Roland N. Horne , Stanford University
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.