Discussion on classification of coalbed reservoir permeability in China

This paper analyzes the permeability of coalbed reservoirs with different ranks and in different regions at home and abroad, which shows that the permeability of same coal rank in foreign countries is 1-2 orders of magnitude higher than that in China, and the difference between the permeability of different coal ranks is insignificant.The study demonstrates that the permeability of coalbed reservoirs is mainly controlled by the amplitude of in-situ stress, and by coal body structure, cleat density and cleat filling due to tectonic evolution and related fluids activities.High in-situ-stress, complex coal body structure and highly filling status are the main factors leading to the permeability of coalbed reservoirs to be low in China.Based on the distribution characteristics of permeability of coalbed reservoirs and related gas production rates, the effective coalbed reservoirs are defined ask> 0.01×10^-15 m² in China and divided into five classes including low permeability (k=0.01×10^-15-0.1×10^-15 m²), medium permeability (k=0.1×10^-15-0.5×10^-15 m²), medium-high permeability (k=0.5×10^-15-1×10^-15 m²), high permeability (k=1×10^-15-5×10^-15 m²) and super high permeability (k> 5×10^-15 m²).The actual production level of CBM wells are affected, besides permeability, by other factors such as the thickness of coals, gas content, fracturing parameters, geological structure, the aquosity of coal measures and drainage schedule.With the progress of CBM development technologies in China, low permeability coal reservoirs with k=0.01×10^-15-0.1×10^-15 m² can achieve an ideal gas production rate.