Motivated by geological carbon dioxide (CO 2 ) storage, many recent studies have investigated the fluid dynamics of solutal convection in porous media. Here we study the convective dissolution of CO 2 in a closed system, where the pressure in the gas declines as convection proceeds. This introduces a negative feedback that reduces the convective dissolution rate even before the brine becomes saturated. We analyse the case of an ideal gas with a solubility given by Henry's law, in the limits of very low and very high Rayleigh numbers. The equilibrium state in this system is determined by the dimensionless dissolution capacity, Π, which gives the fraction of the gas that can be dissolved into the underlying brine. Analytic approximations of the pure diffusion problem with Π > 0, show that the diffusive base state is no longer self-similar and that diffusive mass transfer declines rapidly with time. Direct numerical simulations at high Rayleigh numbers show that no constant flux regime exists for Π > 0; nevertheless, the quantity F/C 2 s remains constant, where F is the dissolution flux and C s is the dissolved concentration at the top of the domain. Simple mathematical models are developed to predict the evolution of C s and F for high-Rayleigh-number convection in a closed system. The negative feedback that limits convection in closed systems may explain the persistence of natural CO 2 accumulations over millennial timescales.
Advances in our understanding of the microbial ecology at sites impacted by light non-aqueous phase liquids (LNAPLs) are needed to drive development of optimized bioremediation technologies, support longevity models, and develop culture-independent molecular tools. In this study, depth-resolved characterization of geochemical parameters and microbial communities was conducted for a shallow hydrocarbon-impacted aquifer. Four distinct zones were identified based on microbial community structure and geochemical data: (i) an aerobic, low-contaminant mass zone at the top of the vadose zone; (ii) a moderate to high-contaminant mass, low-oxygen to anaerobic transition zone in the middle of the vadose zone; (iii) an anaerobic, high-contaminant mass zone spanning the bottom of the vadose zone and saturated zone; and (iv) an anaerobic, low-contaminant mass zone below the LNAPL body. Evidence suggested that hydrocarbon degradation is mediated by syntrophic fermenters and methanogens in zone III. Upward flux of methane likely contributes to promoting anaerobic conditions in zone II by limiting downward flux of oxygen as methane and oxygen fronts converge at the top of this zone. Observed sulfate gradients and microbial communities suggested that sulfate reduction and methanogenesis both contribute to hydrocarbon degradation in zone IV. Pyrosequencing revealed that Syntrophus- and Methanosaeta-related species dominate bacterial and archaeal communities, respectively, in the LNAPL body below the water table. Observed phylotypes were linked with in situ anaerobic hydrocarbon degradation in LNAPL-impacted soils.
Geological carbon storage has the potential to reduce anthropogenic carbon dioxide emissions, if large volumes can be injected and securely retained. Storage capacity is limited by regional pressure buildup in the subsurface. However, natural CO2 reservoirs in the United States are commonly underpressured, suggesting that natural processes reduce the pressure buildup over time and increase storage security. To identify these processes, we studied Bravo Dome natural CO2 reservoir (New Mexico, USA), where the gas pressure is up to 6.4 MPa below the hydrostatic pressure, i.e., less than 30% of the expected pressure. Here, we show that the dissolution of CO2 into the brine reduces the pressure by 1.02 ± 0.08 MPa, because Bravo Dome is isolated from the ambient hydrologic system. This challenges the assumption that the successful long-term storage of CO2 is limited to open geological formations. We also show that the formation containing the reservoir was already 2.85 ± 2.02 MPa underpressured before CO2 emplacement. This is likely due to the overlying evaporite layer, which prevents recharge. Similar underpressured formations below regional evaporites are widespread in the midcontinent of the United States. This suggests the existence of significant storage capacities with properties similar to Bravo Dome, which has contained large volumes of CO2 over millennial time scales.
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