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Date of Award
Doctor of Philosophy (PhD)
Engineered subsurface operations, such as geologic CO2 sequestration (GCS) and CO2-enhanced unconventional oil/gas recovery, are promising strategies to mitigate global warming and meet energy demands. At subsurface sites, CO2-saturated brine–mineral interactions can be affected by aqueous species, either naturally existing or injected, and the interactions may further affect the geophysical properties of rocks and minerals, such as their porosity, permeability, and wettability. Such geophysical properties can control the sealing integrity of the caprock layer, which is essential to safe and efficient long-term CO2 storage. Moreover, the flow and transport of fluids at subsurface sites are also influenced by the geophysical properties. Therefore, to ensure safer and more efficient engineered subsurface operations, it is important to have a better understanding of the coupling of geochemical reactions of rocks and minerals and their geophysical properties under conditions relevant to subsurface environments.
In this work, clay minerals (phyllosilicates) are used as model minerals because of their abundance at subsurface sites used for energy-related engineered operations. The subsurface brine has high salinities, so we first identified the interplay between salinity-induced chemical reactions of biotite (an Fe-bearing mica) and its wettability alteration under high temperature and high pressure conditions. In characterizing the surface physico-chemical properties of reacted biotite, we found that biotite dissolution was enhanced at higher salinities, and biotite surfaces were rougher, more negatively charged, and contained higher densities of hydroxyl groups. All these changes in surface properties made biotite more hydrophilic.
Second, in addition to salinity, we examined the effects of organic and inorganic oxyanions on brine–biotite interactions. Regarding the effects of short-chain carboxylic acid anions, acetate slightly inhibited biotite dissolution and promoted secondary mineral precipitation, mainly due to pH-induced aqueous acetate speciation (mainly acetic acid) and the subsequent surface adsorption of acetic acid to biotite surface Si and Al sites. However, oxalate strongly enhanced biotite dissolution and induced faster and more significant surface morphology changes by forming bidentate mononuclear surface complexes. Our experimental results showed that oxalate selectively attacks edge surface sites and enhances biotite dissolution, thus it increases the relative reactivity ratios of biotite edge surfaces to basal surfaces, while acetate does not impact this relative reactivity. The information on the reactivity differences at biotite edge and basal planes in the presence of organic ligands has implications for subsurface operations in organic-rich sites. Next, we investigated brine–biotite interactions in the presence of inorganic phosphate, which strongly precipitates metal ions. At 95 °C and 102 atm of CO2, biotite dissolution was four times higher with 10 mM phosphate than with the control, 0.1 mM, and 1 mM phosphate. Despite their dissolution differences, in all the phosphate systems, phosphate interacted with Al and Fe, forming surface complexes and precipitating as Fe- or Al-bearing minerals on surfaces and in solutions. The biotite basal surfaces became more hydrophilic after reaction with phosphate, mainly as a result of phosphate adsorption.
Third, we further expanded our investigation into the interactions between clay minerals and phosphonates, which are commonly used as scale inhibitors during engineered subsurface operations. These phosphonates can degrade to increase phosphate concentrations in formation water. We observed that the phosphonate groups enhanced biotite dissolution through both aqueous and surface complexations with Fe, with more significant effects at a higher phosphonate concentration. In particular, surface complexation was more dominant with phosphonates with fewer phosphonate functional groups, and aqueous complexation played a more important role in the presence of phosphonates with more functional groups. The presence of phosphonates also promoted secondary precipitation of Fe- and Al-bearing minerals both in the solution and on the reacted biotite surfaces. Moreover, phosphonate structure (i.e., the number of phosphonate functional groups) affected the distribution, morphology, and phases of secondary precipitates. As a result of phosphonate adsorption, biotite basal surfaces were altered to be more hydrophilic. Next, to better understand the fate and transport of phosphonate scale inhibitors, we elucidated the roles of Fe-bearing phyllosilicates on the chemical stability of phosphonates under subsurface relevant conditions. Three phyllosilicate minerals were tested, showing different effects on DTPMP (diethylenetriaminepenta(methylene)phosphonate) degradation. Muscovite (an Fe-poor phyllosilicate) did not have distinguishable effects on DTPMP degradation. Nontronite (an Fe(III)-rich phyllosilicate) showed slightly promotion effects, and biotite (an Fe(II)-rich phyllosilicate) notably promoted DTPMP degradation. We found that structural Fe(II) within phyllosilicates is key to the redox degradation of DTPMP: Reactive oxygen species (ROS) were generated through the reduction of molecular oxygen by Fe(II) in biotite, and the ROS further degraded DTPMP to form phosphate, formate, and new phosphonates.
Information provided by this study advanced our understanding of the geochemical reactions of clay minerals in the presence of naturally existing aqueous species and the chemical additives introduced during engineered subsurface operations, and illuminated their impacts on the wettability alterations of clay minerals. The findings will help bridge the knowledge gaps between chemical reactions and geophysical property changes of minerals, and they have important implications for designing safer and more efficient energy-related engineered subsurface operations.
Richard L. Axelbaum, Randall T. Cygan, John D. Fortner, Daniel E. Giammar,
Available for download on Friday, April 24, 2020