The researchers conducted experiments on coal samples from southern Qinshui Basin to study how fluid flow affects coal permeability in coalbed methane wells. They found that fast-flowing formation water can remove solid particles, leading to decreased coal permeability. There is no velocity sensitivity during water production and depressurization, but velocity sensitivity occurs during gas production. The critical drainage flow should be under 11.26 m3/d during gas production to prevent pore structure damage. Formation water during gas production causes solid particles to detach and clog pores, reducing coal permeability.