Conductivity of Fracture Proppants in Multiple Layers

Abstract

The fluid conductivity of a hydraulic fracture is critical in determining the effectiveness of the fracturing treatment. Two factors considered here can greatly affect that conductivity. First is the reservoir environment; for example, the presence of hot brine can reduce the permeability of brittle proppants under stress. The second is deviation from Darcy flow, or turbulence. Introduction Fracture conductivity limits the amount of stimulation achieved from the fracturing of many wells. (Fracture conductivity is defined as the width of the fracture multiplied by the apparent permeability of the proppant pack.) In recent years, improved fracturing fluids have made it possible to prop longer fractures, which increases the need for proppants that maintain higher permeability. Theoretical predictions of results from fracturing demonstrate the important relation between stimulation achieved and fracture conductivity.' For example, for long vertical fractures, a tenfold increase in fracture conductivity can lead to a predicted increase in stimulation from two-fold to almost sixfold. For fracture stimulation of wells with initially high productivity, high fracture conductivity is critical to achieving any appreciable stimulation. In the past, fracture conductivity has been measured by placing proppants between slabs of reservoir rock or different metals, applying mechanical stress to the proppants to stimulate the stress that exists in the earth at various depths, and flowing a fluid through the proppant to measure fluid conductivity. 2, 3 Results of those tests are used routinely by industry in designing fracture treatments. Most experiments were performed at room temperature and the fluids were performed at room temperature and the fluids were nitrogen, oil, or water. None of the past work has considered how different fluids contacting the proppant affect conductivity under stress, nor have the proppant affect conductivity under stress, nor have the effects of flow rate on conductivity been investigated. This paper presents data on two phenomena that have not been discussed in the literature but that appear to be very important in determining fracture conductivity with brittle proppants. The first is the effect of the environment (the fluid present and the temperature) on the conductivity of a brittle proppant under stress. The second is the effect of proppant under stress. The second is the effect of flow rate (deviation from Darcy's law) on the flow resistance of proppants. Consideration of these two phenomena is important to improving the design of phenomena is important to improving the design of fracture treatments, to understanding better the incentive for improving proppants, and to properly testing proppants in the laboratory. proppants in the laboratory. The experimental approach of investigating thick layers of proppant was taken here to separate some of the variables important in fracture conductivity from the even more complex problem of the flow in packed fractures between various types of reservoir packed fractures between various types of reservoir rock. This approach has proved advantageous. Hence, the present work applies directly only to vertical fractures packed with several layers of brittle proppant. Multiple layers result when the fracturing proppant. Multiple layers result when the fracturing fluid viscosity is low enough to allow the proppant to settle to the bottom of the vertical fracture during the injection of fluid. Multiple layers also result when the fluid is viscous enough to suspend the proppant, provided the concentration of proppant is proppant, provided the concentration of proppant is high enough and the fracture is wide enough during injection. JPT P. 1101