Reactions between CO2-charged brines and reservoir minerals might either enhance the long-term storage of CO2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. This study uses the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral-fluid reactions,under elevated CO2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2x10-14 and 4x10-16 mol·m-2·s-1, respectively. Fluid ion-activity products are close to equilibrium (e.g. DGr for plagioclase between -2 and -10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on DGr. Local variation in DGr is attributed to the injection and disassociation of CO2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer, feldspar hydrolysis reactions consume H+ and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low DGr would be compatible with far-from-quilibrium rates of ~1x10-13 mol·m-2·s-1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium. Surface carbonate deposits and cementation within the footwall of the local fault systems record multiple injections of CO2 into the Navajo Aquifer and leakage of CO2 from the site over ca. 400,000 years. The d18O, d13C and 87Sr/86Sr of these deposits record rapid rates of CO2 leakage (up to ~1000 tonnes/a) following injection of CO2, but rates differ by an order of magnitude between each fault, due to differences in the fault architecture. Elevated pCO2 enhances rates of feldspar dissolution in the host aquifer and carbonate precipitation in fracture conduits. Silicate mineral dissolution rates decline and carbonate precipitation rates increase as pH and the CO2 charge dissipate. The Sr/Ca of calcite cements record average precipitation rates of ~2x10-6 mol/m2/s, comparable to laboratory derived calcite precipitation rates in fluids with elevated Mn/Ca and Fe/Ca, at cc of ~1 to 3. This suggests that far-from-equilibrium carbonate precipitation, which blocks fracture conduits and causes the leaking system to self-seal, driven by CO2 degassing in the shallow subsurface, can be accurately modeled with laboratory derived rates. Sandstones altered in CO2 leakage conduits exhibit extensive dissolution of hematite grain coatings and are chemically bleached as a result. Measurements of Eh-pH conditions in the modern fluid, and modeling of paleo-Eh-pH conditions using calcite Fe and Mn concentrations, suggests that the CO2-charged groundwaters are reducing, due to their low dissolved O2 content and that pH suppression due to high pCO2 is capable of dissolving and transporting large concentrations of metals. Exhumed paleo-CO2 reservoirs along the crest of the Green River anticline have been identified using volatile hosting fluid inclusions. Paleo-CO2-charged fluids mobilized hydrocarbons and CH4 from deeper formations, enhancing the reductive dissolution of hematite, which produced spectacular km-scale bleached patterns in these sediment.

ABSTRACT Modeling Diffusion and Buoyancy-Driven Convection with Application to Geological CO2 Storage Rebecca Allen Geological CO2 storage is an engineering feat that has been undertaken around the world for more than two decades, thus accurate modeling of flow and transport behavior is of practical importance. Diffusive and convective transport are relevant processes for buoyancy-driven convection of CO2 into underlying fluid, a scenario that has received the attention of numerous modeling studies. While most studies focus on Darcy-scale modeling of this scenario, relatively little work exists at the pore-scale. In this work, properties evaluated at the pore-scale are used to investigate the transport behavior modeled at the Darcy-scale. We compute permeability and two different forms of tortuosity, namely hydraulic and diffusive. By generating various pore ge- ometries, we find hydraulic and diffusive tortuosity can be quantitatively different in the same pore geometry by up to a factor of ten. As such, we emphasize that these tortuosities should not be used interchangeably. We find pore geometries that are characterized by anisotropic permeability can also exhibit anisotropic diffusive tortuosity. This finding has important implications for buoyancy-driven convection modeling; when representing the geological formation with an anisotropic permeabil- ity, it is more realistic to also account for an anisotropic diffusivity. By implementing a non-dimensional model that includes both a vertically and horizontally orientated 5 Rayleigh number, we interpret our findings according to the combined effect of the anisotropy from permeability and diffusive tortuosity. In particular, we observe the Rayleigh ratio may either dampen or enhance the diffusing front, and our simulation data is used to express the time of convective onset as a function of the Rayleigh ratio. Also, we implement a lattice Boltzmann model for thermal convective flows, which we treat as an analog for CO2 storage modeling. Our model contains the multiple- relaxation-time scheme and moment-based boundary conditions to avoid the numer- ical slip error that is associated with standard bounce-back. The model’s accuracy and robustness is demonstrated by an excellent agreement between our results and benchmark data for thermal flows ranging from Ra = 103 to 108. Our thermal model captures analogous flow behavior to that of CO2 through fluid-filled porous media, including the transition from diffusive transport to initiation and development of convective fingering.

Australia’s economy is heavily dependent on coal-based energy and greenhouse gas intensive natural resource extraction and processing industries. As part of an international climate change mitigation effort Australia will have to undergo a national transformation to a low emissions society by mid century. Federal and State Governments in Australia, like their counterparts in other major developed economies, have been persuaded that reliance on fossil fuels in stationary energy industries such as electricity generation and minerals processing will be able to continue with the deployment of a value chain of technologies fitted to these installations for capturing carbon dioxide, transporting it to a disposal site, and then injecting it into subsurface geological formations for permanent storage (carbon capture and storage, or CCS). Understanding the likely effectiveness of CCS for reducing greenhouse gas emissions from stationary energy industries is therefore critical to policy formulation for, and management of, Australia’s emissions mitigation effort and national transformation over the decades ahead. / This thesis aims to offer a clearer understanding of the practicalities, limitations and uncertainties surrounding future CCS use in Australia and of the contribution CCS can make to mitigating emissions from the Australian stationary energy sector in the period to 2050. It considers two central questions: Is CCS a realistic option for emissions mitigation in Australia? Are Australian climate policies formulated to facilitate CCS deployment and optimise its potential contribution? The criteria employed in this thesis for answering these questions are restricted to those having an ascertainable causal impact on the timing, pace and ultimate scale of CCS deployment within Australia. The methodology used for the research is grounded in critical approaches and integrated assessment within a holistic, trans-disciplinary paradigm. / This thesis finds that under Australia’s existing climate policy framework it is unrealistic to expect CCS can contribute more than 75 million tonnes of CO2 per annum to emissions mitigation by 2050. Australia does have sufficient potential geological storage resources to expect some environmentally safe CCS infrastructure could be engineered over time, but commencement of large scale build-out is not likely before 2025. When CCS will become a commercial mitigation option in Australia is unpredictable and dependent more on the political economy of climate change than on Australian research, development and demonstration activities. / The thesis also finds that the existing climate policy framework is increasing rather than decreasing the risks to timing and usefulness of CCS even to the level of 75 million tonnes of CO2 per annum by 2050. This thesis concludes that Australian Governments are not developing the institutional capability to oversee a holistic decarbonisation of the stationary energy sector. This capability is required not only to address the risks to CCS deployment but also to prevent market failures that foreclose an optimal contribution from all other potential mitigation technologies. The thesis proposes that an Australian national CCS company be created with responsibility for CCS integration, transport and storage services in order to develop Australian capability rather than that of international corporations.

Cette étude porte sur l'impact des fuites de CO2 provenant d'un réservoir géologique de stockage de CO2 sur un aquifère d'eau douce. Elle se distingue des autres études, réalisées sur le même thème, par la recherche et l'application, depuis l'échelle du laboratoire jusqu'à celle d'un site pilote, de nouveaux outils de monitoring et d'approche isotopique destinés à la détection précoce de fuite de CO2. Afin de tester ces outils, des échantillons solides et liquides provenant de l'aquifèrestratégique de l'Albien du Bassin de Paris ont été prélevés, analysés et utilisés pour une étude expérimentale en batch. Cette expérience permet de contraindre et de comprendre précisément les interactions eau-roche-CO2 et les réponses isotopiques. Une application grandeur nature en Norvège a permis de mettre en place ce programme isotopique et de suivre l'évolution de la composition isotopique en distinguant les processus et phénomènes naturels et les processus reliés à l'injection de CO2. Ces deux cas d'étude appliqués au contexte de détection de fuite de CO2 ont permis de choisir les outils isotopiques les meilleurs comme indicateurs indirects de la présence de CO2, dans le cas particulier des systèmes étudiés. L'efficience de ces outils isotopiques réside dans l'enregistrement de la trace laissée par la présence de CO2 au cours d'interactions eau/roche/CO2. L'utilisation de tels outils nécessite une méthodologie rigoureuse abordée dans ce manuscrit et nécessite d'être adaptée aux spécificités des sites envisagés. / This study deals with the impact of CO2 leakages out of geological storage into overlying freshwater aquifers. Compared to other existing studies, the major added value of this study lies, on the one hand, in the research of new monitoring tools and isotopic approach in the context of CCS aiming at early and sensitive detection of CO2 leakage and, on the other hand, in the application of these tools at the (limited) laboratory scale as well as at field scale. In order to test these tools, solid and liquid materials were sampled out of the major strategic drinking water Albian aquifer in the Paris Basin (France). We have then precisely characterized and used them within a batch experiment. This experiment yields interesting results which help understanding and constraining precisely the water-rock-CO2 interactions as well as the isotopic responses. A real scale application of the method was then performed in Norway. It was an opportunity to develop this isotopic program and to track the isotopic evolution composition, while differentiating the natural processes and the system response tothe CO2 injection. When applied to the detection of CO2 leakage context, the two case studies open the way for choosing the “best” isotopic tools as indirect indicators of CO2 presence in these specific systems. The efficiency of these isotopic tools comes from the recording of the CO2 footprint all along the water-rock-CO2 interactions. Using such tools imposes a rigorous methodology, which is tackled inthis manuscript. Furthermore, future application will require adapting to the specifics of a proposed site.