Joint analysis using geomechanics, computed X-ray tomography and petrography based on coal samples from a carboniferous basin in Poland

Two coal samples from Polish Upper Silesian Coal Basin with different degrees of coalification were analysed to investigate the influence of coal petrographic parameters on geomechanical tests. The samples were subjected to petrographic analyses such as maceral, microlithotype and vitrinite reflection. Triaxial geomechanical tests were performed, and full stress–strain characteristic was recorded. The P- and S-wave velocities were also measured during the triaxial compression tests, at each stage of the test. The dynamic elastic modulus was determined. Computed X-ray microtomography was conducted in three steps: under initial sample conditions, in a linear portion of the stress–strain curve and after the triaxial compression test. The sample was first scanned (reference image) to determine the geometry of the natural pores and microcracks. The second stage represents the samples after geomechanical testing stopped in the linear portion of the stress–strain curve. The third stage of CT measurements was performed after the triaxial compression test. Samples were analysed with respect to the propagation of cracks and changes in pore space development. CT images obtained at different stages were compared to characterize the specimen’s reaction to stresses. The qualitative and quantitative interpretation was based on comparison of the diameters of pores and cracks, dip angles, strikes, directions and volumes before the test, in the linear portion of the stress–strain curve and after the test. The results indicated a relationship between geomechanical properties and the sum of reactive and inert macerals. It appeared that the coal sample was prone to fracturing in defined plane directions.