Utilizing surface treated nanoparticles for enhanced geologic carbon sequestration
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Geologic carbon sequestration (GCS), the permanent storage of anthropogenic carbon dioxide (CO₂) into deep underground rock formations, has been identified as a key climate change mitigation strategy. However, due to differences in viscosity and density between the injected CO₂ and resident formation brine, CO₂ suffers from significant viscous fingering and gravitational instability. This leads to poor storage capacity and security. The work presented in this thesis shows how certain surface treated silica nanoparticles can work to address these key issues and improve today's GCS technology. Corefloods were performed in a Boise sandstone core with liquid CO₂ and brine or nanoparticle dispersion. CO₂ storage efficiency was achieved through primary drainage and residual (capillary) trapping was achieved after a subsequent brine postflush. Through meticulous measurements in pressure drop, mass balance and CT scanning, it was determined that primary drainage in the presence of surface treated nanoparticles results in significant improvements in CO₂ sweep efficiency and storage capacity through two main mechanisms: 1) the viscosification of CO₂ by the generation of nanoparticle stabilized CO₂-in-brine emulsion during pore scale Roof snap-off events, and 2) the reduction of CO₂ relative permeability through blockage of flow paths by the same nanoparticle stabilized CO₂ droplets. Furthermore, the improvements in CO₂ sweep efficiency resulted in higher levels of CO₂ residual trapping. The improvements in displacement dynamics were highly correlated to nanoparticle concentrations. Low concentrations (0.5 wt%) were shown to reduce viscous fingers and increase CO₂ saturations. At even higher concentrations (5.0wt%), the CO₂-brine displacement was fully stabilized into a shock front; initial CO₂ storage improved by a factor of 2.3 compared to the control while residual trapping increased threefold. Due to the large pore volumes of CO₂ injection sites, experiments were conducted with partial nanoparticle saturations as a method to optimize CO₂ storage while minimizing volume of nanoparticles utilized. It was demonstrated that the same CO₂ storage efficiency and residual trapping values were achieved by using 40% less nanoparticles than fully saturating the core prior to CO₂ injection, suggesting that nanoparticle stabilized CO₂-in-brine emulsions can be formed during Roof snap-off events outside the main CO₂ displacement front. This has significant implications on the economic feasibility for enhanced geologic carbon sequestration. Finally, in a second application of nanoparticles for GCS, the experiments were performed vertically to replicate the scenario of CO₂ leakage. Nanoparticles increased the breakthrough time even during low flow rates where displacements were dominated by buoyancy and capillary heterogeneity. From the experiments tested, nanoparticles should be considered a method to reduce the speed of CO₂ leakage, rather than preventing it completely.