Reports: AC948424-AC9: Enhanced CO2 Sequestration Taking Account of Aqueous Solubility

Roland N. Horne , Stanford University

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, 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.

During the 2010-2011 academic year, research student Sarah Pistone completed the project.  Findings indicated that CO2 solubility in water is very important as it caused residual water saturation to decrease over time. This is suggested to be the result of a phenomenon that was termed “active phase change” (APC), which is apparent only in a soluble gas such as CO2 and will be explained in more detail later in this report. Results were obtained from laboratory experiments where we considered the simplified case of CO2 and water (instead of brine) at ambient conditions in order to investigate the phenomenon of active phase change and its effect on apparent relative permeabilities. The results of this work may be applied to a variety of fields. Relevant sectors include carbon capture and sequestration (CCS), CO2 enhanced geothermal systems (EGS), as well as traditional geothermal systems with natural CO2 in their reservoir fluids, and enhanced oil recovery (EOR) operations. The most common application of this type of research is to CCS, where there are generally four main mechanisms by which CO2 may be “trapped” underground: (1) a structural confining layer that acts as a hydrodynamic barrier to flow, (2) mineralization of carbonate species, (3) disconnected gas phase that is immobile due to capillary effects, and (4) solubility trapping where CO2 dissolves into reservoir fluid, which may be enhanced by gravity effects. “Active phase change” (APC) may serve as a fifth trapping mechanism whereby more CO2 could be stored than would have otherwise been predicted.

A more complete understanding of the conditions that enhance or degrade CO2 mobility would help mitigate scaling or corrosion issues. In addition, CO2 solubility is an important factor in EOR operations that use a water alternating CO2 gas drive. If the CO2 dissolves in water in the formation there will be less gas to mix with the residual oil, thus lower total oil production. Perhaps saturating the water with CO2 prior to injection would help mitigate this problem. Again, and understanding of CO2 solubility in water and APC may help with the design of more productive EOR schemes.

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