• journal_title：Vadose Zone Journal
• Contributor：Curtis M. Oldenburg ; André J. A. Unger
• Publisher：Soil Science Society of America
• Date：2004-
• Format：text/html
• Language：en
• Identifier：10.2113/3.3.848
• journal_abbrev：Vadose Zone Journal
• issn：1539-1663
• volume：3
• issue：3
• firstpage：848
• section：SPECIAL SECTION: RESEARCH ADVANCES IN VADOSE ZONE HYDROLOGY THROUGH SIMULATIONS WITH THE TOUGH CODES

Geologic CO₂ sequestration is being considered as a way to offset fossil fuel–related CO₂ emissions to reduce the rate of increase of atmospheric CO₂ concentrations. The accumulation of vast quantities of injected CO₂ in geologic sequestration sites may entail health and environmental risks from potential leakage and seepage of CO₂ into the near-surface environment. We are developing and applying a coupled subsurface and atmospheric surface-layer modeling capability built within the framework of the integral finite difference reservoir simulator TOUGH2. The overall purpose of the modeling studies is to predict CO₂ concentration distributions under a variety of seepage scenarios and geologic, hydrologic, and atmospheric conditions. These concentration distributions will provide the basis for determining aboveground and near-surface instrumentation needs for CO₂ sequestration monitoring and verification, as well as for assessing health, safety, and environmental risks. A key feature of CO₂ is its large density (ρ = 1.8 kg m⁻³) relative to air (ρ = 1.2 kg m⁻³), a property that may allow small leaks to cause concentrations in air above the occupational exposure limit of 4% in low-lying and enclosed areas such as valleys and basements where dilution rates are low. The approach we take to coupled modeling involves development of T2CA, a TOUGH2 module for modeling the multicomponent transport of water, brine, CO₂, gas tracer, and air in the subsurface. For the atmospheric surface-layer advection and dispersion, we use a logarithmic vertical velocity profile to specify constant time-averaged ambient winds, and atmospheric dispersion approaches to model mixing due to eddies and turbulence. Initial simulations with the coupled model suggest that atmospheric dispersion quickly dilutes diffuse CO₂ seepage fluxes to negligible concentrations, and that rainfall infiltration can cause CO₂ to return to the subsurface as a dissolved component in infiltrating rainwater.