Eungyu Park

Analytical solutions of contaminant transport from finite one-, two-, and three-dimensional sources in a finite-thickness aquifer.

Analytical study of contaminant transport from a finite source in a finite-thickness aquifer is most useful in hydrological and environmental sciences and engineering but rarely investigated in previous studies. This paper provides analytical solutions of contaminant transport from one-, two-, and three-dimensional finite sources in a finite-thickness aquifer using Greens function method. A library of unpublished analytical solutions with different finite source geometry is provided. A graphically integrated MATLAB script is developed to calculate the temporal integrations in the analytical solutions and obtain the final solutions of concentration. The analytical solutions are examined by reproducing the solutions of some special cases discussed in previous studies. The sensitivities of the line source solutions to source geometry, dispersion coefficients, and distance to the source are tested. The contaminant concentration in the near field is found to be sensitive to the source geometry and anisotropy of the dispersion coefficients. The contaminant concentration in the far field is found to be much less sensitive to the source geometry. The physical insights of the analytical solutions are interpreted.

On the horizontal-well pumping tests in anisotropic confined aquifers

A method that directly solves the boundary problem of flow to a horizontal-well in an anisotropic confined aquifer is provided. This method solves the point source problem first, and then integrates the point source solution along the horizontal well axis to obtain the horizontal well solution. The short and long time approximations of drawdowns are discussed and are utilized in the semilog analysis of the drawdown. A closed-form analytical solution of geometrical skin effect at the wellbore is derived. Type curves and derivative type curves of horizontal pumping wells are generated using the chow program. This program also calculates the drawdown at any given observation well at any given time. The horizontal-well type curves are different from the vertical-well type curves at early time, reflecting the different nature of flow to a horizontal-well and to a vertical-well. The horizontal-well type curves converge to the vertical-well type curves at late time, showing the similar nature of flow to a horizontal-well and to a vertical-well at late time. The sensitivity of the type curves and derivative type curves on monitoring well location, aquifer anisotropy, horizontal well depth, and horizontal well length is tested. These type curves and derivative type curves can be used in the matching point method for interpreting the pumping test data.

Hydraulics of a finite-diameter horizontal well with wellbore storage and skin effect

We have obtained solutions of groundwater flow to a finite-diameter horizontal well including wellbore storage and skin effect in a three-dimensionally anisotropic leaky aquifer. These solutions improve previous line source solutions by considering realistic well geometry and offer better description of drawdown near the horizontal well. These solutions are derived on the basis of the separation of the source and the geometric functions. The source function is analyzed using Laplace transformation, and the geometric function is derived based on the method of superposition. The solution in a confined aquifer is derived as a special case of the solution in a leaky aquifer. The graphically integrated computer program FINHOW is written to generate type curves of groundwater flow to a finite-diameter horizontal well. The influence of the finite-diameter of the well, the wellbore storage, the skin effect, the leakage parameter, and the aquifer anisotropy is thoroughly analyzed. The well diameter, the wellbore storage, the skin effect, and the aquifer anisotropy substantially affect the near-well early time drawdown if compared to the line source solution, but they have negligible influence upon the far field or late time drawdown. This research provides a better tool for interpreting finite-diameter horizontal well pumping tests.

Hydraulics of horizontal wells in fractured shallow aquifer systems

Abstract An analysis of groundwater hydraulic head in the vicinity of a horizontal well in fractured or porous aquifers considering confined, leaky confined, and water-table aquifer boundary conditions is presented. Solutions for hydraulic heads in both leaky confined and water table aquifers are provided. The fracture model used in this study is the standard double-porosity model. The aquitard storage is included in the formula. Solutions for the confined and unconfined conditions, fractured and porous conditions, wellbore storage, and skin effect are compared. Several findings of this study are, (1) the influence of wellbore storage and skin upon the drawdown for a fractured confined aquifer is similar to that for a porous confined aquifer, (2) aquitard storage affects the intermediate time the most by delaying the drawdown, and (3) there is a significant difference between the type curves of fractured and porous confined aquifers in most aquifer boundary conditions because of the contribution of matrix storage, and such a difference disappears at the later time.

Horizontal well hydraulics in leaky aquifers

Abstract This paper presents a general study of horizontal well hydraulics for three aquifer types: a leaky confined aquifer, a leaky water table aquifer, and a leaky aquifer under a water reservoir. Semi-analytical solutions are obtained for cases that exclude and include the aquitard storage. The type curves and derivative type curves for these different conditions are provided. A graphically integrated MATLAB program named HW_LEAK is written to facilitate numerical calculations and generation of the type curves and derivative type curves. This study shows that (1) derivative type curves are more sensitive to the aquitard parameters than the type curves; and that (2) drawdown is sensitive to the aquitard/aquifer thickness ratio and the hydraulic conductivity ratio at the intermediate and later time. Both curves are less sensitive to the aquitard/aquifer specific storage ratio, while the degree of sensitivity of the drawdown to the aquitard parameters is high in a leaky confined aquifer, moderate in a water table aquifer, and low in an aquifer under a water reservoir.

Modeling of Spatiotemporal Thermal Response to CO 2 Injection in Saline Formations: Interpretation for Monitoring

We evaluated the thermal processes with numerical simulation models that include processes of solid NaCl precipitation, buoyancy-driven multiphase SCCO2 migration, and potential non-isothermal effects. Simulation results suggest that these processes—solid NaCl precipitation, buoyancy effects, JT cooling, water vaporization, and exothermic SCCO2 reactions—are strongly coupled and dynamic. In addition, we performed sensitivity studies to determine how geologic (heat capacity, brine concentration, porosity, the magnitude and anisotropy of permeability, and capillary pressure) and operational (injection rate and injected SCCO2 temperature) parameters may affect these induced thermal disturbances. Overall, a fundamental understanding of potential thermal processes investigated through this research will be beneficial in the collection and analysis of temperature signals collectively measured from monitoring wells.

Dissolved plume attenuation with DNAPL source remediation, aqueous decay and volatilization — Analytical solution, model calibration and prediction uncertainty

A vertically-integrated analytical model for dissolved phase transport is described that considers a time-dependent DNAPL source based on the upscaled dissolution kinetics model of Parker and Park with extensions to consider time-dependent source zone biodecay, partial source mass reduction, and remediation-enhanced source dissolution kinetics. The model also considers spatial variability in aqueous plume decay, which is treated as the sum of aqueous biodecay and volatilization due to diffusive transport and barometric pumping through the unsaturated zone. The model is implemented in Excel/VBA coupled with (1) an inverse solution that utilizes prior information on model parameters and their uncertainty to condition the solution, and (2) an error analysis module that computes parameter covariances and total prediction uncertainty due to regression error and parameter uncertainty. A hypothetical case study is presented to evaluate the feasibility of calibrating the model from limited noisy field data. The results indicate that prediction uncertainty increases significantly over time following calibration, primarily due to propagation of parameter uncertainty. However, differences between the predicted performance of source zone partial mass reduction and the known true performance were reasonably small. Furthermore, a clear difference is observed between the predicted performance for the remedial action scenario versus that for a no-action scenario, which is consistent with the true system behavior. The results suggest that the model formulation can be effectively utilized to assess monitored natural attenuation and source remediation options if careful attention is given to model calibration and prediction uncertainty issues.

Non-parametric simulations-based conditional stochastic predictions of geologic heterogeneities and leakage potentials for hypothetical CO2 sequestration sites

The present study focuses on understanding the leakage potentials of the stored supercritical CO2 plume through caprocks generated in geostatistically created heterogeneous media. For this purpose, two hypothetical cases with different geostatistical features were developed, and two conditional geostatistical simulation models (i.e., sequential indicator simulation or SISIM and generalized coupled Markov chain or GCMC) were applied for the stochastic characterizations of the heterogeneities. Then, predictive CO2 plume migration simulations based on stochastic realizations were performed and summarized. In the geostatistical simulations, the results from the GCMC model showed better performance than those of the SISIM model for the strongly non-stationary case, while SISIM models showed reasonable performance for the weakly non-stationary case in terms of low-permeability lenses characterization. In the subsequent predictive simulations of CO2 plume migration, the observations in the geostatistical simulations were confirmed and the GCMC-based predictions showed underestimations in CO2 leakage in the stationary case, while the SISIM-based predictions showed considerable overestimations in the non-stationary case. The overall results suggest that: (1) proper characterization of low-permeability layering is significantly important in the prediction of CO2 plume behavior, especially for the leakage potential of CO2 and (2) appropriate geostatistical techniques must be selectively employed considering the degree of stationarity of the targeting fields to minimize the uncertainties in the predictions.

Migration behavior of supercritical and liquid CO2 in a stratified system: Experiments and numerical simulations

Multiple scenarios of upward CO2 migration driven by both injection-induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core-flooding system with a 2-D X-ray scanner. Two reservoir-type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 50°C), where CO2 exists in a supercritical state and (2) the deep-sea sediment reservoir scenario (28 MPa and 25°C), where CO2 is stored in the liquid phase. The core-flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO2 tended to find a preferential flow path (low capillary entry pressure and high-permeability (high-k) path) and bypass the unfavorable pathways, leaving low CO2 saturation in the low-permeability (low-k) layers. No distinctive fingering was observed as the CO2 moved upward, and the CO2 movement was primarily controlled by media heterogeneity. The CO2 saturation in the low-k layers exhibited a more sensitive response to injection rates, implying that the increase in CO2 injection rates could be more effective in terms of storage capacity in the low-k layers in a stratified reservoir. Under the deep-sea sediment condition, the storage potential of liquid CO2 was more than twice as high as that of supercritical CO2 under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure buildup, CO2 saturation, and CO2 flux. Finally, we showed that a high gravity number ( Ngr) tended to be more influenced by the heterogeneity of the porous media.

A shallow water table fluctuation model in response to precipitation with consideration of unsaturated gravitational flow

A precise estimation of groundwater fluctuation is studied by considering delayed recharge flux (DRF) and unsaturated zone drainage (UZD). Both DRF and UZD are due to gravitational flow impeded in the unsaturated zone, which may nonnegligibly affect groundwater level changes. In the validation, a previous model without the consideration of unsaturated flow is benchmarked. The model is calibrated using multi-year groundwater data, and consistent model parameter statistics are obtained and validated. The estimation capability of the new model is superior to the benchmarked model as indicated by the significantly improved representation of groundwater level with physically interpretable model parameters.