The scaling of processes governing ground-water flow and mass transport in the near subsurface is being linked to the hierarchical sedimentary architecture found within geo-reservoirs. The PIs have developed a code that generates three-dimensional models that represent the heterogeneity of hydrogeologic attributes arising from hierarchical sedimentary architecture in braid-belt deposits. The sedimentary architecture represented ranges in scale from centimeters to kilometers. This architecture does not represent all deposits, aquifers, or reservoirs, but it does represent important ones, including the Fortymile Wash alluvium at Yucca Mountain, glaciofuvial aquifers in the northern United States, the Ringold Formation at the Hanford site, and the Ivishak Formation, an Alaskan north-slope petroleum reservoir. The model is not meant to represent any one site exactly, but to represent important aspects of heterogeneity common to many sites. The full model represents a domain 2.5 km by 2.5 km by 10 m thick with voxels of 1 cubic centimeter on a 62.5 trillion-voxel lattice. While parts of the domain have been created on desktop workstations, we are currently collaborating with scientists at the Pacific Northwest National Laboratory (PNNL) to generate the full 62.5 trillion node domain using the high performance supercomputer located at the Environmental Molecular Sciences Laboratory (EMSL) at PNNL. The full model domain will be freely distributed to the research community as a resource for testing ideas related to the upscaling problem, the inverse problem, and for other computational research requiring high-resolution grids. In our work, we are generating simulations to test the hypothesis that the percolation of highly permeable open-framework gravels must be understood not only from their proportions and geometry, but also from the proportions, geometry, and spatial distribution of the strata at multiple scales (unit bars, compound bars, channel fills, etc.). Furthermore, we are generating numerical transport simulations to test the hypothesis that solutions to upscaling problems may require accounting for the presence of percolation, as well as the hierarchy of unit types defined at different scales. If successful, the full model domain and numerical tracer tests will be freely distributed to the research community. Thus, the research will promote the testing of emerging ideas in computational research on ground water flow and transport. The hypotheses we will test using these models are relevant to heterogeneity and upscaling issues in environmental cleanup and to enhanced petroleum recovery. The project also benefits human resources and education in science and engineering through the training of a post-doctoral scholar in the areas of high-performance computing, hydrogeology, and sedimentology, and through mentoring from university and national laboratory scientists.
The broader goal of this research was to better understand processes of fluid flow and mass transport through the natural, porous, subsurface. This included the processes of transport and dispersion of contaminants in ground-water flow within the near subsurface. The near-subsurface zone is critical because it is most associated with protecting and managing sources of drinking water, and with the restoration of contaminated environmental sites. Also of interest are the processes associated with multi-phase fluid flow in deeper geologic formations. Understanding deeper-seated processes is important to evaluating strategies for CO2 injection in deep sedimentary basins for long term sequestration. Subsurface transport in sedimentary aquifers cannot be understood without understanding, characterizing, and modeling physical and chemical heterogeneity. This heterogeneity is strongly related to stratification of sediment during deposition. The recent literature in sedimentology contains new ideas on how fluvial sediments are organized, among different scales, along with a quantification of attributes that control flow and transport within such deposits. The results from highly detailed field studies of multi-scale and hierarchical sedimentary architecture commonly found in fluvial deposits were represented in a three-dimensional digital model. This digital model was used in computational experiments in order to better understand how flow and transport processes are controlled by hierarchical stratal architecture across a range of relevant scales. The use of high-performance computing facilitated representing a range of scales from centimeters up to kilometers. The digital model represents subsurface heterogeneity in some important aquifers, including those at a number of Department of Energy environmental cleanup sites, and also heterogeneity within some georeservoirs targeted for injection of CO2 for geologic sequestration. The formation of pathways for preferential flow through the subsurface was shown to be defined by the proportions, geometries and juxtapositoning tendencies of the strata across the hierarchy of scales. The number and directional nature of preferential flow pathways were shown change with scale within a deposit. The geologic structure creating preferential-flow pathways, which reduces spatial entropy of flow parameters relative to random heterogeneity, was shown to increase the relative entropy in the residence time for solute transport. A Lagrangian-based stochastic theory for plume dispersion in hierarchical and multiscale porous media was tested against numerical tracer tests. The theory proved robust and it facilitated discernment and quantification of how each scale of heterogeneity contributed to the total plume dispersion. Computational experiments also gave new insight into how capillary trapping in multi-phase fluid flow is affected at different scales within the hierarchy of stratal architecture, and how trapping relates to paleoflow directions.
