Fracture propagation and evolution in lignite under high-temperature steam

To investigate the mechanism of fissure expansion in brown coal under high-temperature steam conditions and its regulatory effect on permeability, this study utilizes brown coal from the Huolin River in Inner Mongolia as the research subject. Using a self-developed high-temperature, high-pressure steam convection heating experimental apparatus, the study simulates the in-situ gasification environment of brown coal. Employing a multi-scale, multi-parameter collaborative experimental approach, the research comprehensively utilizes micro-CT three-dimensional reconstruction, scanning electron microscopy (SEM) for microscopic morphology observation, uniaxial compression mechanical testing, and permeability experiments. Quantitative parameters such as fracture volume fraction, average length, and spatial connectivity are used to characterize fracture development, while the thermal deformation coefficient α is employed to characterize the phase transformation characteristics of thermal deformation in the coal body. A fracture propagation evaluation system based on multi-source data is established. The results indicate that: (1) High-temperature steam significantly transforms micro-fractures (100–500 μm) into medium-to-large fractures, with the average length increasing from 305.38 μm to 746.92 μm. (2) The surface of lignite evolves from dense to porous, exhibiting a stepwise fracture development pattern (“initiation-expansion-“Y” type network-overall failure”), accompanied by enhanced permeability. (3) The spatial distribution of fractures demonstrates a “central radiation-edge expansion” characteristic, with 400 ℃ identified as the critical temperature for full fracture network connectivity—porosity increases from 27.1% to 50.2%, and the fracture volume fraction reaches 18.75%. (4) At 400 ℃, thermal deformation undergoes a phase transition: the thermal deformation coefficient α shifts from －1.2% to 0.8%, resulting in the coal body transitioning from contraction to expansion, which elevates permeability by 1.1 times. This research provides theoretical guidance for optimizing seepage channels and process control in in-situ lignite gasification mining.