Christine Doughty

An active fracture model for unsaturated flow and transport in fractured rocks

The unsaturated zone at Yucca Mountain, a potential repository site of high-level nuclear waste, is a complex hydrologic system in which a variety of important flow and transport processes is involved. To quantify these processes as accurately as possible is a theoretically challenging and practically important issue. In this study, we propose a new formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18–27% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under ambient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensitivity analyses are performed to investigate effects of the “activity” of connected fractures on flow and transport behavior in unsaturated rocks.

Modeling Supercritical Carbon Dioxide Injection in Heterogeneous Porous Media

We investigate the physical processes that occur during the sequestration of carbon dioxide (CO2) in liquid-saturated, brine-bearing geologic formations using the numerical simulator TOUGH2. CO2 is injected in a supercritical state that has a much lower density and viscosity than the liquid brine it displaces. In situ, the supercritical CO2 forms a gas-like phase, and also partially dissolves in the aqueous phase, creating a multi-phase, multi-component environment that shares many important features with the vadose zone. The flow and transport simulations employ an equation of state package that treats a two-phase (liquid, gas), three-component (water, salt, CO2) system. Chemical reactions between CO2 and rock minerals that could potentially contribute to mineral trapping of CO2 are not included. The geological setting considered is a fluvial/deltaic formation that is strongly heterogeneous, making preferential flow a significant effect, especially when coupled with the strong buoyancy forces acting on the gas-like CO2 plume. Key model development issues include vertical and lateral grid resolution, grid orientation effects, and the choice of characteristic curves.

Investigation of conceptual and numerical approaches for evaluating moisture, gas, chemical, and heat transport in fractured unsaturated rock

Abstract We investigate the utility and appropriateness of various conceptual and numerical approaches for modeling several flow and transport processes in the unsaturated zone (UZ) at Yucca Mountain, NV, using a one-dimensional (1-D) column extracted from a three-dimensional (3-D) UZ site-scale model. Simulations of steady-state and transient moisture flow, transient gas flow, tracer transport, and thermal loading scenarios are performed, using a variety of numerical approaches to treat fracture–matrix (F–M) interactions, including an effective continuum model (ECM) and several varieties of dual-continua models. For the dual-continua models, we investigate the effect of varying the number of matrix gridblocks per fracture gridblock, the formulation applied for calculating F–M interface area, and whether or not global matrix-to-matrix flow occurs (dual-permeability vs. dual-porosity models). The key findings of the work based on a 1-D column are as follows. For steady-state moisture flow, most of the numerical and conceptual models provide similar results for saturation and fracture flow profiles. The ECM adequately models the steady-state processes because the system is not too far away from F–M equilibrium. For both transient moisture flow and transient transport in a steady flow field, the ECM is not adequate in general. Within the dual-continua models, the formulation for F–M interface area can have a major effect on the hydrodynamic response to an infiltration pulse and tracer arrival at various horizons, with fracture responses becoming earlier as F–M interface area decreases. The number of matrix blocks also has a significant effect on response time, with the more accurate multi-matrix-gridblock models yielding slower fracture response times. For transient gas flow arising from barometric pressure variations, the ECM adequately models the process, because F–M gas flow occurs rapidly compared to the time scale of the barometric pressure variations. For thermal loading, preliminary studies indicate that the ECM does not capture all the significant physical processes, because rapid fluid and heat flow can occur in the fractures before the matrix has a chance to equilibrate.

Conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone

A conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone was developed based on the results of lithological studies and a series of ponded infiltration tests conducted at the Box Canyon site near the Idaho National Engineering and Environmental Laboratory. The infiltration tests included one 2-week test in 1996, three 2-day tests in 1997, and one 4-day test in 1997. For the various tests, initial infiltration rates ranged from 4.1 cm/d (4.75 ×10−7 m/s) to 17.7 cm/d (2.05×10−7 m/s) and then decreased with time, presumably because of mechanical or microbiological clogging of fractures and esicular basalt in the near-surface zone, as well as the effect of entrapped air. The subsurface moisture redistribution was monitored with tensiometers, neutron logging, time domain reflectrometry, and ground-penetrating radar. A conservative tracer, potassium bromide, was added to the pond water at a concentration of 3 g/L to monitor water flow with electrical resistivity probes and water sampling. Analysis of the data shows evidence of preferential flow rather than the propagation of a uniform wetting front. We propose a conceptual model describing the saturation-desaturation behavior of the basalt, in which rapid preferential flow occurs through the largest vertical fractures, followed by a gradual wetting of other fractures and the basalt matrix. Fractures that are saturated early in the tests may become desaturated thereafter, which we attribute to the redistribution of water between fractures and matrix. Lateral movement of water takes place within horizontal fracture and rubble zones, enabling development of perched water bodies.

The impact of geological heterogeneity on CO2 storage in brine formations: a case study from the Texas Gulf Coast

Abstract Geological complexities such as variable permeability and structure (folds and faults) exist to a greater or lesser extent in all subsurface environments. In order to identify safe and effective sites in which to inject CO2 for sequestration, it is necessary to predict the effect of these heterogeneities on the short- and long-term distribution of CO2. Sequestration capacity, the volume fraction of the subsurface available for CO2 storage, can be increased by geological heterogeneity. Numerical models demonstrate that in a homogeneous rock volume, CO2 flowpaths are dominated by buoyancy, bypassing much of the rock volume. Flow through a more heterogeneous rock volume disperses the flow paths, contacting a larger percentage of the rock volume, and thereby increasing sequestration capacity. Sequestration effectiveness, how much CO2 will be sequestered for how long in how much space, can also be enhanced by heterogeneity. A given volume of CO2 distributed over a larger rock volume may decrease leakage risk by shortening the continuous column of buoyant gas acting on a capillary seal and inhibiting seal failure. However, where structural heterogeneity predominates over stratigraphic heterogeneity, large columns of CO2 may accumulate below a sealing layer, increasing the risk of seal failure and leakage.

Using surface deformation to image reservoir dynamics

The inversion of surface deformation data such as tilt, displacement, or strain provides a noninvasive method for monitoring subsurface volume change. Reservoir volume change is related directly to processes such as pressure variations induced by injection and withdrawal. The inversion procedure is illustrated by an application to tiltmeter data from the Hijiori test site in Japan. An inversion of surface tilt data allows us to image flow processes in a fractured granodiorite. Approximately 650 barrels of water, injected 2 km below the surface, produces a peak surface tilt of the order of 0.8 microradians. We find that the pattern of volume change in the granodiorite is very asymmetrical, elongated in a north‐northwesterly direction, and the maximum volume change is offset by more than 0.7 km to the east of the pumping well. The inversion of a suite of leveling data from the Wilmington oil field in Long Beach, California, images large‐scale reservoir volume changes in 12 one‐ to two‐year increments from 1...

Flow and transport in hierarchically fractured rock

We construct multiple realizations of hierarchical fracture networks with fractal dimensions between one and two, then simulate single-well pumping tests and natural-gradient tracer tests on them. We calculate averages and standard deviations of test results over the multiple realizations, and show individual results for selected cases to highlight key features of flow and transport through hierarchically fractured rock. These studies are intended to illustrate the range of possible behavior that can be obtained during fracture-dominated hydraulic and tracer tests, and provide insights into how to interpret field responses. The fractal dimension of the fracture network itself is generally larger than the fractal dimension of the flow field arising during a well test. The performance measures of the natural-gradient tracer tests, including the total flow through the fracture network, tracer travel time, front width, and maximum breakthrough concentration, can all be correlated to fractal dimension. Although some of the features observed in the flow and transport behavior within the hierarchically fractured rock have been observed by other authors using non-fractal fracture network concepts (e.g. channelized flow with early breakthrough times, crossing breakthrough curves), others arise directly from the fractal nature of the fracture network, in which variability occurs on all scales (e.g. front width and maximum breakthrough concentration that are constant over a wide range of fractal dimensions). Generally, transport simulations show large variability within a given realization and among realizations with the same fractal dimension, even in networks whose dimension is close to two. This finding is consistent with the large variability in experimental results observed at fractured rock field sites.

Groundwater “fast paths” in the Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers

Preferential flow paths are expected in many groundwater systems and must be located because they can greatly affect contaminant transport. The fundamental characteristics of radiogenic isotope ratios in chemically evolving waters make them highly effective as preferential flow path indicators. These ratios tend to be more easily interpreted than solute-concentration data because their response to water-rock interaction is less complex. We demonstrate this approach with groundwater {sup 87}Sr/{sup 86}Sr ratios in the Snake River Plain aquifer within and near the Idaho National Engineering and Environmental Laboratory. These data reveal slow-flow zones as lower {sup 87}Sr/{sup 86}Sr areas created by prolonged interaction with the host basalts and a relatively fast flowing zone as a high {sup 87}Sr/{sup 86}Sr area.

A semianalytical solution for heat-pipe effects near high-level nuclear waste packages buried in partially saturated geological media

Abstract The emplacement of a strong heat source, such as a high-level nuclear waste package, in a partially saturated permeable medium will give rise to the development of a heat pipe. The present paper analyzes a simplified version of this problem that has a steady state solution, for radial geometry. The solution is obtained in semianalytical form, and is compared to the analogous solution for a linear heat pipe. Various applications are presented for porous as well as for fractured-porous media with different hydrologie properties. The parameters determining heat-pipe length and the question of whether the vicinity of the heat source dries up are discussed. The semianalytical solution is verified by numerical simulations that show the transient evolution from uniform initial conditions to the eventual steady state.

A similarity solution for two-phase water, air, and heat flow near a linear heat source in a porous medium

Placement of a heat source in a partially saturated geologic medium causes strongly coupled thermal and hydrologic behavior. To study this problem, a recently developed semianalytical solution for two-phase flow of water and heat in a porous medium has been extended to include an air component and to incorporate several physical effects that broaden its range of applicability. The problem considered is the placement of a constant-strength linear heat source in an infinite homogeneous medium with uniform initial conditions. Under these conditions the governing partial differential equations in radial distance r and time t reduce to ordinary differential equations through the introduction of a similarity variable η = r/t1/2. The resulting equations are coupled and nonlinear, necessitating a numerical integration. The similarity solution developed here is used to investigate various physical phenomena related to partially saturated flow in low-permeability rock, such as vapor pressure lowering, pore level phase change effects, and an effective continuum representation of fractured/porous media. Application to several illustrative problems arising in the context of high-level nuclear waste disposal at Yucca Mountain, Nevada, indicates that fluid flow, phase changes, and latent heat transfer may have a significant impact on conditions at the repository.