Surface-coated silica nanoparticles for conformance control of buoyancy-driven CO₂ flow
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In light of growing concerns over rising atmospheric concentrations of greenhouse gases like carbon dioxide (CO₂), carbon capture and storage (CCS) has been suggested as a means to reduce the rate of net addition of CO₂ to the atmosphere. One potential CCS method involves injecting CO₂ into deep saline aquifers, where they are designed to reside for long periods of time. High-pressure and high-temperature CO₂/brine flow through porous media is the subject of active research, but faithfully recreating the conditions and forces found deep in the subsurface remains a challenge. In particular, the role of buoyant forces in transporting CO₂ must be studied further, since the long-term migration of CO₂ is dominated by buoyancy. This study consists of two parts. Chapter 1 discusses buoyancy as relevant to the context of CO₂ sequestration and prior methods used to study buoyancy-dominated flow. Four methods to experimentally recreate buoyancy-driven flow in high-pressure corefloods are presented: “inject low and let rise,” progressive pressure increase, simplified Darcy’s Law, and the Buckley-Leverett approach. Chapter 2 investigates the potential of using surface-coated silica nanoparticles to improve the conformance of CO₂ during flow through aquifers. The Buckley-Leverett approach is used to determine a single buoyancy-driven flow rate, and a vertical coreflood is conducted using this flow rate. Core-average saturation and pressure drop measurements across the core are measured, and the in-situ CO₂ distribution is visualized by taking axial X-ray CT scans of the core during the experiment. The effect of the nanoparticles is studied by conducting the experiment with three different nanoparticle concentrations: 0 wt% (as a control), 0.5 wt%, and 5 wt%. The addition of 0.5 wt% of nanoparticles (NP) does not markedly improve the conformance of CO₂ when compared to the control. However, at concentrations of 5 wt% NP, steady-state and residual CO₂ saturation increases, sweep efficiency increases, and CO₂ mobility decreases significantly when compared to the control. The lack of effectiveness of the 0.5 wt% formulation may be due to the influence of perpendicular-to-flow bedding layers that are present in the cross-bedded sandstone core used in the experiments. There are mixed indications regarding the suitability of the Buckley-Leverett approach to predicting the buoyancy-driven flow regime.