Visualising fluid transport efficiency in rough fractures : towards predicting subsurface fracture flow

Abstract

Successful geological storage, ranging from anthropogenic waste (e.g. Carbon Dioxide & nuclear materials) to energy (e.g. Hydrogen) storage, relies not only upon fluid transport efficiency within geological formations but also on the ability of overlying formations to contain injected fluids over years to millennia. Interconnected fault and fracture systems may compromise these low-permeability geological seals, enabling fluid escape from storage reservoirs. Identifying the degree to which faults and fractures present realistic leakage geometries is key information for accurate risk assessment of any prospective storage site. This thesis presents a systematic investigation into the properties that impact single- and two-phase fluid flow in single rough fractures. We utilise micrometre-scale imaging techniques, primarily laboratory- and synchrotron-based X-ray micro-computed tomography, to visualise and quantify the internal geometries of 3D-printed and natural geological fractures. Fracture aperture measurements in both materials demonstrate single fracture distributions to be lognormal, which facilitates significant flow complexities. For two-phase flow, we observe deviation from typical invasion percolation behaviour under capillary-dominated conditions. Quantification of the relative roughness (aperture standard deviation/aperture mean) reveals that connected fluid invasion occurs in aperture regions where the relative roughness ≤ 0.56. These results can inform numerical modelling and forecasting of flow in rough fractures.