Laboratory investigation of CO₂ storage by mineral carbonation | A laboratory investigation of CO₂ storage by mineral carbonation

Achieving geological carbon mineral storage (CMS) through engineered in-situ mineralisation has become established as a climate mitigation technology over the last ten years. Despite the success of various demonstration projects, uncertainties remain over the mechanisms of carbonate formation within reservoirs, and grain scale- processes which could aid or inhibit ongoing mineral precipitation and thus affect the long-term viability of geological repositories. This thesis presents the results of a campaign of laboratory experiments designed to further knowledge about the physio- chemical phenomena which underpin the mineralisation reaction. Original research is presented in three thematically linked journal articles. The first paper presents the results of a suite of batch experiments conducted over the course of several years. These investigate the effects of time, temperature, CO₂ partial pressure (pCO₂), and fluid chemistry on carbonate precipitation in the CO₂-water-basalt system. I show that for CO₂-water-basalt experiments at low temperature and pCO₂ ranges (45-85 C and 0.04 – 1 bar pCO₂), carbonates do not precipitate in experiments of up to one year’s duration. Instead, minor clay and zeolite alteration are the only discernible mineralogical changes from high sensitivity analytical methods. Only experiments with the addition of 0.5 M NaHCO₃ solution display the precipitation of carbonates over measured timescales. I use these results to deduce that formation fluid pH is a critical factor when assessing the viability of potential CMS reservoirs. The second paper presents the design, construction, and operational demonstration of bespoke reservoir simulation apparatus I have developed during the PhD to address a gap in the published literature. This compact reaction cell design allows for the simulation of CMS reservoir environments with realistic physical conditions; whole rock cores and fluid flow. The cell is x-ray transparent and allows the collection of time-series microtomography (4DμCT) data which tracks physical changes in the rock as the mineralisation reaction proceeds. I demonstrate the functionality of the cell and include a complete set of technical drawings for other researchers to reproduce the cell. The third paper presents the results of a 93-day operando experiment simulating a basalt-hosted CMS reservoir environment using the methodology presented in the previous chapter. I document the entire lifecycle of the mineralisation reaction using a combination 4DμCT and a host of other microanalytical techniques, investigating the diagenesis of newly formed carbonates within the basalt sample. I show the development of a nascent fracture network and argue that the data present a strong case for an underlying self-sustaining process of reactive surface generation, driven by precipitation of carbonates. I show a bulk increase in sample porosity of up to 2% and a final volume fraction of carbonate minerals of up to 1.56% over the duration of the experiment. The 4DμCT data presented are novel in terms of both the underlying fidelity to CMS reservoir environments, and the duration of reaction investigated. The presented results have applied value in reservoir characterization, understanding fundamental physical processes, and validation of subsurface models. The presented methodologies and results offer scope for a continued programme of experimental work to further knowledge of CMS reservoir environments.