Influence of nodular structures on fracture development in fine-grained rocks: numerical simulations based on the discrete element method

Abstract

Discrete element experiments were performed to simulate fracturing processes in nodule-bearing, fine-grained rocks. Two models that contain a collection of bonded circular particles were subjected to uniaxial compression to yield shortening and fracturing of the particle assembly. Model I with soft nodules produced incremental amount of microfractures within nodules during the early stage of deformation, followed by the generation of macro sized fractures between neighbouring nodules by microfracture coalescence. The nodule-linking fractures constituted through-going fractures that cross-cut nodules. Model II with stiff nodules produced two conjugate sets of fractures that are predominantly localized within the rock matrix. The fractures cross-cut nodule-rock interfaces and broke contact bonds of nodule surfaces and their enclosing rocks, creating opening voids between them. The nodules, as mechanical heterogeneities, responded mechanically differently to the remote stress from the rock matrix. The distorted local stress field induced by the nodules can explain the varied levels of fracture development in nodules from the rock matrix. This study demonstrates the significant impact of nodular structures on the local stress field and fracture development within nodular horizons, which is believed to be instructive for fracture analysis in nodule-bearing fine-grained rocks that constitute caprocks, reservoir or source rocks for hydrocarbons.