Abstract:

The exploration and exploitation of coalbed methane have gradually turned into deep coal seams. Taking Yichuan of Shaanxi and Shizhuang of Shanxi as the study area, collecting shallow and deep coal samples and integrating with some experiments, this paper analyzes the difference of pore structures between shallow coal seams and deep coal seams and the adsorption characteristics of deep medium-high rank coal reservoirs. Mineral and lithotype test, mercury intrusion method, NMR, low-temperature nitrogen adsorption test and isothermal adsorption test had been used for this study with those coal samples (100-1,800 m). The results with the increase of the depth are as follows: (1)The percentage of adsorption pore shows a trend of increase while the number of seepage pore decreases. The mercury injection curve has a trend toward type Ⅲ. The main peak area of low NMR T2 relaxation is gradually increasing and even can be up to 90% of all peak areas when the depth is more than 1,700 m; the zero signal sections among main peaks show that the ability of connectivity between micropores and mesopores is worse. The curves of low-temperature nitrogen adsorption have a trend from typeⅠto typeⅡ. The above results show that the abilities of pore connectivity, permeability and physical properties of reservoirs are becoming worse. (2)Coal reservoir surface area and pore volume have their high-value ranges near 1,000 m. Then, they decrease just as the predecessors’ point of view, which is due to the small pores. (3)The adsorption ability of deep coal seam is influenced by positive effect of pressure and negative effect of temperature. The maximum adsorption capacity increases when the pressure becomes high, but the adsorption capacity has a decrease of 0.25 cm³/g while the temperature rises 1 ℃. The effect of temperature on Langmuir pressure is different from predecessors’ views. There is an inflexion point in temperature (35 ℃) and Langmuir pressure will not rise until the temperature reach it.

Key words: deep coal seam, medium-high rank coal, pore structure, permeability, adsorption, CBM, coal reservoir