关键词: 煤岩气, 孔隙结构, 全孔径拼接, 孔缝特征, 吸附气

Abstract: Compared with traditional coalbed methane reservoirs, the coal rock reservoir in the Dinan Bulge of Junggar Basin is generally deeper (>2500 m). In order to understand the pore structure characteristics of the coal rocks in this area and its influence on the occurrence and migration of coal gas, the coal rock of Jurassic Xishanyao Formation in the well DM-2-101 is taken as the research object. The coal quality, coal rank, physical properties (stress sensitivity, density, porosity and permeability) and gas bearing properties of the coal rocks were detected and analyzed. The characteristics of pore development of coal rocks were analyzed by casting sheet, full-diameter CT scanning, scanning electron microscopy (SEM), diffraction and energy spectrum analysis, and the pore distribution was quantitatively characterized by micrometer CT scanning, high pressure mercury injection, carbon dioxide adsorption and nitrogen adsorption. The results show that coal rock in well DM-2-101 has the characteristics of low moisture, extremely low ash and low volatile content. The maceral components of the coal rock are mainly inertinite and vitrinite, with an average content of 48.07% and 45.82%, respectively. There are numerous hydrostatic fractures, interlayer fractures, cell pores and a few pores in reservoir space, with the domination of cell pores. The specific surface area is mainly the contribution of micropores (<10 nm), which accounts for more than 95%. The pore volume is mainly the contribution of large pores (>1000 nm), and its volume proportion reaches 62.97%-65.92%. The second was micropores, which accounted for 24.98%- 20.39% of the volume. The overlying pressure has an exponential decreasing relationship with porosity and permeability, and the porosity and permeability under overlying pressure can reach 8.23% and 1.47 mD respectively, which indicates that the coal rock can form an effective reservoir, but there is a certain heterogeneity. The pore size distribution of coal rock affects its adsorption properties and gas content. Adsorbed gas is widespread in coal rock, which is mainly adsorbed around micropores. The free gas mainly accumulates in some large pores. This study clarified the pore distribution characteristics at multi-scale levels of deep coal rock reservoirs through multi-methods with full-aperture quantitative characterization, which provided data and theoretical support for the occurrence and production mechanism of coal rock gas, and had certain guiding significance for the evaluation of coal rock gas resource potential of the Jurassic Xishanyao Formation in Junggar Basin. 

Key words: coal rock gas, pore structure, full aperture splicing, pore and fracture characteristics, adsorbed gas