Assessment of Fluid Flow and Solute Transport Characteristics through a Rough - Walled Fracture with Flow Imaging

Abstract

Understanding fluid flow and solute transport in a rough-walled fracture is important in many problems such as petroleum and geothermal reservoir exploitation, geological storage of CO2 and siting of radioactive waste repositories. In order to understanding of fracture flow, we conducted the direct measurement of flow velocity across rough-walled fractures at Reynolds number (Re) of 0.014 to 0.086. The results were used for an order of magnitude analysis to evaluate assumptions underlying the Stokes and the Reynolds equations, which are derived from simplifying the Navier - Stokes equations. Even at very rough subregions, viscous forces were at least 2 orders of magnitude greater than inertial forces, indicating that the Stokes equations are valid for Re < 0.1. However, the assumption made in the derivation of the Reynolds equation that second derivative term with respect to z is dominant over other viscous terms was not satisfied even at moderate roughness for Re < 0.1. The Reynolds equation overestimated flow rate. Also, microscopic observation of solute transport through a rough-walled fracture was made to assess the evolution of eddies and their effect on non-Fickian tailing, A noteworthy phenomenon was observed that as the eddy grew, the particles were initially caught in and swirled around within eddies, and then cast back into main flow channel, which reduced tailing. This differs from the conventional conceptual model, which presumes a distinct separation between mobile and immobile zones. Fluid flow and solute transport modeling within the 3-D fracture confirmed tail shortening due to mass transfer by advective paths between the eddies and the main flow channel, as opposed to previous 2-D numerical studies that showed increased tailing with growing eddies.