Experimental study on the velocity sensitivity of coal reservoir during coalbed methane drainage in southern Qinshui Basin

Determination of the velocity sensitivity in coal reservoirs during the different production stages of coalbed methane wells is fundamentally crucial to adopt appropriate drainage technologies. To address this need, simulation experiments of coal samples from southern Qinshui Basin in China were conducted to test the variation of coal permeability with fluid flow. The pore structures were tested before and after the simulation experiment by using mercury injections, and the pore shape was observed using scanning electron microscope (SEM). The results show that formation water with fast flow may remove solid particles and that there is no velocity sensitivity under the experimental conditions of different coal samples and formation waters during the water production and depressurization stages of the coalbed methane well. There is a trend of the velocity sensitivity in the coalbed methane reservoir showing high concentration of solid particles during the stages of water production and depressurization. Coal permeability decreases with the increase of the fluid flow, there are different levels of velocity sensitivity in the coalbed methane reservoir during gas production of the coalbed methane well. The critical drainage flow should be within 11.26 m(3)/d during gas production of the coalbed methane well. The generation of the velocity sensitivity will make the pore structure of the coalbed methane reservoir poorly. During the stage of gas production, the formation water produces poorly, and the solid particles adhered to the surface of coal easily fall off and are deposited in the transition pore and micropore, which further results in the decrease of coal permeability.