Small dip, big impact: How 1° strata inclination affects density-driven flow in anisotropic rocks

Abstract

Density-driven flow caused by fluid density contrasts in saturated porous rocks plays a critical role in many geological applications, including mineral exploration, hydrogen or carbon storage, and the migration of brines or hydrocarbons. Yet, the influence of anisotropic permeability on density-driven flow, as is common in sedimentary basins, is generally overlooked. By expressing the gravitational force acting on fluids with contrasting densities in anisotropic saturated porous media, we analytically estimate the velocity and angle of a propagating plume as functions of the dip angle of the permeability tensor and the ratio of longitudinal to transverse permeability anisotropy (r). The sensitivity of the plume angle to the bedding angle is greatest in the common case of near-horizontal layers. We demonstrate how, for large permeability anisotropy (e.g., r >100), a dip of only 1° dramatically affects plume migration compared to the perfectly horizontal case. This highlights the need for accurate representation of strata orientation and associated anisotropy in fluid-flow simulations. These results could affect mineral exploration strategies in sedimentary basins, where the slope and anisotropy of sedimentary strata could result in dense mineralizing brines migrating closer or farther from their source than previously expected. Overall, these findings are pertinent to the propagation of both lighter and denser plumes, e.g., supercritical CO2 or brines, revealing where and how fast these plumes can migrate through the dipping, anisotropic strata in sedimentary basins.