Variation law of compressive strength and Young’s modulus of coal rock with coal rank and their significance for CBM development

In order to analyze the variation law of compressive strength and Young′s modulus of coal under the control of coal rank, suggestions for coalbed methane development in different coal rank areas are put forward. Through rock uniaxial compression test and coal quality test, the mechanical parameters (compressive strength, Young's modulus) and Ro,max data of 20 coal rock samples from 18 mining areas in China were obtained, and some pre-test data were synthesized, based on which, the variation law of compressive strength and Young's modulus of coal rock from low coal rank to high coal rank (Ro,max=0.33%~3.44%) and its influence on fracturing and drainage were discussed systematically. The results show that coal rank significantly affects the development of coal rock cleats and pores. Lignite and sub-bituminous coal(Ro,max≤0.50%) have less developed cleats, but have the largest porosity and mainly macropores.. During the evolution to the middle coal rank, the porosity of the coal rock decreases. After the middle coal rank (0.65%≤Ro,max), the porosity of the coal rock slightly increases and stabilizes, but the cleat gradually develops to the maximum around Ro,max=1.50%. When coal rock further evolves to a high coal rank (1.90% ≤Ro,max), the coal macromolecular functional groups re-polymerize, resulting in the cleat retreat.. Under the control of coal rank, the compressive strength and Young's modulus of coal rock show an "M" -shaped variation law with the increase of coal rank. When Ro,max≤0.5% (mainly lignite, sub-bituminous coal),Ro,max≥3.0% (mainly anthracite) and 1.0%≤Ro,max≤2.0%(mainly coking coal, lean coal), the compressive strength and 〖JP2〗Young's modulus of coal rock are low and easy to crack, but the short and wide cracks are not easy to extend, and the conductivity is poor. It's necessary to appropriately improve the fracturing scale to increase the length of crack, and to control drainage speed to prevent the fast decay of production capacity. When 0.5%≤Ro,max≤1.0% (mainly gas coal, fat coal) and 2.0%≤Ro,max≤3.0% (mainly lean coal), the compressive strength and Young's modulus of coal rock are relatively high, which requires a high cracking pressure, but the crack is easy to extend. During the fracturing, the scale should be controlled to prevent it from penetrating through the water barrier on the roof and floor of the coal seam. Drainage speed should be appropriately improved as well.