured calcite abundances using reactive transport modeling. Our model considers the following essential factors affecting calcite precipitation: (1) infiltration, (2) the ambient geothermal gradient, (3) gaseous CO(2) diffusive transport and partitioning in liquid and gas phases, (4) fracture-matrix interaction for water flow and chemical constituents, and (5) water-rock interaction. Over a bounding range of 2-20 mm/year infiltration rate, the simulated calcite distributions capture the trend in calcite abundances measured in a deep borehole (WT-24) by the USGS. The calcite is found predominantly in fractures in the welded tuffs, which is also captured by the model simulations. Simulations showed that from about 2 to 6 mm/year, the amount of calcite precipitated in the welded Topopah Spring tuff is sensitive to the infiltration rate. This dependence decreases at higher infiltration rates owing to a modification of the geothermal gradient from the increased percolation flux. The model also confirms the conceptual model for higher percolation fluxes in the fractures compared to the matrix in the welded units, and the significant contribution of Ca from water-rock interaction. This study indicates that reactive transport modeling of calcite deposition can yield important constraints on the unsaturated zone infiltration-percolation flux and provide useful insight into processes such as fracture-matrix interaction as well as conditions and parameters controlling calcite deposition.

Calibration of Yucca Mountain unsaturated zone flow and transport model using porewater chloride data

In this study, porewater chloride data from Yucca Mountain, NV are analyzed and modeled by three-dimensional chemical transport simulation and analytical methods. The simulation modeling approach is based on a continuum formulation of coupled multiphase fluid flow and tracer transport processes through fractured porous rock using a dual-continuum concept. Infiltration rate calibrations were performed using the porewater chloride data. Model results of chloride distributions were improved in matching the observed data with the calibrated infiltration rates. Statistical analyses of the frequency distribution for overall percolation fluxes and chloride concentration in the unsaturated zone system demonstrate that the use of the calibrated infiltration rates had an insignificant effect on the distribution of simulated percolation fluxes but significantly changed the predicted distribution of simulated chloride concentrations. An analytical method was also applied to model transient chloride transport. The method was verified by three-dimensional simulation results to be capable of capturing major chemical transient behavior and trends. Effects of lateral flow in the Paintbrush nonwelded unit on percolation fluxes and chloride distribution were studied by three-dimensional simulations with increased horizontal permeability. The combined results from these model calibrations furnish important information for the UZ model studies, contributing to performance assessment of the potential repository.

Modeling Reactive Multiphase Flow and Transport of Concentrated Solutions

Abstract A Pitzer ion-interaction model for concentrated aqueous solutions was added to the reactive multiphase flow and transport code TOUGHREACT. The model is described and verified against published experimental data and the geochemical code EQ3/6. The model is used to simulate water-rock-gas interactions caused by boiling and evaporation within and around nuclear waste emplacement tunnels at the proposed high-level waste repository at Yucca Mountain, Nevada. The coupled thermal, hydrological, and chemical processes considered consist of water and air/vapor flow, evaporation, boiling, condensation, solute and gas transport, formation of highly concentrated brines, precipitation of deliquescent salts, generation of acid gases, and vapor-pressure lowering caused by the high salinity of the concentrated brine.

A sequential implicit algorithm of chemo-thermo-poro-mechanics for fractured geothermal reservoirs

We describe the development of a sequential implicit formulation and algorithm for coupling fluid-heat flow, reactive transport, and geomechanics. We consider changes in pore volume from dissolution caused by chemical reactions, in addition to coupled flow and geomechanics. Moreover, we use the constitutive equations for the multiple porosity model for fractured geothermal reservoirs, employing failure-dependent permeability dynamically and updating it every time step. The proposed sequential algorithm is an extension of the fixed-stress split method to chemo-thermo-poro-mechanics, facilitating the use of existing flow-reactive transport and geomechanics simulators.We first validate a simulator that employs the proposed sequential algorithm, matching the numerical solutions with the analytical solutions such as Terzaghis and Mandels problems for poro-mechanics and the reference solutions of chemo-poro-mechanics and chemo-thermo-poro-mechanics in the 1D elastic problems. We also perform convergence test, and the proposed algorithm shows fast convergence, when full iteration is taken, and first order accuracy in time for the staggered approach.We then investigate two test cases: 2D multiple porosity elastic and 3D single porosity elastoplastic problems, and explore the differences in coupled flow and geomechanics with and without reactive transport. We find that the change in pore-volume induced by mineral dissolution can impact on fluid pressure and failure status, followed by significant changes in permeability and flow variables, showing strong interrelations between flow-reactive transport and geomechanics. HighlightsIntegration of flow-reactive transport-geomechanics simulators.A fast and convergent sequential implicit algorithm.Practical coupling between reactive transport and thermoporomechanics.Simulation of geochemistry in coupled flow and geomechanics.

Modeling acid-gas generation from boiling chloride brines

BackgroundThis study investigates the generation of HCl and other acid gases from boiling calcium chloride dominated waters at atmospheric pressure, primarily using numerical modeling. The main focus of this investigation relates to the long-term geologic disposal of nuclear waste at Yucca Mountain, Nevada, where pore waters around waste-emplacement tunnels are expected to undergo boiling and evaporative concentration as a result of the heat released by spent nuclear fuel. Processes that are modeled include boiling of highly concentrated solutions, gas transport, and gas condensation accompanied by the dissociation of acid gases, causing low-pH condensate.ResultsSimple calculations are first carried out to evaluate condensate pH as a function of HCl gas fugacity and condensed water fraction for a vapor equilibrated with saturated calcium chloride brine at 50-150°C and 1 bar. The distillation of a calcium-chloride-dominated brine is then simulated with a reactive transport model using a brine composition representative of partially evaporated calcium-rich pore waters at Yucca Mountain. Results show a significant increase in boiling temperature from evaporative concentration, as well as low pH in condensates, particularly for dynamic systems where partial condensation takes place, which result in enrichment of HCl in condensates. These results are in qualitative agreement with experimental data from other studies.ConclusionThe combination of reactive transport with multicomponent brine chemistry to study evaporation, boiling, and the potential for acid gas generation at the proposed Yucca Mountain repository is seen as an improvement relative to previously applied simpler batch evaporation models. This approach allows the evaluation of thermal, hydrological, and chemical (THC) processes in a coupled manner, and modeling of settings much more relevant to actual field conditions than the distillation experiment considered. The actual and modeled distillation experiments do not represent expected conditions in an emplacement drift, but nevertheless illustrate the potential for acid-gas generation at moderate temperatures (<150°C).