Equivalent upscaled hydro-mechanical properties of a damaged and fractured claystone around a gallery (Meuse/Haute-Marne Underground Research Laboratory)

Abstract

In this work, we present calculations and analyses of equivalent continuum (upscaled) coefficients describing the damaged, fissured and fractured claystone around an underground gallery. We focus here on mechanical and coupled hydro-mechanical properties of the damaged claystone (the upscaled Darcy permeability of the same claystone was studied in a previous paper focused on hydraulics without mechanical deformations). Concerning the geometric structure of the damaged clay stone around the cylindrical excavation, we use a hybrid 3D geometric model of fissuring and fracturing, comprising (a) a set of 10 000 statistical fissures with radially inhomogeneous statistics (size, thickness and density increasing near the wall), and (b) a deterministic set of large curved 'chevrons' fractures, periodically spaced along the axis of the drift according to a 3D chevron pattern. The hydro-mechanical coefficients calculated here are second-and fourth-rank tensors, which are displayed using ellipsoids. For simplicity, we also calculate equivalent isotropic coefficients extracted from these tensors: Young's modulus (E), bulk modulus (K), Lamé shear modulus (m), Poisson's ratio (n), Biot coefficient (B, stress-pressure coupling) and Biot modulus (M, pressure-fluid production coupling). All of these coefficients are affected by the degree of damage and fracturing, which increases near the wall of the gallery. Both 3D and '2D transverse' distributions are analysed, on grids of 3D cubic voxels and 2D pixels, respectively. Global coefficients upscaled over the entire damaged and fractured zone are also analysed. Other types of averages are presented, for example, upscaled values over a cylindrical annular shell at various radial distances from the gallery wall. The relation to the degree of fracturing is discussed, including for instance the effect of fracturing on bulk and shear stiffnesses, and on the hydro-mechanical coupling coefficients of the damaged claystone.

Upscaling 3D coupled hydro mechanical properties of fractured porous rocks

“…The present paper develops in detail and validates the full mathematical theory and tensorial equations of the upscaling technique used in Ref. 14.…”

“…Ababou et al 1 introduced a permeable porous matrix for permeability upscaling in 2D, and later Cañamón et al [11][12][13] generalized that method for 3D hydromechanical upscaling, in which only the fluid-filled cracks contributed to H-M pressure/stress coupling. Later, 14 extended the previous method so that both the fluid-filled cracks and the saturated porous matrix contribute to Hydro-Mechanical pressure/stress coupling (and to fluid production/pressure coupling). The present paper develops in detail and validates the full mathematical theory and tensorial equations of the upscaling technique used in Ref.…”