Discrete micro-physics interactions determine fracture apertures

Abstract

An important question arises in relation to a rock-mass that is disrupted by an array of fractures, namely: how to quantify the evolving spatial arrangement of fracture apertures that are a major factor in bulk fluid flow processes. The approach herein employs a discrete micro-physics model of the rock texture, enabling the formulation of analytical expressions that explicitly define the fluids//geomechanics interactions that occur at the micro-scale. The resulting macro-scale responses of the model define the stress, bulk strain, and pressure states that characterise the porous rock. Via extending the discrete model by introducing a planar discontinuity, the fracture-normal bulk strain determines the status of the fracture aperture, as a consequence of the movement of the rock//fracture interface. The micro-physics model shows that a closed fracture cannot change to an open fracture by pressure changes alone; instead, bulk strain must elongate the porous rock in a direction normal to the fracture. Once opened, fracture apertures respond to changes in fluid pressure. A realistic context, within which the required bulk strain occurs, is the discontinuum geomechanics of fractured rock-mass systems, for which previous simulations exhibit a range of emergent local states that relate to the conditions, identified via the micro-physics, as being the essential controls on aperture evolution.Article highlightsDiscrete rock-texture model underpins micro-physics expressions that lead to macro-scale material response of matrix//fractureClosed fracture cannot open without local elongation normal to fracture; high pressure alone does not open fractureOpen fracture changes aperture with changing pressure