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Date of Award
Doctor of Philosophy (PhD)
Geologic CO2 sequestration (GCS) is considered a promising method to reduce anthropogenic CO2 emission. Assessing the supercritical CO2 (scCO2)¬–gas or liquid phase water (g, l)–mineral interactions is critical to evaluating the viability of GCS processes. This work contributes to our understanding of geochemical reactions at CO2¬–water (g, l)–mineral interfaces, by investigating the dissolution of aluminosilicates in CO2-acidified water (l). Plagioclase and biotite were chosen as model minerals in reservoir rock and caprock, respectively. To elucidate the effects of brine chemistry, first, the influences of cations in brine including Na, Ca, and K, have been investigated. In addition to the cations, the effects of abundant anions including sulfate and oxalate were also examined. Besides the reactions in aqueous phase, we also examine the carbonation of silicates in water (g)-bearing supercritical CO2 (scCO2) under conditions relevant to GCS. For the metal carbonation, in particular, the effects of particle sizes, water, temperature, and pressure on the carbonation of wollastonite were systematically examined.
For understanding the cations effects in brine, the impacts of Na concentrations up to 4 M on the dissolution of plagioclase and biotite were examined. High concentrations of Na significantly inhibited plagioclase dissolution by competing adsorption with proton and suppressing proton-promoted dissolution. Ca has a similar effect to Na, and their effects did not suppress each other when Na and Ca co-existed. For biotite, the inhibition effects of Na coupled with an enhancing effect due to ion exchange reaction between Na and interlayer K, which cracked the basal surfaces of biotite. The K in aqueous phase significantly inhibited the dissolution. If the biotite is equilibrated with NaCl solutions initially, the biotite dissolved faster than the original biotite and the dissolution was inhibited by Na and K in brine. The outcomes improve our current knowledge of silicates dissolution to the high salinity conditions in subsurface environments.
In addition to cations, the role of anions in geochemical reactions in subsurfaces are important. This study investigated the anion effects by studying sulfate and oxalate. Sulfate formed monodentate surface complexes with the Al sites on plagioclase surface and enhanced the dissolution. Oxalate was also found to enhance the plagioclase dissolution. Co-existing oxalate and sulfate suppressed the effects of sulfate on plagioclase dissolution. This information provides useful insights for understanding the roles of sulfate and organic compounds on the CO2¬–water–mineral interactions during scCO2 enhanced oil recovery. The results also aid in formulating a scientific guideline of the proper amount of SO2 co-injection with CO2.
Water in GCS sites can exist in water-bearing scCO2 in addition to the aqueous phase in brine. Thus, it is important to understand the effects of water-bearing scCO2 on the carbonation of silicates. To address the gap between the nano- and micro-sized particles used in the laboratory to the large grains in field sites, we utilized wollastonite and investigated the effects of particle sizes on the wollastonite carbonation in water-bearing scCO2. The thickness of the reacted layer on the particle surfaces was found to be constant for different sized particles. The amorphous silica layer formed act as a diffusion barrier for water-bearing scCO2. In addition, the reaction extent was higher with more water, lower temperature, and higher pressure. Further, higher water saturation percentage and lower temperature can lead to the formation of more permeable amorphous silica layers.
This thesis included the investigations of both liquid phase and vapor phase water that contacted with scCO2, and the effects of cations and anions on both formation and caprock minerals. The findings from this work improve our knowledge of the geochemical reactions at CO2¬–water–mineral interfaces, which will help us design a safer GCS operation and assess the impacts of GCS on the environmental safety and quality.
Richard Axelbaum, Alexander Bradley, John Fortner, Daniel Giammar,
Available for download on Friday, April 19, 2019