Thermal-hydrological Processes Research Articles

A Novel Generalized Clapeyron Equation-Based Model for Capturing the Soil Freezing Characteristics Curve of Saline Soil: Validation by Small Sample Lab and Field Experiments

The soil freezing characteristic curve (SFCC) describes the relationship between the freezing point and unfrozen water content, which are two critical parameters in depicting the heat, solute, and water transport in frozen soil. In this paper, we propose a novel Generalized Clapeyron Equation (GCE)-based model, the GCE-Salt Model, to better capture the SFCC in frozen soil in the presence of solute. It keeps the matric potential Ψf in the GCE as its original meaning and incorporates the effect of solute potential in the equilibrium freezing temperature. The performance of our GCE-Salt Model was validated by both lab and field experimental data and compared with related models (Combined Model and GCE-Tan Model). The GCE-Salt Model performed exceptionally well in extremely saline soil and it performed well in both non-saline and saline soil. (1) Our GCE-Salt Model could capture the SFCC of non-saline soil equally as well as the Combined Model (NSE = 0.866); (2) our GCE-Salt Model performed similarly well as the Combined Model and a little better than the GCE-Tan Model for the slightly to highly saline soil (NSE ≥ 0.80 for three models); and (3) our GCE-Salt Model (NSE = 0.919) beat the Combined Model (NSE = 0.863) and the GCE-Tan Model (NSE = 0.62) in capturing the SFCC of extremely saline soil, mainly because the inherent expression of our GCE-Salt Model can more accurately capture the freezing point. Our findings highlight the effect of solute potential on the ice–water change and could improve the understanding of the effect of freezing and thawing on the thermal–hydrological processes, structure of saline soil, and landscape evolution in cold regions.

Thermal-hydraulic-mechanical analysis of enhanced geothermal reservoirs with hybrid fracture patterns using a combined XFEM and EDFM-MINC model

The long-term fluid circulation of Enhanced Geothermal Systems (EGS) involves complex coupled Thermal-Hydrological-Mechanical (THM) processes dominated by both hydraulic and activated natural fractures. The simulation of such multiphysical processes is known to be a challenging problem. In this work, we have developed an integrated numerical approach to advance the multiphysical simulation of EGS. In our model, the hydraulic fractures of arbitrary shapes in response to pressure changes and thermal strains are handled by the three-dimensional (3D) eXtended Finite Element Method (XFEM). The induced/natural fractures are incorporated into the model and treated as one continuum of the Multiple INteracting Continua (MINC) for the investigation of their impacts. A TOUGH -based program, TOUGH2-EGS, is utilized to simulate the Thermal-Hydrological processes. The 3D Embedded Discrete Fracture Method (EDFM), compatible with the 3D XFEM, is adopted to model the hydraulic fractures. Sensitivity analyses are performed for this model in terms of injection temperature and thermal expansivity. The hybrid EDFM and MINC model is established and analyzed for an EGS with both hydraulic and induced/natural fractures. The convergence performance of the single-fracture model shows that an appropriate stiffness coefficient is essential for this model and different choices of the coefficient values result in distinct behaviors. Our numerical results illustrate that the fracture aperture is opened by the cold fluid injection and the reservoir performance is dominated by the thermal stress/strain, which highlights the necessity of multiphysical simulation. In the hybrid model, the enhancement of the natural fracture permeability notably delays the thermal breakthrough by allowing injected fluid to contact more hot reservoirs. The novelties of this work lie in three aspects. Firstly, the combined 3D XFEM and EDFM is capable of handling arbitrary fracture shapes in a 3D EGS model. Secondly, the hybrid hydraulic and induced/natural fracture model enables us to establish the stimulated reservoir volume of the EGS and investigate the operational and geological parameters. Thirdly, the introduction of fracture stiffness stabilizes the numerical solution.

Integrated Hydrologic Modelling of Groundwater-Surface Water Interactions in Cold Regions

Groundwater-surface water (GW-SW) interaction, as a key component in the cold region hydrologic cycle, is extremely sensitive to seasonal and climate change. Specifically, the dynamic change of snow cover and frozen soil bring additional challenges in observing and simulating hydrologic processes under GW-SW interactions in cold regions. Integrated hydrologic models are promising tools to simulate such complex processes and study the system behaviours as well as its responses to perturbations. The cold region integrated hydrologic models should be physically representative and fully considering the thermal-hydrologic processes under snow cover variations, freeze-thaw cycles in frozen soils and GW-SW interactions. Benchmarking and integration with scarce field observations are also critical in developing cold region integrated hydrologic models. This review summarizes the current status of hydrologic models suitable for cold environment, including distributed hydrologic models, cryo-hydrogeologic models, and fully-coupled cold region GW-SW models, with a specific focus on their concepts, numerical methods, benchmarking, and applications across scales. The current research can provide implications for cold region hydrologic model development and advance our understanding of altered environments in cold regions disturbed by climate change, such as permafrost degradation, early snow melt and water shortage.

Characteristics of land surface thermal-hydrologic processes for different regions over North China during prevailing summer monsoon period

The observation stations of Northern China are divided into three regions: the arid Northwest China, the Loess Plateau, and the cool Northeast China. The consistencies, differences, and associated mechanisms of land surface thermal-hydrologic processes among the three regions were studied based on the normalization of major variables of land surface thermal-hydrologic processes, using data collected during prevailing summer monsoon period (July and August, 2008). It is shown that differences of surface thermal-hydrologic processes are remarkable among the three regions because of different impacts of summer monsoon. Especially their soil wet layers occur at different depths, and the average albedo and its diurnal variations are distinctly different. Surface net short-wave radiation in the Loess Plateau is close to that in the cool Northeast China, but its surface net long-wave radiation is close to that in the arid Northwest China. And the ratio of net radiation to global solar radiation in the cool Northeast China is higher than the other two regions, though its temperature is lower. There are obvious regional differences in the ratios of surface sensible and latent heat fluxes to net radiation for the three regions because of distinct contribution of sensible and latent heat fluxesto land surface energy balance. The three regions are markedly different in the ratio of water vapor flux to pan evaporation, but they are consistent in the ratio of water vapor flux to precipitation. These results not only indicate different influences of climate and environmental factors on land surface thermal-hydrologic processes in the three regions, but also show that summer monsoon is important in the formation and variation of the pattern of land surface thermal-hydrologic processes.

Modeling Thermal-Hydrologic Processes for a Heated Fractured Rock System: Impact of a Capillary-Pressure Maximum

Various thermal-hydrologic models have been developed to simulate thermal-hydrologic conditions in emplacement drifts and surrounding host rock for the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. The modeling involves two-phase (liquid and gas) and two-component (water and air) transport in a fractured-rock system, which is conceptualized as a dual-permeability medium. Simulated hydrologic processes depend upon calibrated system parameters, such as the van Genuchten α and m, which quantify the capillary properties of the fractures and rock matrix. Typically, these parameters are not calibrated for strongly heat-driven conditions, i.e., conditions for which boiling and rock dryout occur. The objective of this study is to modify the relationship between capillary pressure and saturation, P c(S), for strongly heated conditions that drive saturation below the residual saturation (S → 0). We offer various extensions to the van Genuchten capillary-pressure function and compare results from a thermal-hydrologic model with data collected during the Drift-Scale Test, an in situ thermal test at Yucca Mountain, to investigate the suitability of these various P c extension methods. The study suggests that the use of extension methods and the imposition of a capillary-pressure cap (or maximum) improve the agreement between Drift-Scale Test data and model results for strongly heat-driven conditions. However, for thermal-hydrologic models of the Yucca Mountain nuclear waste repository, temperature and relative humidity are insensitive to the choice of extension method for the capillary-pressure function. Therefore, the choice of extension method applied to models of drift-scale thermal-hydrologic behavior at Yucca Mountain can be made on the basis of numerical performance.

Modeling of thermally driven hydrological processes in partially saturated fractured rock

This paper is a review of the research that led to an in‐depth understanding of flow and transport processes under strong heat stimulation in fractured, porous rock. It first describes the anticipated multiple processes that come into play in a partially saturated, fractured porous volcanic tuff geological formation when it is subject to a heat source such as that originating from the decay of radionuclides. The rationale is then given for numerical modeling being a key element in the study of multiple processes that are coupled. The paper outlines how the conceptualization and the numerical modeling of the problem evolved, progressing from the simplified to the more realistic. Examples of numerical models are presented so as to illustrate the advancement and maturation of the research over the last 2 decades. The most recent model applied to in situ field thermal tests is characterized by (1) incorporation of a full set of thermal‐hydrological processes into a numerical simulator, (2) realistic representation of the field test geometry in three dimensions, and (3) use of site‐specific characterization data for model inputs. Model predictions were carried out prior to initiation of data collection, and the model results were compared to diverse sets of measurements. The approach of close integration between modeling and field measurements has yielded a better understanding of how coupled thermal hydrological processes produce redistribution of moisture within the rock, which affects local permeability values and subsequently the flow of liquid and gases. The fluid flow, in turn, will change the temperature field. We end with a note on future research opportunities, specifically those incorporating chemical, mechanical, and microbiological factors into the study of thermal and hydrological processes.

Results from an International Simulation Study on Coupled Thermal, Hydrological, and Mechanical Processes near Geological Nuclear Waste Repositories

As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermal-mechanical responses can be predicted with a relatively high confidence level.

Modeling Seepage into Heated Waste Emplacement Tunnels in Unsaturated Fractured Rock

Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that-for the first several hundred years after emplacement-will be heated to above-boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above-boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. In this paper, we describe numerical analyses of the coupled thermal-hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock-drift interface-independent of the thermal conditions-a second barrier exists to downward percolation at above-boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual-permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions.

Experimental and Numerical Investigation of Flow Phenomena in Nonisothermal, Variably Saturated Bentonite–Crushed Rock Mixtures

Mixtures of sodium bentonite and crushed rock are being examined as components of the engineered barrier system in a geologic repository of high‐level nuclear waste. Laboratory experiments were performed to determine the thermal and unsaturated hydraulic properties of bentonite–crushed diorite mixtures. Water‐retention curves were obtained from conventional pressure cell and evaporation experiments. In addition, transient data from heating and gas injection experiments on laboratory columns were analyzed using inverse modeling techniques. Measured pressures, temperatures, and drained‐water volumes were jointly inverted to estimate absolute permeability, thermal conductivity, specific heat, and capillary strength parameters. Simultaneous matching of all available data—specifically the gas breakthrough at the top of the column—proved difficult, pointing toward aspects of the experimental design and the conceptual model that need to be refined. The analysis of sensitivity coefficients and the correlation structure of the parameters revealed the importance of accurately capturing coupled thermal hydrological processes within the column as well as the details of the experimental apparatus, such as heat losses and storage of water and gas in the measuring burette. The parameters estimated using different experimental and analytical procedures were consistent with one another, providing backfill material properties useful for the simulation of gas‐ and heat‐generating nuclear waste repositories.

Conceptual evaluation of the potential role of fractures in unsaturated processes at Yucca Mountain

A wide array of field observations, in situ testing, and rock and water sampling (and subsequent analyses) within the unsaturated zone (UZ) of Yucca Mountain demonstrate the importance of fractures to flow and transport in the welded tuffs. The abundance of fractures and the spatial variability in their hydraulic properties, along with the heterogeneity within lithologic formations, make evaluation of unsaturated processes occurring within the potential repository horizon complex. Fracture mapping and field testing show that fractures are well connected, yet considerable variation is seen within and between units comprising the potential repository horizon with regard to fracture trace length, spacing, permeability, and capillarity. These variations have important implications for the distribution and movement of water and solutes through the unsaturated zone. Numerical models designed to assess such phenomena as unsaturated flow, transport, and coupled thermal–hydrological processes each require their own conceptual model for fracture networks, in order to identify the subset of all fractures that is relevant to the particular study. We evaluate several process-dependent conceptual models for fractures and identify the relevant fracture subsets related to these processes.

Uncertainties in coupled thermal–hydrological processes associated with the Drift Scale Test at Yucca Mountain, Nevada

Understanding thermally driven coupled hydrological, mechanical, and chemical processes in unsaturated fractured tuff is essential for evaluating the performance of the potential radioactive waste repository at Yucca Mountain, Nevada. The Drift Scale Test (DST), intended for acquiring such an understanding of these processes, has generated a huge volume of temperature and moisture redistribution data. Sophisticated thermal–hydrological (TH) conceptual models have yielded a good fit between simulation results and those measured data. However, some uncertainties in understanding the TH processes associated with the DST still exist. This paper evaluates these uncertainties and provides quantitative estimates of the range of these uncertainties. Of particular interest for the DST are the uncertainties resulting from the unmonitored loss of vapor through an open bulkhead of the test. There was concern that the outcome from the test might have been significantly altered by these losses. Using alternative conceptual models, we illustrate that predicted mean temperatures from the DST are within 1 °C of the measured mean temperatures through the first 2 years of heating. The simulated spatial and temporal evolution of drying and condensation fronts is found to be qualitatively consistent with measured saturation data. Energy and mass balance computation shows that no more than 13% of the input energy is lost because of vapor leaving the test domain through the bulkhead. The change in average saturation in fractures is also relatively small. For a hypothetical situation in which no vapor is allowed to exit through the bulkhead, the simulated average fracture saturation is not qualitatively different enough to be discerned by measured moisture redistribution data. This leads us to conclude that the DST, despite the uncertainties associated with open field testing, has provided an excellent understanding of the TH processes.

Experimental and Numerical Investigation of Flow Phenomena in Nonisothermal, Variably Saturated Bentonite–Crushed Rock Mixtures

Mixtures of sodium bentonite and crushed rock are being examined as components of the engineered barrier system in a geologic repository of high-level nuclear waste. Laboratory experiments were performed to determine the thermal and unsaturated hydraulic properties of bentonite/crushed diorite mixtures. Water-retention curves were conventionally obtained from pressure cell and evaporation experiments. In addition, transient data from heating and gas injection experiments on laboratory columns were analyzed using inverse modeling techniques. Measured pressures, temperatures, and drained-water volumes were jointly inverted to estimate absolute permeability, thermal conductivity, specific heat, and capillary strength parameters. Simultaneous matching of all available data specifically the gas breakthrough at the top of the column proved difficult, pointing towards aspects of the experimental design and the conceptual model that need to be refined. The analysis of sensitivity coefficients and the correlation structure of the parameters revealed the importance of accurately capturing coupled thermal hydrological processes within the column as well as the details of the experimental apparatus, such as heat losses and storage of water and gas in the measuring burette. The parameters estimated using different experimental and analytical procedures were consistent with one another, providing backfill material properties useful for the simulation of gas- and heat-generating nuclear waste repositories.

A field study for understanding thermally driven coupled processes in partially saturated fractured welded tuff

As part of a multi-laboratory team, we are carrying out two in situ thermal tests — the single Heater Test and Drift Scale Test, in an underground facility at Yucca Mountain, Nevada, USA, the proposed site for a high-level nuclear waste repository. Our objective in these tests is to gain a more in-depth understanding of the coupled thermal–hydrological–mechanical–chemical processes likely to exist in the fractured rock mass around a geological repository . These coupled processes are monitored continuously by numerous sensors emplaced in boreholes, while cross-hole radar tomography , neutron logging , electrical resistivity tomography, and interference air-permeability tests all serve to measure moisture change in the rock mass. Thermal–hydrological processes for both tests have been simulated (using a 3-D numerical model) and compared to the extensive data set. In this paper, we present examples to illustrate how an iterative approach requiring close integration of modeling and measurements enables us to track the complex coupled processes we seek to understand. The main manifestation of coupled thermal-hydrological processes is in the time evolution of the drying and condensation zones. Good agreement exists between model predictions and measurements, specifically the decrease in air-permeability values within zones of increased liquid saturation in the fractures and the increase of radar velocity in cross-hole radar survey in zones of decreased matrix liquid saturation. A heat-pipe signature in the temperature data arising from liquid–vapor counter-flow occurs in both the measurements and simulated results. The good agreement between predictions from the numerical simulations and measurements in the thermal tests indicates that our basic understanding of the thermal-hydrological processes in a potential repository at Yucca Mountain is sound. However, detailed behavior is impacted by site-specific heterogeneity, in the form of discrete fractures that are not likely to be predictable a priori. One emphasis of the on-going Drift Scale Test is to build on the present understanding and to assess the impact of heterogeneity to the repository performance.

Predictions and observations of the thermal–hydrological conditions in the Single Heater Test

The Single Heater Test (SHT) is one of two in-situ thermal tests included in the site characterization program for the potential underground nuclear waste repository at Yucca Mountain. Coupled thermal–hydrological–mechanical–chemical processes in the fractured rock mass around the heater were monitored by numerous sensors emplaced among 30 boreholes. Periodic active testing of cross-hole radar tomography, neutron logging, electrical resistivity tomography, and interference air permeability tests probed the change of moisture content in the rock mass. Thermal–hydrological processes in the SHT have been simulated using a 3-D numerical model and compared to the monitored data. The good agreement between the temperature data and simulated results indicates that the thermal–hydrological responses of the SHT in the 9 months of heating are well-represented by the coupled thermal–hydrological numerical model. The dominant heat transfer process is by conduction and the signature of vapor and liquid counter flow is subtle in the temperature data. The simulated result of a dry-out zone of about 1 m (at the end of the heating phase) around the heater hole, and a condensation zone of increased liquid saturation outside of the dry-out zone, is consistent with the radar tomography and air permeability data. Tomography data and post-test laboratory measurements indicate that the moisture content is larger below than above the heater horizon, suggesting gravity drainage of condensate in the fractures. Model studies show that gravity drainage occurs in simulations using the dual permeability conceptual model, but is absent in the effective-continuum model, where matrix and fractures are required to be in thermodynamic equilibrium at all times.

Discrete-Fracture Modeling of Thermal-Hydrological Processes at Yucca Mountain and the Llnl G-Tunnel Heater Test

AbstractAn in situ heater test was performed at G-Tunnel, Nevada Nuclear Test Site, to investigate the thermal-hydrological response of unsaturated, fractured volcanic tuff under conditions similar to those at Yucca Mountain. The NUFT flow and transport code was used to model the test using discrete-fracture and equivalent-continuum approaches. Nonequilibrium fracture flow and thermal buoyant gas-phase convection were found to be the likely causes for observed lack of condensate imbibition into the matrix. The potential repository at Yucca Mountain was also modeled. Disequilibrium fracture flow is predicted to occur for less than a hundred years after emplacement followed by a period of fracture-matrix equilibrium, during which the equivalent-continuum and discrete-fracture models give almost identical results.

Hydrogeological Regimes and Their Subsurface Thermal Effects

The symposium on which this monograph is based was organized for the purposes of bringing together two bodies of experts from the hydrological and geothermal communities to tackle the problem of the coupled thermalhydrological processes in the subsurface.For many years, workers interested in the measurement and interpretation of terrestrial heat flow have been well aware of more efficient modes of heat transport than conduction but assumed that if the temperature‐depth, or total thermal resistance‐depth (Bullard) plots obtained using borehole data were linear, then perturbing influences were insignificant. In other words, linearity of such plots was implicitly used as a test of reliability—so much so that in some cases where the data appeared to have systematic deviation, due to perturbing sources of unknown form and strength, the data were massaged until linear plots were obtained, thus allowing inferences to be made about the perturbing source and the conductive heat flow densities [Allis and Garland, 1976; Francheteau et al., 1984; Jaupart et al., 1985].

Constraints from fluid inclusions on sulfide precipitation mechanisms and ore fluid migration in the Viburnum Trend lead district, Missouri

Measurements on fluid inclusions in hydrothermal dolomite cements place constraints on sulfide precipitation mechanisms and on the thermal-hydrologic processes which formed the Viburnum Trend Mississippi Valley-type lead district. Homogenization temperatures and freezing point depressions were determined for fluid inclusions in Bonneterre Dolomite-hosted dolomite cements in mine samples, as well as drill core from up to 13 km outside of the district. A well-defined cathodoluminescent zonation distinguishes dolomite growth zones in the Vi-burnurn Trend as older or younger than main-stage mineralization (octahedral galena) and facilitates correlation with other dolomites outside the Viburnum Trend.Homogenization temperatures and salinities in samples from mines are not systematically different from those of samples outside of the district. Medians of homogenization temperature distributions differ by not more than 25 degrees C, so that a temperature gradient, if present, should not have exceeded approximately 25 degrees C within the study area. These observations are interpreted to indicate that the Viburnum Trend was not strongly thermally anomalous with respect to surrounding country rock and that fluid flow occurred on a broad scale through not only the Lamotte Sandstone but through the overlying Cambrian carbonates as well.The absence of a significant, recognizable decrease in temperature either vertically within the section or east-west across the district, coupled with the minor amount of silica in the district, argues against cooling as a primary cause of sulfide precipitation. Fluids whose primary aquifer was the Lamotte Sandstone, predominantly a quartz arenite, should have been in equilibrium with quartz. Quartz in the Viburnum Trend occurs as a minor, drusy, vug-lining phase, but the district lacks the intense silicification found in other Mississippi Valley-type districts such as Tri-State (Oklahoma, Kansas, Missouri). Quartz solubility is strongly temperature dependent and, under equilibrium conditions, a decrease of 10 degrees C or more should have precipitated at least as many moles of silica as galena (assuming a galena solubility of between 1 and 10 ppm). Clearly this is not the case, as galena is far more abundant than quartz in the Viburnum Trend.Ice final-melting temperatures (T m ) in fluid inclusions generally range from -14 degrees to -27 degrees C for primary dolomite-hosted inclusions. Using these T m values and cation ratios for the inclusion fluids, absolute concentrations for the individual cations and chloride were calculated using the thermochemical model of Spencer et al. (1990). The corresponding high but variable salinities, 3.9 to 5.9 chloride molality, are evidence for the presence of more than one distinct fluid during mineralization.In a reduced sulfur mineralization model with Pb carried as chloride complexes, dilution is also a possible sulfide precipitation mechanism. The difference in Pb solubility (for an equal quantity of reduced sulfur) in the extremes of the chloride concentration range, 3.9 vs. 5.9 molal, reaches 1 ppm only for pH values below approximately 4.5. Accepting 1 ppm as a minimum metal concentration for a viable ore-forming fluid, dilution only appears capable of precipitating sulfides in a fluid with pH near the lower limit of values considered geologically reasonable or attainable.Dolomite cements hosting warm (approximately 105 degrees -125 degrees C) saline fluid inclusions are ubiquitous in the porous dolomitic facies of the Bonneterre Dolomite. Based on stratigraphic reconstructions, however, it is unlikely that the Bonneterre was buried deeper than 1.5 km. The distribution of warm inclusions beyond the Viburnum Trend district implies that fluid migration was regional in scale. Fluid inclusion temperatures inconsistent with typical basement heat-flow-controlled geothermal gradients (25 degrees -35 degrees C/km) may be explained by long-distance migration of warm, basin-derived brines. Elevated temperatures observed in fluid inclusions at shallow stratigraphic depths are consistent with a gravity flow hydrologic system characterized by rapid flow rates and the capacity for advective heat transport.