Modelling stress-dependent permeability in fractured rock including effects of propagating and bending fractures

The influence of in-situ stresses on flow processes in fractured rock is investigated using a novelmodelling approach. The combined finite-discrete element method (FEMDEM) is used to model thedeformation of a fractured rock mass. The fracture wall displacements and aperture changes aremodelled in response to uniaxial and biaxial stress states. The resultant changes in flow properties ofthe rock mass are investigated using the Complex Systems Modelling Platform (CSMPþþ). CSMPþþisused to model single-phase flow through fractures with variable aperture and a permeable rock matrix.The study is based on a geological outcrop mapping of a low density fracture pattern that includes therealism of intersections, bends and segmented features. By applying far-field (boundary) stresses to asquare region, geologically important phenomena are modelled including fracture-dependent stressheterogeneity, the re-activation of pre-existing fractures (i.e. opening, closing and shearing), thepropagation of new fractures and the development of fault zones. Flow anisotropy is investigated undervarious applied stresses and matrix permeabilities. In-situ stress conditions that encourage a closing offractures together with a more pervasive matrix-dominated flow are identified. These are comparedwith conditions supporting more localised flow where fractures are prone to dilatational shearing andcan be more easily exploited by fluids. The natural fracture geometries modelled in this work are notperfectly straight, promoting fracture segments that dilate as they shear. We have demonstrated theintroduction of several realistic processes that have an influence on natural systems: fractures canpropagate with wing cracks; there is the potential for new fractures to connect with existing fractures,thus increasing the connectivity and flow; blocks can rotate when bounded by fractures, bent fractureslead to locally different aperture development; highly heterogeneous stress distributions emergenaturally