Sung-Hoon Ji

Wettability-dependent DNAPL migration in a rough-walled fracture.

The effect of wettability on the migration of dense non-aqueous phase liquids (DNAPLs) through a rough-walled fracture was investigated. The migration characteristics of DNAPL were found to be strongly dependent on the wettability. For a fracture with a hydrophilic surface, DNAPL migrated through larger apertures as disconnected blobs when the groundwater flow regime was linear (Re=1). However, for non hydrophilic surfaces DNAPL did not migrate in the same way as for the hydrophilic surface. The intermediate-wet surface, with a contact angle of approximately 90 degrees , makes gravity pressure dominant over the capillary pressure, resulting in the fastest DNAPL migration. DNAPL was retained on the hydrophobic fracture, where the capillary barrier of larger apertures forced the DNAPL to migrate through the smaller apertures. In the nonlinear flow regime of Re=60, DNAPL generally migrated downward as a result of the inertial pressure of flowing water for all the wettability conditions, but the local downward migration paths were still determined by the capillary pressure, which resulted in the fastest and slowest migration on the hydrophilic and the hydrophobic fractures, respectively. This study implies that the hydrophilic and intermediate-wet surfaces will be favorable for DNAPL and oil recovery.

Influence of ambient groundwater flow on DNAPL migration in a fracture network: Experiments and simulations

[1] We consider the influence of ambient groundwater flow on the migration of DNAPL within a fracture network. In context of a modified invasion percolation (MIP) growth algorithm, we formulate a mechanistic model that includes capillary and gravity forces as well as viscous forces within the DNAPL and the ambient groundwater. The MIP model is verified against laboratory experiments, which show good agreement in DNAPL migration path through a two-dimensional fracture network. The results of both simulations and laboratory experiments suggest that ambient groundwater flow can be a significant factor controlling DNAPL migration path, velocity, and channeling pattern in a fracture network. INDEX TERMS: 5104 Physical Properties of Rocks: Fracture and flow; 5139 Physical Properties of Rocks: Transport properties; 5114 Physical Properties of Rocks: Permeability and porosity. Citation: Ji, S.-H., I. W. Yeo, K.-K. Lee, and R. J. Glass, Influence of ambient groundwater flow on DNAPL migration in a fracture network: Experiments and simulations, Geophys. Res. Lett., 30(10), 1504, doi:10.1029/2003GL017064, 2003.

Effects of the Correlation Length on the Hydraulic Parameters of a Fracture Network

We consider the influences of correlation length and aperture variability on the REV, the equivalent permeability of a fracture network, and the uncertainty in the equivalent permeability using a two-dimensional orthogonal bond percolation model. The percolation threshold, correlation length, effective conductivity, and coefficient of variation of the effective conductivity are investigated over statistically representative multiple realizations with Monte Carlo simulations in 2D fracture networks that have log-normally distributed individual fracture permeabilities. We show that although the aperture variability is large, the REV and the correlation length are similar near the percolation threshold. In contrast, when the fracture density is much larger than the percolation threshold they diverge as the aperture variability increases. We characterize the effects of correlation length and aperture variability on effective conductivity with a simple function. From the coefficient of variation analysis, the correlation length can be a criterion for evaluating which conceptual model is appropriate for describing the flow system for a given fracture network when aperture variability is sufficiently small. However, discrete fracture network models are recommended for flow simulation models because of the difficulty of REV estimation and the uncertainty in equivalent hydraulic parameters when aperture variability is large.

Derivative-Assisted Classification of Fractured Zones Crossing a Deep Borehole

In this study, the derivative analysis using the derivative of drawdown with respect to log-time was utilized to determine candidates for hydraulic conductor domains (HCDs). At a 500-m deep borehole in the study site, the fractured rocks crossing the borehole were first classified in fractured and nonfractured zones by core logging and geophysical loggings, such as acoustic televiewing, density, and flow loggings. After conducting the hydraulic tests such as constant head withdrawal and recovery tests at the fractured zones and the nonfractured zones, the derivative analyses were carried out, of which the results were evaluated to determine the candidates for HCDs. For the nonfractured zones, the diagnostic plot has only a big hump indicating poor connection of the background fractures to the permeable geologic media, while those of the candidates for HCDs show various flow regimes. On the basis of these results, the candidates for HCDs among the fractured zones were determined. From discussion on the results, the combination of the spacing analysis and derivative analysis following a hydraulic test is recommended for determining the candidates for HCDs rather than other geophysical loggings.

Hydraulic conceptualization of a single fracture using hydraulic interference tests at a deep underground condition

In this study, we evaluated the conceptualization approaches to represent heterogeneous hydraulic property of a single fracture using geostatistics and stochastic method. Interference tests at the single fracture, which was identified at the depth of about 250 m from the ground surface in the Olkiluoto island, Finland, were simulated. The transmissivity data from the test boreholes were geostatistically analyzed, and the analysis results were used to reproduce the transmissivity field of the single fracture by the ordinary krigging and conditional random generation approaches. The groundwater flow models were calibrated by comparing the observed flow rates at the boreholes during the interference tests to the simulated ones. The simulation results show that the discrepancies between the simulated and observed flow rates became larger as the observed flow rates were large, which indicates that the ordinary krigging and conditional random generation approaches for conceptualization of the single fracture were not be able to sufficiently reflect the real hydraulic property. Nevertheless, the observed flow rates were included in the ranges of simulated ones when using the conditional random generation approach, which shows that the conditional random generation approach is applicable although the number of observations is too small to reenact the real hydraulic property of a single fracture.

Appropriate Domain Size for Groundwater Flow Modeling with a Discrete Fracture Network Model

When a discrete fracture network (DFN) is constructed from statistical conceptualization, uncertainty in simulating the hydraulic characteristics of a fracture network can arise due to the domain size. In this study, the appropriate domain size, where less significant uncertainty in the stochastic DFN model is expected, was suggested for the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) site. The stochastic DFN model for the site was established, and the appropriate domain size was determined with the density of the percolating cluster and the percolation probability using the stochastically generated DFNs for various domain sizes. The applicability of the appropriate domain size to our study site was evaluated by comparing the statistical properties of stochastically generated fractures of varying domain sizes and estimating the uncertainty in the equivalent permeability of the generated DFNs. Our results show that the uncertainty of the stochastic DFN model is acceptable when the modeling domain is larger than the determined appropriate domain size, and the appropriate domain size concept is applicable to our study site.