We present a set of experiments to investigate the development of solid volume fraction (??) variations in the plane of a fracture in the direction perpendicular to the flow. We injected a concentrated suspension (??=0.5) into the same parallel fracture in all the experiments, however, each experiment included one or two thin-plate obstructions at different locations and orientations within the fracture. Though the development of ??-gradients in the plane of the fracture was not observed in the experimental system, we did observed a surprising transient behavior of these concentrated suspensions. A pressure transient was observed at all flow rates in all experiments and suggests that the solid volume fraction distribution across the fracture as well as the distribution in the plane of the fracture affect the pressure response to a given flow rate. This pressure transient is related to the timescale associated with a solid distribution front reaching the fracture and can have major implications when designing experiments or interpreting pressure data from field operations.
Flow of concentrated suspensions is important to a range of natural processes and engineered systems. Fluids from natural hazards (i.e. mud flows, avalanches, magma flows) are typically characterized by a large volume of suspended solids . Industrial applications where suspended-solid flow is important include hydraulic fracturing for oil and gas production [2, 3], and environmental remediation . The rheology, and flow behavior, of the carrier fluid is significantly different than the rheology of the suspension, which is strongly influenced by the solid volume fraction (??). At high shear rates, the presence of the solids typically causes shear-thinning behavior, whereas at low shear rates, a yield stress is common . This is especially important in confined geometries, i.e. fractures, tubes, etc., where the walls can exert large shear stresses.