Impact of permeability anisotropy misalignment on flow rates predicted by hydrogeological models

Abstract

The notion of permeability is critical to compute underground fluid flow. In most cases rock permeability is anisotropic, due to physical processes including gravitational compaction, which often results in the principal permeability directions being approximately horizontal and vertical in undeformed rocks. However, rocks often are tilted and/or deformed over time, therefore permeability orientation varies. Anisotropic permeability with varying orientation is hard to quantify in three-dimensional (3D) models and is therefore sometimes approximated, for convenience, by setting the principal permeability directions to horizontal and vertical, and assuming that corresponding errors in fluid flow might be negligible when the change in orientation is minimal. This study shows how minor misalignment of the permeability tensor can lead to large errors in fluid flow magnitude and corresponding transport times for strongly anisotropic rocks. It also provides a method to set anisotropic permeability orientation appropriately in geometrically complex 3D models using implicit 3D geological modelling. The misalignment is particularly costly when fluid flow is localised in thin channels, where a misalignment of just 5° leads to errors of two orders of magnitude for anisotropy ratios (between the largest and smallest principal values of the permeability tensor) of 104. It is therefore recommended to set anisotropic permeability accurately, using longitudinal and transverse components along with their respective orientations, rather than horizontal and vertical components. This approach will become increasingly important as 3D models gain realism in their representation of complex geometries.