The impacts of flow velocity on permeability and porosity of coals by core flooding and nuclear magnetic resonance: Implications for coalbed methane production

The fluid flow velocity has a significant effect on the coalbed methane (CBM) production by influencing the porosity and permeability of coals during the drainage process. In this work, the fluid velocity sensitivity experiments combined with the nuclear magnetic resonance (NMR) technology were performed to investigate the impacts of various flow velocities on permeability and porosity. The results show that the permeability of different rank coals has various characteristics with the increase of flow velocity. For low rank coals, the permeability always increases first then decreases with the increase of flow velocity. However, the permeability gradually decreases with the increase of flow velocity for medium and high rank coals. For the same rank coals, the higher initial permeability is, the more severe permeability damage is. Additionally, the porosity variation reflected by NMR T2 spectrum indicates that T2 between 10 ms and 200 ms is the main reduction space of seepage paths. The influence of flow velocity on permeability is mainly due to the blockage of fluid seepage space by coal fines. Moreover, the effects of dewatering rate on CBM production were further discussed combined with field production wells. The dropping rate of working fluid level at the single-phase water stage plays an important role on the production of coal fines and gas production, especially at the late production stage. Decreasing dewatering rate at the two-phase (water and gas) stage can improve the CBM production effectively. To improve the effect of flow velocity on petrophysical properties of CBM reservoir, the dewatering rate of CBM wells should be slow and stable. Therefore, this study could be conducive to CBM production or mining safety.