A liquid-solid model to optimize the application of friction reducers for hydraulic fracturing/cutting in the underground coal mine

Friction reducers are widely applied to reduce friction losses in the natural oil and gas industry. The application of hydraulic fracturing in underground coal mines results in a new issue for friction reducers: The hydraulic fluid, mixed with a friction reducer, should reduce friction loss to maintain the hydraulic pressure while simultaneously discharging coal particles to clean the borehole. Most previous studies have focused on the former aspect, ignoring the discharge capacity of coal particles. To compensate for this shortcoming, a coupled flow model of coal particles and hydraulic fluid is proposed considering the interactions between the two phases and characteristics of the coal particles. The model was implemented and solved using the finite element method approach (COMSOL Multiphysics), and the impact of the injection velocity, hydraulic fluid viscosity, and stacking angle of coal particles was investigated. Before simulation, an experiment was conducted to determine the viscosity range and velocity dependency of the hydraulic fluid with a flow-loop instrument. A field application was conducted based on simulation results. The viscosity of 2.5 mPa s is a critical value around which the maximum friction reduction efficiency can be achieved, and the discharge capacity of coal particles can be enhanced significantly above this value. The applications of hydraulic fracturing or hydraulic cutting should be coupled with the viscosity of the hydraulic fluid. The injection velocity should be sufficiently high, and the time interval of the water spray should be sufficiently large for the low-viscosity hydraulic fluid. The stacking angle of the coal particles has little influence on the long-term discharge process. An upward borehole is recommended for the consideration of coal particle discharge because of the gravity effect. This work provides constructive suggestions and guidance for underground hydraulic fracturing and hydraulic cutting.

Automated summary

This study investigates the impact of friction reducers on hydraulic pressure and coal particle discharge in underground coal mines. A coupled flow model of coal particles and hydraulic fluid was proposed, considering the interactions between the two phases and characteristics of the coal particles. The model was implemented and solved using the finite element method approach, and the results showed that the viscosity of the hydraulic fluid is a critical factor for maximum friction reduction efficiency and enhanced coal particle discharge capacity. The findings provide constructive suggestions for underground hydraulic fracturing and cutting applications.