Experimental and numerical study of coal mechanical properties during coalification jumps

The mechanical characteristics of coal reservoirs are important parameters in the hydraulic fracturing of coal. In this study, coal samples of different ranks were collected from 12 coal mines located in Xinjiang and Shanxi, China. The coal ranks were identified with by the increased Maximum vitrine reflectance (R-o,R-max) value. The triaxial compression experiments were performed to determine the confining pressure effect on the mechanical properties of coal samples of different ranks. The numerical approaches, including the power function, arctangent, and exponential function models, were used to find the correlation between coal elastic modulus and the confining pressure. The fitting equations of compressive strength and elastic modulus of coal ranks were constructed under different confining pressures. The results showed that the coal compressive strength of different ranks has a positive linear correlation with the confining pressure. The coal elastic modulus and confining pressure showed an exponential function. Poisson's ratio of coal and confining pressure show negative logarithmic function. The stress sensitivity of the coal elastic modulus decreases with the increase of confining pressure. The coalification jump identifies that the compressive strength, elastic modulus, and stress sensitivity coefficient of coal have a polynomial relationship with the increase of coal ranks. The inflection points in coalification at R-o,R-max = 0.70%, 1.30%, and 2.40%, are the first, second, and third coalification jumps. These findings provide significant support to coal fracturing during CBM production.