A simulation study on the suspension and transport patterns of proppants in long distance pipelines for integrated well and ground fracturing

Integrated well and ground fracturing is a developmental trend for the staged fracturing of underground long boreholes in coal mines. Using the integrated fracturing technology, the fracturing fluids, after being pressurized using a ground fracturing pump, enter long boreholes in underground coal mines through ground penetrating wells and long distance pipelines, aiming to achieve fracturing with high injection rates of fracturing fluids. The suspension and transport patterns of proppants in long distance pipelines hold critical significance for optimizing parameters related to proppant adding and avoiding sand plugging in pipelines. This study evaluated the rheological properties and proppant carrying capacity of fracturing fluids through laboratory experiments. Based on the Euler-Euler-based granular flow theory, this study constructed a numerical simulation model to investigate the suspension and transport patterns of proppants in horizontal pipelines and their influencing factors. Moreover, this study explored the flow regimes in which fracturing fluids carry proppants and the calculation model of critical settling velocities. The results are as follows: (1) Adding 1% of friction reducer can increase the viscosity of active water-based fracturing fluids by 3‒5 times. A lower proppant density, a higher fracturing fluid viscosity, and a higher proppant concentration result in a lower proppant settling velocity in fracturing fluids. (2) The flow of proppants in horizontal pipelines is subjected to multiple factors. A lower flow rate of fracturing fluids, as well as a higher density and larger particle size of proppants, can lead to more severe proppant settling at the pipeline bottom and a low proppant carrying capacity. (3) With an increase in the pipeline diameter, the position where the proppant volume fraction at the pipeline outlet cross-section peaks would shift from the middle to lower part of the pipeline to the bottom of the pipeline, accompanied by aggravated pipeline wear caused by proppant flow. (4) A larger proppant concentration corresponds to stronger interactions between proppants, which can reduce the proppant carrying capacity of fracturing fluids. (5) The model recommended by the Technical Code of Dredging Engineering was employed to calculate the critical settling velocity of proppants when active water-based fracturing fluids were adopted. The calculation results show that the critical injection rate of fracturing liquids required for proppant carrying increases exponentially with the pipeline diameter and that the critical injection rate can be reduced by increasing the fracturing fluid viscosity. The model of the critical injection rate of fracturing fluids built in this study and the proppant transport patterns obtained in this study can assist in the optimization of and the matching between pipeline diameter and the injection rate of fracturing fluids, thus providing theoretical support for integrated well and ground fracturing.