Date of Award

8-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Murdoch, Lawrence C

Committee Member

Falta , Ronald W

Committee Member

DeVol , Timothy A

Abstract

ABSTRACT Storage of supercritical phase CO2 in deep saline aquifers is being considered to reduce greenhouse gases in the atmosphere, and this process is expected to increase the pressure in these deep aquifers. One potential consequence of pressurization is an increase in the upward flux of saline water. Saline groundwater occurs naturally at shallow depths in many sedimentary basins, so an upward flux of solutes could degrade the quality of aquifers, and threaten aquatic ecosystems where groundwater discharge is important. The objective of this research is to evaluate the impacts associated with increasing the upward flux of saline water as a result of CO2 storage, or other effects. The approach was to develop and evaluate simulations of salt concentration in a fresh water aquifer overlying saline groundwater that is subjected to changes in flux. The first task was to verify the solution of benchmark problems of density-dependent flow using the computational codes COMSOL Multiphysics. The COMSOL code was then used to analyze idealized 2D and 3D geometries representing the essential details of a shallow, fresh water aquifer underlain by a saline ground water in a sedimentary basin. The analysis was conducted in two stages, one that simulated the development of a fresh water aquifer by flushing out salt water, and another that simulated the effect of a pulse-like increase in the upward flux from the basin. The effects of saline encroachment were evaluated using a sensitivity analysis of key parameters, and the results were formulated in both dimensioned and dimensionless form. The results indicate that the depth of the fresh/salt interface is a function of the recharge rate, duration of fresh water flushing/basin flux rate, density of the saline water, as well as the formation anisotropy. The fresh/salt interface was found to be more uniform and shallower as the density of the salt water increased. Increased upward flux of saline water raised the fresh/salt interface, and it increased the salinity of water discharging to streams. However, the magnitudes of these effects appeared to be small. For example, the mass loading of chloride to streams only exceeded the TMDL for chloride at a reference location under extreme conditions. Nevertheless, the increases in chloride concentration in the fresh water system are long-lived, so this effect should be considered when designing a CO2 storage project. The significant contributions of this study include 1) identification of important controls on the encroachment of saline ground water on overlying fresh water resources; 2) evaluation of the expected impacts posed by increased flux from saline aquifers caused by CO2 storage.

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