Natural Rock Fractures: From Aperture to Fluid Flow

Abstract

Fractures provide preferential flow paths and establish the internal “plumbing” of the rock mass. Fracture surface roughness and the matedness between surfaces combine to delineate the fracture geometric aperture. New and published measurements show the inherent relation between roughness wavelength and amplitude. In fact, data cluster along a power trend consistent with fractal topography. Synthetic fractal surfaces created using this power law, kinematic constraints and contact mechanics are used to explore the evolution of aperture size distribution during normal loading and shear displacement. Results show that increments in normal stress shift the Gaussian aperture size distribution toward smaller apertures. On the other hand, shear displacements do not affect the aperture size distribution of unmated fractures; however, the aperture mean and standard deviation increase with shear displacement in initially mated fractures. We demonstrate that the cubic law is locally valid when fracture roughness follows the observed power law and allows for efficient numerical analyses of transmissivity. Simulations show that flow trajectories redistribute and flow channeling becomes more pronounced with increasing normal stress. Shear displacement induces early aperture anisotropy in initially mated fractures as contact points detach transversely to the shear direction; however, anisotropy decreases as fractures become unmated after large shear displacements. Radial transmissivity measurements obtained using a torsional ring shear device and data gathered from the literature support the development of robust phenomenological models that satisfy asymptotic trends. A power function accurately captures the evolution of transmissivity with normal stress, while a logistic function represents changes with shear displacement. A complementary hydro-chemo-mechanical study shows that positive feedback during reactive fluid flow heightens channeling.

Pore-scale phenomena in carbon geological storage (Saline aquifers—Mineralization—Depleted oil reservoirs)

“…The evolution of mineral dissolution in a porous medium is inherently related to pore size variability and the presence of preferential flow pathways which deliver the majority of the reactants in advective regimes: 10% of pores may be responsible for more than 50% of the total flow rate in sediments (Jang et al, 2011), and flow channeling is even more pronounced in fractured rock masses (National Academies of Sciences Engineering and Medicine, 2020; Cardona et al, 2021). Therefore, dissolution and transport are coupled, and may involve fabric changes.…”

Fluid-Driven Instabilities in Granular Media: From Viscous Fingering and Dissolution Wormholes to Desiccation Cracks and Ice Lenses

“…Inherent pore size variability results in preferential flow pathways across sediments; in fact, as few as 10% of pores may be responsible for more than 50% of the total flow rate (Jang et al, 2011). Note: Flow channeling is even more pronounced in fractures and fractured rock masses (National Academies of Sciences, Engineering, and Medicine, 2020;Cardona et al, 2021). These preferential flow paths will also deliver the majority of the reactants to cause either dissolution or precipitation within the soil mass.…”

A New Local-Global Upscaling Method for Flow Simulation in Naturally Fractured Reservoirs

“…Accurate modeling of fluid flow in fractured formations is crucial for various environmental and energy applications, such as the oil recovery process in fractured reservoirs, geological CO2 sequestration, geothermal energy extraction, and water management (Cardona et al, 2021;Geiger et al, 2010;Hoteit et al, 2019;Hoteit & Firoozabadi, 2008a, 2008bVahrenkamp et al, 2020;Younes et al, 2021). Detailed geological characterization of fractured reservoirs has been commonly described by the discrete-fracture model (DFM), in which matrix and fractures are explicitly represented using unstructured grids.…”