Quantitative Characteristics of Nanoscale Pores in Gas-Bearing Volcanic Rocks of the Yingcheng Formation in the Songnan Gas Field

A detailed study is conducted on the nanoscale pores in the volcanic rocks of the Yingcheng Formation in the Songnan gas field in the Songliao Basin of China. Mercury intrusion porosimetry (MIP) and the nitrogen adsorption method are used to analyze the distribution characteristics of nanoscale pores and their capacity to adsorb methane molecules. Volcanic reservoirs in the Songnan gas field consist of rhyolite, tuff, and volcanic breccia. The results of the MIP method indicate that the microscopic pores in the rhyolite, tuff, and volcanic breccia are mainly >1000 nm, 100-1000 nm, and <100 nm, respectively. The nanoscale pore diameters in the tuff exhibit positive correlations with specific surface area and specific pore volume. The results of the nitrogen adsorption method reveal that the average pore diameter in the volcaniclastic rocks (i.e., the tuff and breccia) is larger than that in the rhyolite. The quantity and geometric shapes of pores are the main factors influencing the ability of the rhyolite and volcanic breccia to contain gas, whereas the pore diameter mainly influences this ability in the tuff. The lower limit of pore diameter that can effectively adsorb methane molecules in the volcanic reservoir of the Songnan gas field is 22 nm. In pores ranging in diameter from 22 nm to 1000 nm, the specific pore volume reaches a maximum of 11 cm(3)/kg, thus indicating that nanoscale pores can act as good gas reservoirs. This study of nanoscale pores provides a new direction for the exploration of volcanic rock reservoirs. The lower limit of pore diameter for adsorption can be used as an important parameter in gas reservoir research.