The effect of clastic sedimentary structures at multiple scales on fluid flow and transport: High-resolution numerical experiments

Many aquifers and petroleum reservoirs are hosted by elastic sedimentary rocks, which typically possess complex internal structures and are hierarchically organized at various scales of complexity and length. Although sedimentary patterns are well known and their hydraulic properties of the composing lithologies, i.e., porosities and conductivities, are intrinsically coupled, they are rarely utilized in flow and transport simulations. The purpose of this study is to integrate sedimentological information and modeling by using a stratigraphic simulator to generate input for a new flow simulator. The stratigraphic simulator provides maps of the distribution and orientation of the sedimentary units, which are used to align tensor properties such as hydraulic conductivities. The resulting fully three-dimensional nine-component conductivity tensor is suitable to approximate subnode-scale properties like grain fabric, small-scale bedding or lamination, or fractures. Flow-restricting interfaces can be applied to mimic thin but low-permeable units. The flow simulator allows the assessment of potentiometric heads, velocities, flowline and advective front geometries, as well as the upscaling of conductivity of complex multidimensional model domains, using different driving head gradient directions and local conductivity tensors.; The results emphasize the importance of small-scale structured sedimentary heterogeneity. Complex two- and three-dimensional structures with moderate conductivity ratios cause appreciable differences in upscaled conductivity tensors, which are enhanced when the local conductivities are slightly anisotropic. However, depending on the local conductivity tensors, the potentiometric heads and the flowline pattern can be uniform and the upscaled hydraulic conductivity can be isotropic, even though the model domain is heterogeneous. The amplitude and pattern of the head perturbation is controlled by the geometry, orientation, and magnitude of the local conductivity tensor, which defines the direction of the local fluid flow and ultimately controls the flowline, associated advective front morphology, and the “structure-induced dispersion.” The potentiometric head and velocity pattern of locally slightly anisotropic models and high contrast locally isotropic are similar. The location and magnitude of the maximum velocity is dependent on the local conductivity tensor, the conductivity unit geometry, and the local head gradient. A “geometric hydraulic barrier” is generated where fast tensor components of adjacent nodes are at high angles.