On Water Flow in Hot Fractured Rock -- A Sensitivity Study on theImpact of Fracture-Matrix Heat Transfer

Dual-continuum models have been widely used in modeling flowand transport in fractured porous rocks. Among many other applications,dual-continuum approaches were utilized in predictive models of thethermal-hydrological conditions near emplacement tunnels (drifts) atYucca Mountain, Nevada, the proposed site for a radioactive wasterepository in the U.S. In unsaturated formations such as those at YuccaMountain, the magnitude of mass and heat exchange between the twocontinua fracture network and matrix is largely dependent on the flowcharacteristics in the fractures, because channelized finger-type flowstrongly reduces the interface area between the matrix surfaces and theflowing liquid. This effect may have important implications, for example,during the time period that the fractured rock near the repository driftswould be heated above the boiling point of water. Depending on themagnitude of heat transfer from the matrix, water percolating down thefractures will either boil off in the hot rock region above drifts or maypenetrate all the way to the drift walls and possibly seep into the opencavities. In this paper, we describe a sensitivity analysis using avariety of approaches to treat fracture-matrix interaction in athree-dimensional dual-continuum setting. Our simulation example is alaboratory heater experiment described in the literature that providesevidence of rapid water flow in fractures, leading to drift seepagedespite above-boiling conditions in the adjacent fractured rock. Theexperimental finding can only be reproduced when the interface area forheat transfer between the matrix and fracture continua is reduced toaccount for flow channeling

Combining multiple lower-fidelity models for emulating complex model responses for CCS environmental risk assessment

Numerical modeling is essential to support natural resource management and environmental policy-making. In the context of CO2 geological sequestration, these models are indispensible parts of risk assessment tools. However, because of increasing complexity, modern numerical models require a great computational effort, which in some cases may be infeasible. An increasingly popular approach to overcome computational limitations is the use of surrogate models. This paper presents a new surrogate modeling approach to reduce the computational cost of running a complex, high-fidelity model. The approach is based on the simplification the high-fidelity model into computationally efficient, lower-fidelity models and on linking them with a mathematical function (linking function) that addresses the discrepancies between outputs from models with different levels of fidelity. The resulting linking function model, which can be developed with small computational effort, can be efficiently used in numerical applications where multiple runs of the original high-fidelity model are required, such as for uncertainty quantification or sensitivity analysis. The proposed approach was then applied to the development of a reduced order model for the prediction of groundwater quality impacts from CO2 and brine leakage for the National Risk Assessment Partnership (NRAP) project

A Semi-Analytical Solution for Large-Scale Injection-Induced PressurePerturbation and Leakage in a Laterally Bounded Aquifer-AquitardSystem

A number of (semi-)analytical solutions are available to drawdown analysis and leakage estimation of shallow aquifer-aquitard systems. These solutions assume that the systems are laterally infinite. When a large-scale pumping from (or injection into) an aquifer-aquitard system of lower specific storativity occurs, induced pressure perturbation (or hydraulic head drawdown/rise) may reach the lateral boundary of the aquifer. We developed semi-analytical solutions to address the induced pressure perturbation and vertical leakage in a 'laterally bounded' system consisting of an aquifer and an overlying/underlying aquitard. A one-dimensional radial flow equation for the aquifer was coupled with a one-dimensional vertical flow equation for the aquitard, with a no-flow condition imposed on the outer radial boundary. Analytical solutions were obtained for (1) the Laplace-transform hydraulic head drawdown/rise in the aquifer and in the aquitard, (2) the Laplace-transform rate and volume of leakage through the aquifer-aquitard interface integrated up to an arbitrary radial distance, (3) the transformed total leakage rate and volume for the entire interface, and (4) the transformed horizontal flux at any radius. The total leakage rate and volume depend only on the hydrogeologic properties and thicknesses of the aquifer and aquitard, as well as the duration of pumping or injection. It was proven that the total leakage rate and volume are independent of the aquifer's radial extent and wellbore radius. The derived analytical solutions for bounded systems are the generalized solutions of infinite systems. Laplace-transform solutions were numerically inverted to obtain the hydraulic head drawdown/rise, leakage rate, leakage volume, and horizontal flux for given hydrogeologic and geometric conditions of the aquifer-aquitard system, as well as injection/pumping scenarios. Application to a large-scale injection-and-storage problem in a bounded system was demonstrated

Discrete dilatant pathway modeling of gas migration through compacted bentonite clay

A coupled multiphase fluid flow and discrete fracturing model is applied to simulate bench-scale gas migration experiments on compacted bentonite. The numerical modeling is based on the linking of the multiphase fluid flow simulator TOUGH2 with a Rigid-Body-Spring Network model, which enables a discrete (lattice) representation of elasticity and individual fractures. The evolution of a complex network of dilatant flow paths is modeled through opening and breakage of lattice interface bonds between porous-elastic matrix elements. To achieve a good match with the experimental results, including an abrupt gas breakthrough along with pressure and stress responses, it was necessary to calibrate model parameters for (1) air-entry pressure, (2) shear and tensile failure of lattice interface bonds, (3) moisture swelling/shrinkage effects on stress, and (4) aperture-dependent permeability of dilatant flow paths. Our best-fit conceptual model considers a pervasive network of discrete flow paths propagating from the gas injection point, whereas some of the experimental data indicate the potential for heterogeneous and unstable flow paths

Reply to Comments by Veling on “A Semi-Analytical Solution for Large-Scale Injection-Induced Pressure Perturbation and Leakage in a Laterally Bounded Aquifer–Aquitard System” by Zhou, Birkholzer, and Tsang

Veling (2010) pointed to 'a serious mistake' and 'mathematical inconsistency' in Zhou et al. (2009) because the dimensionless flow equations in Equation 4 (in terms of dimensionless hydraulic head rise in the aquifer and the aquitard) would give rise to additional terms when back converting to the groundwater flow equations, in the case that initial conditions for hydraulic head were spatially variable. He added, however, that the conclusions of the paper remain valid when uniform initial conditions are assumed. We accept this comment because we have indeed assumed uniform initial conditions in the system but failed to state this explicitly in the publication, partially because this assumption is very common in groundwater hydrology when deriving analytical and semi-analytical solutions. The same assumption was employed, for example, by Veling in Veling and Maas (2009), as stated 'For the ease of presentation we assume from here on that {phi}{sub i0} (r, z) ... are all equal to zero. An arbitrary initial function ... will complicate the solution, but not essentially'. We shall emphasize that with this assumption, our semi-analytical solutions and their derivations are correct

Impacts of elevated dissolved CO2 on a shallow groundwater system: reactive transport modeling of a controlled-release field test

One of the risks that CO2 geological sequestration imposes on the environment is the impact of potential CO2/brine leakage on shallow groundwater. The reliability of reactive transport models predicting the response of groundwater to CO2 leakage depends on a thorough understanding of the relevant chemical processes and key parameters affecting dissolved CO2 transport and reaction. Such understanding can be provided by targeted field tests integrated with reactive transport modeling. A controlled-release field experiment was conducted in Mississippi to study the CO2-induced geochemical changes in a shallow sandy aquifer at about 50 m depth. The field test involved a dipole system in which the groundwater was pumped from one well, saturated with CO2 at the pressure corresponding to the hydraulic pressure of the aquifer, and then re-injected into the same aquifer using a second well. Groundwater samples were collected for chemical analyses from four monitoring wells before, during and after the dissolved CO2 was injected. In this paper, we present reactive transport models used to interpret the observed changes in metal concentrations in these groundwater samples. A reasonable agreement between simulated and measured concentrations indicates that the chemical response in the aquifer can be interpreted using a conceptual model that encompasses two main features: (a) a fast-reacting but limited pool of reactive minerals that responds quickly to changes in pH and causes a pulse-like concentration change, and (b) a slow-reacting but essentially unlimited mineral pool that yields rising metal concentrations upon decreased groundwater velocities after pumping and injection stopped. During the injection, calcite dissolution and Ca-driven cation exchange reactions contribute to a sharp pulse in concentrations of Ca, Ba, Mg, Mn, K, Li, Na and Sr, whereas desorption reactions control a similar increase in Fe concentrations. After the injection and pumping stops and the groundwater flow rate decreases, the dissolution of relatively slow reacting minerals such as plagioclase drives the rising concentrations of alkali and alkaline earth metals observed at later stages of the test, whereas the dissolution of amorphous iron sulfide causes slowly increasing Fe concentrations

Reactive transport simulations to study groundwater quality changes in response to CO2 leakage from deep geological storage

AbstractAs an effort to evaluate risks associated with geologic sequestration of CO2, this work assesses the potential effects of CO2 leakage on groundwater quality. Reactive transport simulations are performed to study the chemical evolution of aqueous Pb and As after the intrusion of CO2 from a storage reservoir into a shallow confined groundwater resource. The simulations use mineralogies representative of shallow potable aquifers in the USA; both 2D (depth-averaged) and 3D simulations are conducted. Sensitivity studies are also conducted for variation in hydrological and geochemical conditions, as well as several other critical parameters. Model results suggest that a significant increase of aqueous lead (Pb) and arsenic (As) may occur in response to CO2 intrusion, but in most sensitivity cases their concentrations remain below the EPA specified maximum contaminant levels (MCLs). Adsorption/desorption from mineral surfaces significantly impacts the mobilization of Pb and As. Results from the 3D model agree fairly well with the 2D model in cases where the rate of CO2 intrusion is relatively small (so that the majority of CO2 readily dissolves in the groundwater), whereas discrepancies between 2D and 3D models are observed when the CO2 intrusion rate is comparably large

A comparative simulation study of coupled THM processes and their effect on fractured rock permeability around nuclear waste repositories

This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings: (a) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (b) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal-mechanically-induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal-mechanically-induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermal-mechanically-induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts

A comparative simulation study of coupled THM processes and their effect on fractured rock permeability around nuclear waste repositories

Abstract This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings: (a) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (b) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermalmechanically-induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal-mechanically-induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermalmechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts

Imaging and quantification of spreading and trapping of carbon dioxide in saline aquifers using meter-scale laboratory experiments

The role of capillary forces during buoyant migrati on of CO2 is critical towards plume immobilization within the post-injection phase of a geological carbon sequestration operation. However, the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in-situ plume evolution. To overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic CO2 plumes in the presence of a background hydraulic gradient, we conducted two unique long-te rm experiments in a 2.44 m × 0.5 m tank. X-ray attenuation allowed measuring the evolution of a CO2-surrogate fluid saturation, thus providing direct insight into capillarity- and buoyancy-domin ated flow processes occurring under successive drainage and imbibition conditions. The comparison of saturation distributions between two experimental campaigns suggests that layered-type h eterogeneity plays an important role on non- wetting phase (NWP) migration and trapping, because it leads to (i) longer displacement times (3.6 months vs. 24 days) to reach stable trapping c onditions, (ii) limited vertical migration of the plume (with center of mass at 39% vs. 55% of aquife r thickness), and (iii) immobilization of a larger fraction of injected NWP mass (67.2% vs. 51. 5% of injected volume) as compared to the homogenous scenario. While these observations confirm once more the role of geological heterogeneity in controlling buoyant flows in the s ubsurface, they also highlight the importance of characterizing it at scales that are below seismic resolution (1-10 m)