Speaker
Description
Global climate change due to high dependence on fossil fuel has necessitated the need to deploy decarbonization technologies. Carbon capture and storage (CCS), a technology that stores CO2 permanently in the subsurface, is largely seen as necessary to reduce CO2 emissions from large-scale industrial sources. As CCS technology has become commercially viable, recent years have witnessed a large expansion of CCS projects driven by innovative incentive mechanisms and sustainable government support. Although mitigating global warming is a long-term goal, current engineering methods for CCS projects are focused on short-term operational parameters, typically 20-30 years. While CO2 is expected to remain securely trapped in the subsurface, the long-term fate of injected CO2 over thousands of years has yet to be investigated.
An important trapping mechanism for injected CO2 is residual trapping, where a large fraction of injected CO2 is disconnected into ganglia trapped in the pore spaces by capillary forces. A widely accepted theory is that residual trapping is responsible for permanent entrapment, as the trapped CO2 ganglia is assumed to remain immobile until they eventually dissolve into the reservoir brine. However, field observation that supports this theory is hardly available due to long time scales of dissolution. In previous studies, we have shown that the residually trapped CO2 is thermodynamically unstable due to capillary heterogeneity. This instability induces Ostwald ripening among residually trapped CO2, thereby redistributing the gas phase across the system despite the gas phase remaining immobile. This redistribution causes residually trapped CO2 to re-aggregate and potentially remobilize, undermining the long-term security of geological CO2 storage.
In this study, we evaluate the thermodynamic stability of residually trapped CO2 from a fundamental perspective. By analyzing the chemical potential of the residually trapped CO2, we identify that in addition to capillary heterogeneity, both hydrostatic pressure and geothermal gradients lead to thermodynamic instability of residually trapped CO2. These driving forces induce non-convective transport of dissolved CO2 in the aqueous phase, thereby redistributing the residually trapped CO2 throughout the system in different directions. The resulting non-convective transport is a combination of multiple mass transfer mechanisms, including molecular diffusion, sedimentation of solutes and the Soret effect.
Furthermore, we study the characteristics of the non-convective transport and the resulting redistribution of residually trapped CO2 through numerical simulation. Results indicate that hydrostatic pressure dominates the redistribution of residually trapped CO2 by inducing molecular diffusion of dissolved CO2. This diffusive flux is upward such that the gas phase depletes in the bottom and accumulates at the top under the seal. Geothermal gradients can somewhat mitigate this accumulation by reducing the CO2 solubility gradient. This diffusive flux depletes residually trapped gas in a rate of 10^5 years / m approximately. Although the non-convective transport is slow, the residually trapped CO2 accumulates under the seal inevitably in a shorter period. Re-aggregation and remobilization of residually trapped CO2 under the seal is highly likely to occur, which will be characterized in our future study.
References
Li, Yaxin, Charlotte Garing, and Sally M. Benson. "A continuum-scale representation of Ostwald ripening in heterogeneous porous media." Journal of Fluid Mechanics 889 (2020).
Garing, Charlotte, et al. "Pore-scale capillary pressure analysis using multi-scale X-ray micromotography." Advances in Water Resources 104 (2017): 223-241.
de Chalendar, Jacques A., Charlotte Garing, and Sally M. Benson. "Pore-scale modelling of Ostwald ripening." Journal of Fluid Mechanics 835 (2018): 363-392.
| Time Block Preference | Time Block C (18:00-21:00 CET) |
|---|---|
| Acceptance of Terms and Conditions | Click here to agree |
