Modes of free convection in fractured low-permeability media

C T Simmons, J M Sharp, D A Nield

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    [1] Abundant data indicate significant fluid and solute fluxes across low-permeability shales in the Gulf of Mexico, the precise mechanisms of which are poorly understood. In this study, we analyze possible modes of intrafracture and interfracture free convection that may occur across fractured low-permeability layers, such as shales. Fracture spacing, fracture aperture, shale thickness, and the density gradient across the shale unit are shown to play key roles in the governing fluid and solute transport processes. All modes of free convection (parallel to the fracture plane, perpendicular to the fracture plane, and convection between fractures on the larger layer scale) are theoretically possible for reasonable hydrogeologic parameters. Free convection parallel to the fracture plane is shown to be the dominant (and most likely) mode of free convection, requiring only very modest salinity differences for onset to occur. Least likely is convection perpendicular to the fracture plane. The results presented here suggest that free convection may not be uncommon in thick shale sequences, such as in the Gulf of Mexico Basin. An important consequence of these findings is that analyses that do not consider both interfracture and intrafracture convection modes may significantly underestimate the likelihood of the occurrence of free convection. These findings have important implications for the study of free convection and solute transport processes in fractured low-permeability media and associated numerical modeling analyses.
    Original languageEnglish
    Article numberW03431
    Number of pages8
    JournalWater Resources Research
    Issue number3
    Publication statusPublished - 2008


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