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This research is addressing the fundamental behavior of carbon dioxide (CO₂) sequestration into water aquifers, with a specific examination of the joint effects of relative permeability and gas dissolution into and evolution from the water phase.  The interaction of solubility effects with hydrodynamic effects is not currently well understood, and there is good potential to enhance the effectiveness of carbon dioxide sequestration if these fundamental issues are investigated.  The work is making use of experimental observations of carbon dioxide displacements in rock cores and in micromodel visualization cells.  These observations have been used to develop and verify conceptual models based on statistical thermodynamics.  The concepts of solubility sequestration have been investigated by others, (for example, Pruess and Garcia, 2002) however the importance of the interaction between dissolution and relative permeability seems to have been missed.  We refer to this interaction as “active phase change”, and have been investigating its influence on the displacement and storage of CO₂ in aquifers.  Experimental observations of the differences between N₂-water and CO₂-water flow in our preliminary research have shown that apparent relative permeability is affected strongly by CO₂ dissolution – the consequences of these phenomena on sequestration are unknown but are likely to be of considerable significance. The overall goal of this project has been to investigate the importance of active phase change and mass transfer on CO₂-water two-phase flow and determine the impacts this phenomenon has on CO₂ sequestration. Importantly, the issue of dissolution phenomena in CO₂-water systems may allow for improved sequestration of CO₂ without needing to go to the extreme depths required to achieve supercriticality, while reducing the risk of CO₂ escaping to the atmosphere.

We have discovered that the solubility of CO₂ has an impact on the two-phase flow characteristics of CO₂ and water, which brings into question the current understanding and underlying assumptions associated with CO₂ sequestration. This impact was first discovered during a low pressure experiment, where alternating drainage and imbibition floods were performed with N₂ and CO₂. The final connate water saturation in the CO₂ experiment went far below the final connate water saturation in the N₂ experiment and below what standard capillary entry pressure models would suggest. This led to the term ‘active phase change’, in which the simultaneous flow of two soluble phases causes differences in two-phase flow characteristics when compared to classic two-phase flow models governed by interfacial tension alone. We visually confirmed the concept of active phase change by using a silicon micromodel of Barea sandstone, and witnessed the transport of CO₂ through the pore-network by dissolution and bubble nucleation. The concept of active phase change was then tested theoretically by examining the physics of phase change and phase nucleation. It was found that during two-phase flow that supersaturated conditions are possible and these conditions would explain active phase change. Therefore, we addressed the impact active phase change may have during CO₂ sequestration by coupling a statistical rate theory model with a relative permeability model for a simple steam-water system and found that active phase transport can be very significant for the vapor phase at high liquid saturations.