Proppant distribution in a fracture subjected to normal stress

Abstract:

During the shut-in stage of hydraulic fracturing, aperture decreases, which traps proppant between the fracture walls. The resulting proppant distribution determines the hydraulic conductivity of the fracture. If proppant is uniformly distributed throughout the fracture, proppant permeability dictates the resulting fracture conductivity. However, if proppant is distributed in isolated islands surrounded by proppant-free regions, fracture conductivity may be enhanced. Optimizing the creation of such islands requires a quantitative understanding of the factors (normal stress, solid concentration, carrier fluid composition) that control the distribution of proppant and its evolution during closure. We use a 15 x 15-cm bench-scale transparent fracture that allows fracture closure by applying normal stress to quantify the distribution of proppant between the fracture walls after shut-in. Eight actuators apply a constant force to an aluminum frame supporting the fracture surfaces and four LVDTs measure displacements. Preliminary constant-normal-stress experiments (closed inlet and outlet) show a direct correlation between the force applied by the actuators and the fluid pressure. Furthermore, for some fluid-solid combinations, proppant in the constant-aperture fracture develop isolated islands surrounded by open regions. In the absence of a confining stress, these open regions may be mobilized during flow-back, so ongoing experiments measure the evolution of these proppant islands during shut-in and flow-back at increased normal stress. Our experiments provide mechanistic insights to proppant pack evolution necessary for optimizing fracture conductivity enhancement during and after fracture shut-in.