Calculation of free gas content of low rank coal reservoirs: a case study of the southeast margin of Junggar basin

There are great amount of low-rank coalbed methane (CBM) resources in China, accounting for about 43.5% of total CBM resources in the country. However, our limited knowledge of the characteristics of low-rank CBM affects its exploration and development. By analyzing the pore and fracture characteristics, adsorption characteristics, including gas and other aspects of low rank coal reservoirs of the southern margin of Junggar basin, we think that the southern margin of Junggar basin has good adsorption properties, developed macro fracture and micro fracture. Compared with the middle, high rank coal reservoir, the porosities in this area are mainly mesopores and macropores, these porosities provide a place for free gas occurrence. We also estimated free gas content of Xishanyao Formation in the southeast margin of Junggar basin by gas equation. We reached the conclusions that the total gas content of coal seam is 2.85~8.94m³/t, 6.12m³/t on average, which includes free gas content of about 2.89%~5.14%, 3.90% on average. The estimation of free gas content provides a reliable basis for more scientific and reasonable exploration and development of coalbed methane in the study area.