Fluid-Escape Structures in the Subsurface: Minimum Flow Velocities of Pressure-driven Slurry Injection during Natural Hydraulic Fracturing

Abstract

Field studies of natural hydraulic fractures and fluid-escape structures yield data on the maximum clast size observed at each outcrop, data that were then used to constrain the minimum flow velocity of the sediment slurry injected into a hydraulic fracture during a fluid-escape event. Because grain concentrations at the time of injection are not recorded in rock outcrops, flow velocities were examined over a range of reasonable grain concentrations to evaluate the velocities necessary to carry the largest observed clasts as grain concentration varied. Analysis of the data revealed several tends in fluid-escape structures: 1) Most injectites have maximum clast sizes below 30 cm and most slurries have minimum flow velocities < 3 m s-1, 2) As the concentration of suspended particles increases, dependence on flow velocity decreases, and 3) As maximum clast size decreases below several centimeters, dependence on flow velocity greatly decreases. Although these trends in the data result from field observations, the effects of the numerical methods also cannot be ignored. This study represents a first attempt to evaluate the roles of maximum clast size versus grain concentration in estimating the minimum flow velocity for an escaping fluid slurry.