Study on suspension and migration law of proppant in long-distance pipeline for ground-coalmine combined hydraulic fracturing

Ground-coalmine combined hydraulic fracturing is one of the development trend of staged fracturing of long boreholes in underground coal mines. After the fracturing fluid is pressurized by the ground fracturing pump, it enters the long boreholes in underground coal mines through the through-well and long pipeline to conduct large-pump-rate fracturing. The suspension and migration law of proppant in the long pipeline is of great significance for optimizing the sanding parameters and avoiding sand blockage in the pipeline. The rheological properties and sand carrying properties of fracturing fluid were evaluated by laboratory tests. Then, based on the Euler-particle flow theory, a numerical simulation model was constructed to study the transport law of proppant in horizontal pipe. The flow pattern of fracturing fluid and the calculation model of critical settlement velocity were discussed. The results show that the addition of 1% friction reducer can increase the viscosity of active water fracturing fluid by 3~5 times. The smaller the proppant density, the larger the fracturing fluid viscosity and sand concentration, and the smaller the settling velocity of proppant in fracturing fluid. The flow of proppant in horizontal pipe is affected by many factors. The smaller the flow rate of fracturing fluid, the larger the density and particle size of proppant, and the more serious the deposition of proppant at the bottom of the pipe, and the worse the sand carrying effect is. With the increase of pipeline diameter, the maximum volumetric concentration of proppant at the outlet moves from the middle and lower part of the pipeline to the bottom of the pipeline, and the flow of proppant aggravates the wear of the pipeline. The larger the sand concentration is, the stronger the interaction between proppants is, and the sand carrying capacity of fracturing fluid is reduced. The model recommended by dredging technical specification is preferred to calculate the critical settling velocity of proppant under the condition of active water fracturing fluid. With the increase of pipeline diameter, the required critical sand carrying pump rate increases exponentially. Increasing the viscosity of fracturing fluid can reduce the critical sand carrying pump rate. According to the critical pump rate model of sand carrying migration, the pipeline diameter and fracturing fluid pump rate can be optimized and matched. The research results can provide theoretical support for ground-coalmine combined hydraulic fracturing.