Previous numerical and experimental proppant-flowback investigations concluded that a proppant pack is stable when the fracture-width/proppant-diameter ratio (h/d) is between three and six. The experiments described in the full-length paper found that proppant flowback is a function of fluid flow rate, particle size, fracture width, and closure stress. Proppant-free-water-flowback rate was found to decrease as closure stress was increased from zero to 1,000 psi.