Discrete element method modeling of inherently anisotropic rocks under uniaxial compression loading

Abstract

SUMMARYA new numerical approach is proposed in this study to model the mechanical behaviors of inherently anisotropic rocks in which the rock matrix is represented as bonded particle model, and the intrinsic anisotropy is imposed by replacing any parallel bonds dipping within a certain angle range with smooth-joint contacts. A series of numerical models with β = 0°, 15°, 30°, 45°, 60°, 75°, and 90°are constructed and tested (β is defined as the angle between the normal of weak layers and the maximum principal stress direction). The effect of smooth-joint parameters on the uniaxial compression strength and Young's modulus is investigated systematically. The simulation results reveal that the normal strength of smooth-joint mainly affects the behaviors at high anisotropy angles (β > 45°), while the shear strength plays an important role at medium anisotropy angles (30°-75°). The normal stiffness controls the mechanical behaviors at low anisotropy angles. The angle range of parallel bonds being replaced plays an important role on defining the degree of anisotropy.Step-by-step procedures for the calibration of micro parameters are recommended. The numerical model is calibrated to reproduce the behaviors of different anisotropic rocks. Detailed analyses are conducted to investigate the brittle failure process by looking at stress-strain behaviors, increment of micro cracks, initiation and propagation of fractures. Most of these responses agree well with previous experimental findings and can provide new insights into the micro mechanisms related to the anisotropic deformation and failure behaviors. The numerical approach is then applied to simulate the stress-induced borehole breakouts in anisotropic rock formations at reduced scale. The effect of rock anisotropy and stress anisotropy can be captured.

Evolution of stress‐induced borehole breakout in inherently anisotropic rock: Insights from discrete element modeling

“…Microscopic observations reveal that the discontinuities of intrinsic anisotropy are not necessarily continuous or straight (see Figure a) [ Ambrose et al ., ]. In order to explicitly represent the microstructure of this type of rock, a novel approach has been proposed by the authors [ Duan and Kwok , ; Duan et al ., ] based on the bonded particle model [ Potyondy and Cundall , ] and the smooth joint model [ Ivars et al ., ; Mas Ivars et al ., ] as illustrated in Figure b. To construct the inherently anisotropic model, a series of rigid discs are first created and bonded at their contacts to represent the rock matrix (bonded particle model).…”

“…Systematic parametric studies have been conducted to evaluate the effect of smooth joint properties (including stiffness, tensile strength, cohesion, friction angle, friction coefficient, and number of weak layers) on the macroscopic response of anisotropic rock model under both uniaxial compression and Brazilian test conditions, based on which a step‐by‐step calibration procedure is proposed for the efficient selection of microparameters [ Duan and Kwok , , ; Duan et al ., ]. The diverse macroscopic responses between different rock types are found to be controlled by the difference between weak layer properties on particle scale.…”

“…Various failure modes, i.e., elastic mismatch, sliding wing crack, and compression‐induced tensile crack, are found to be the dominant mechanisms when β = 0°, 45°, and 90°, respectively. It is the transition of these micromechanisms that controls the anisotropic behaviors at sample scale [ Duan and Kwok , ; Duan et al ., ; Kwok and Duan , ]. This numerical approach provides a new way to study the behaviors of inherently anisotropic rocks which can link the strength anisotropy, deformation behaviors, and failure patterns on sample scale to the microstructure and particle‐scale properties.…”

“…These cracks around the borehole serve as starting points for more severe breakouts forming as buckling failure. Propagation of these shear failures gives rise to the occurrence of tensile failure of parallel bond, which initiates from their tips and propagates in the direction perpendicular to the weak layers due to the tensile stress field induced by bedding slippage (Figures b and c) [ Amann et al ., ; Duan et al ., ; Rawling et al ., ]. The interaction between the shear failure of smooth joint and tensile failure of parallel bond results in a four‐lobed breakout pattern (Figure d).…”

Numerical analysis of the effect of natural microcracks on the supercritical CO2 fracturing crack network of shale rock based on bonded particle models

“…Our model just ensures that the macroscopic mechanical properties are in agreement with Table . Because the focus of our study was not the effect of shale anisotropy on induced fracture geometry, our model did not fully reproduce the exact behaviors of all types of anisotropic rocks, such as in these anisotropy ratios: , , , and , as defined in the literature . Of course, anisotropy of shale has influence on induced fracture geometry; we will investigate these effects in our future studies.…”