Experimental study of water blocking damage on coal

Water blocking damage, caused by the invasion of external fluids into the coal seams during hydraulic fracturing, is one of the reasons causing coalbed methane wells uneconomic in most areas of China. This paper presents an experimental study of water blocking damage on coal based on the measurements of surface tension, contact angle, gas-phase permeabilities and adsorption and desorption of nitrogen on synthetic coal samples. Distilled water, KCl, AN, and KCl+AN solutions were used as testing liquids in this study. Results show that the hydraulic fracturing fluid with surfactant AN causes the lowest damage to the reservoir permeability; the surfactant AN reduces the surface tension of the liquid and increases the wettability of liquid to coal reservoir; and the surfactant AN inhibits the occurrence of capillary and Jamin effects efficiently. It is concluded that the surfactant AN can reduce the water blocking damage rates of coal reservoirs; and the hydraulic fracturing fluid mixed with 1.5% KCl and 0.05% AN reduces the effect of water blocking damage on the coal permeability.