Frac sand crushing characteristics and morphology changes under high compressive stress and implications for sand pack permeability

Abstract

Sand consisting of round quartz grains is widely used as a proppant during hydraulic fracturing to produce natural gas from tight shale formations. This paper presents results from sand characterization and crushing tests on Jordan Formation frac sand. It includes an assessment of grain-size reduction, changes in particle shape, and reduction in void ratio. It also examines the implications for permeability reduction through a sand pack caused by the closure stress on a hydraulic fracture. The sand from two size ranges (0.6 to 0.71 mm and 0.5 to 0.6 mm) was tested dry under applied compressive stresses of up to 40 MPa in a crushing cup. The overall sand pack stress–strain response becomes softer as grains are crushed. The particle shape shifts from nearly spherical grains to diametrically split grains and then to small elongated and angular fragments for the smaller particle sizes. The permeability of the sand pack reduces by more than 40% at a 20 MPa stress, which is mainly caused by a decrease in void ratio due to compaction. The permeability reduces by over 70% at a 40 MPa stress, which is primarily caused by void ratio decrease, reduction in particle size, and a shift away from spherical particle shapes. Compared to the ISO 13503-2 standard where only the sand crushing percentage after crushing tests is measured, this paper demonstrates that more information can be extracted from sand crushing tests and that sand pack permeability can be assessed to optimize frac sand selection. A sand with a larger size range has a higher crushing percentage, but is more permeable compared to a sand with a smaller size range. This further indicates that frac sand selection based only on sand crushing percentage is not sufficient to achieve better sand pack permeability.