PREFERENTIAL FLOW PATHWAYS IN SEDIMENTARY AQUIFERS. MODELING CONNECTIVITY

The study of hydraulic connectivity and preferential flow pathways within aquifer systems has included a variety of approaches including the use of flow and transport based metrics, the use of critical path analyses, and the use of facies-based approaches linked with the percolation theory. The focus here is on the facies-based approach, and face-to-face cell connectivity as defined within 3-D finite-difference flow models. Such models are common, and their grids are cubic lattices within the percolation theory. The percolation theory shows that a cluster of high-permeability facies will be connected across an infinite cubic lattice if occurring in volume fractions (proportions) between 1.0 and 0.3116 within the model, even without a spatial correlation structure (the independent, random, infinite-domain case). The literature contains a number of models created by a variety of different geologic simulation methodologies, demonstrating that with geologic structure, connectivity can exist across the boundaries of finite lattices at proportions well below 0.3116. Connectivity exists at proportions as low as 0.09 among the studies reviewed. Complex pathways emerge from simple correlation structures, including two-point spatial bivariate structures, especially when multiple scales of stratal architecture are represented. Proportion is an important parameter in this context, and is in fact one of the better defined attributes among those derived from well-log data.