Leakage and Sepage of CO2 from Geologic Carbon SequestrationSites: CO2 Migration into Surface Water
Geologic carbon sequestration is the capture of anthropogenic carbon dioxide (CO{sub 2}) and its storage in deep geologic formations. One of the concerns of geologic carbon sequestration is that injected CO{sub 2} may leak out of the intended storage formation, migrate to the near-surface environment, and seep out of the ground or into surface water. In this research, we investigate the process of CO{sub 2} leakage and seepage into saturated sediments and overlying surface water bodies such as rivers, lakes, wetlands, and continental shelf marine environments. Natural CO{sub 2} and CH{sub 4} fluxes are well studied and provide insight into the expected transport mechanisms and fate of seepage fluxes of similar magnitude. Also, natural CO{sub 2} and CH{sub 4} fluxes are pervasive in surface water environments at levels that may mask low-level carbon sequestration leakage and seepage. Extreme examples are the well known volcanic lakes in Cameroon where lake water supersaturated with respect to CO{sub 2} overturned and degassed with lethal effects. Standard bubble formation and hydrostatics are applicable to CO{sub 2} bubbles in surface water. Bubble-rise velocity in surface water is a function of bubble size and reaches a maximum of approximately 30 cm s{sup -1} at a bubble radius of 0.7 mm. Bubble rise in saturated porous media below surface water is affected by surface tension and buoyancy forces, along with the solid matrix pore structure. For medium and fine grain sizes, surface tension forces dominate and gas transport tends to occur as channel flow rather than bubble flow. For coarse porous media such as gravels and coarse sand, buoyancy dominates and the maximum bubble rise velocity is predicted to be approximately 18 cm s{sup -1}. Liquid CO{sub 2} bubbles rise slower in water than gaseous CO{sub 2} bubbles due to the smaller density contrast. A comparison of ebullition (i.e., bubble formation) and resulting bubble flow versus dispersive gas transport for CO{sub 2} and CH{sub 4} at three different seepage rates reveals that ebullition and bubble flow will be the dominant form of gas transport in surface water for all but the smallest seepage fluxes or shallowest water bodies. The solubility of the gas species in water plays a fundamental role in whether ebullition occurs. We used a solubility model to examine CO{sub 2} solubility in waters with varying salinity as a function of depth below a 200 m-deep surface water body. In this system, liquid CO{sub 2} is stable between the deep regions where supercritical CO{sub 2} is stable and the shallow regions where gaseous CO{sub 2} is stable. The transition from liquid to gaseous CO{sub 2} is associated with a large change in density, with corresponding large change in bubble buoyancy. The solubility of CO{sub 2} is lower in high-salinity waters such as might be encountered in the deep subsurface. Therefore, as CO{sub 2} migrates upward through the deep subsurface, it will likely encounter less saline water with increasing capacity to dissolve CO{sub 2} potentially preventing ebullition, depending on the CO{sub 2} leakage flux. However, as CO{sub 2} continues to move upward through shallower depths, CO{sub 2} solubility in water decreases strongly leading to greater likelihood of ebullition and bubble flow in surface water. In the case of deep density-stratified lakes in which ebullition is suppressed, enhanced mixing and man-made degassing schemes can alleviate the buildup of CO{sub 2} and related risk of dangerous rapid discharges. Future research efforts are needed to increase understanding of CO{sub 2} leakage and seepage in surface water and saturated porous media. For example, we recommend experiments and field tests of CO{sub 2} migration in saturated systems to formulate bubble-driven water-displacement models and relative permeability functions that can be used in simulation models.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE. Assistant Secretary for Fossil Energy, NationalEnergy Technologies Laboratory
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 878528
- Report Number(s):
- LBNL-57768; R&D Project: G20705; BnR: AA3010000; TRN: US0602410
- Country of Publication:
- United States
- Language:
- English
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