Effect of hydrochemical properties on the migration of coal fines in propped fractures

The problem of coal fines restricts the efficient development of coalbed methane in China. The coal sample of No. 3 coal seam in the Shanxi Formation in the Hancheng mining area was selected for carrying out a physical simulation experiment on the effect of different hydrochemical properties on the migration of coal fines in the propped fractures. The change in the permeability of fracture diversion system and the output mass concentration of coal fines were investigated, the influence of different hydrochemical properties on coal fines migration in the propped fractures was explored. Experimental results show that in the displacement condition of Na₂SO₄ solution, the output mass concentration of coal fines decreases with the increase of salinity. Under the condition of high salinity, the coal fines have strong agglomeration and sedimentation, which can block the crack channels. In the displacement condition of NaHCO₃ solution, the output mass concentration of coal fines increases with the increase of salinity. With the increase of salinity, OH⁻ is generated by hydrolysis, which increases the negative charge on the surface of coal fines particles, enhances the repulsive force between coal fines, alleviates the coal fines aggregation effect, and produces part of coal fines particles. Under the condition of 6 000 mg/L solution of Na₂SO₄ and NaHCO₃ solution, the fluid has more distribution space and the highest permeability. In the displacement condition of the MgCl₂ solution, the output mass concentration of coal fines increases with the increase of salinity. High salinity is conducive to the aggregation of coal fines. The internal relative balance of the experimental sample is broken, and the output of coal fines increases. The clogging degree of coal fines inside the experimental sample is roughly the same, and the permeability value does not change much. Under the condition of the same salinity (10 000 mg/L), the mass concentration of coal fines output in the experiments of different hydrochemical types is: NaHCO₃ > Na₂SO₄ > MgCl₂, and the average permeability of the experimental sample is: MgCl₂ > NaHCO₃ > Na₂SO₄. The degree of coal fines aggregation affects the output of coal fines to a certain extent. The agglomeration effect of Na₂SO₄ and NaHCO₃ is lower than that of MgCl₂, and part of the coal fines will migrate. The degree of clogging at the end of the experimental sample is higher than that of the MgCl₂ solution. The hydrolysis of NaHCO₃ generates OH⁻, and the coal fines are in an alkaline environment, which improves the clogging of the experimental sample and increases the permeability. According to the three trends of the rising state, falling state and slight fluctuation state of permeability fluctuation in the simulation experiment, combined with the measured permeability value, the period time of the permeability change trend was counted, and the relationship between the mass concentration of coal fines output and the permeability of coal samples was analyzed. The R² of the regression model is 0.834, indicating that the fluctuation of the coal fines migration affects the experimental permeability.