Analysis of fracture structure evolution of bituminous coal subjected to in situ steam pyrolysis combined with in situ micro-computed tomography technology

Underground in situ pyrolysis mining of coal will be a new trend for utilizing coal resources in the future. Steam heating is a feasible and key technology used in this process. Investigating the evolution mechanism of microstructural parameters during coal in situ steam pyrolysis is essential. The influence of external stress on microstructural parameters of coal has not been considered in previous studies. Herein, the permeability and fracture structure parameters of bituminous coal subjected to in situ steam pyrolysis were studied using a high-temperature and high-pressure triaxial testing machine combined with in situ micro-computed tomography (micro-CT) technology. Moreover, the advantages of steam pyrolysis were revealed by comparing it with the natural pyrolysis process. Results showed that (1) the evolution law of fracture ratio and the proportion of large fractures (equivalent fracture diameter R >100 μm) with temperature is consistent. The proportion of large fractures has a considerable impact on the fracture ratio. (2) From 25°C to 600°C, the permeability and fracture structure parameters of bituminous coal exhibit an “increase – decrease–increase” trend with increasing temperature. The threshold temperature points for changing fracture structure parameters are 300°C and 400°C. The promoting effect of pyrolysis on fracture structure parameters competes with the inhibition effect of external stress on fracture development, determining the evolution law of fracture structure parameters with temperature. (3) The threshold temperature zone where the microstructural parameters change considerably is 400°C–600°C. The fracture ratio, proportion of large fractures, and permeability of coal increase considerably. (4) Unlike natural pyrolysis, steam pyrolysis promotes the advancement of threshold temperature point. After 450°C, steam pyrolysis promotes the development of large fractures (R >100 μm) and the fracture ratio is about twice that observed in case of natural pyrolysis. These findings provide theoretical support for the engineering practice of coal in situ underground pyrolysis mining and are of great importance to the development of micromechanics.