Study on fracturing monitoring and expansion law of underground gasification coal based on different gasification channel types

During the underground coal gasification process, the thermal stress produced at high temperatures will lead to coal cracking, which may cause gas leakage. Therefore, a real-time monitoring and regularity research of coal cracking is of significance for the long-term and stable operation of underground coal gasification systems. Acoustic emission (AE) technology is used to monitor coal fracturing activity in experimental models of different gasification channel types (coaxial gasification channel, coaxial gasification channel with bottom cross-hole, and V-type connection hole gasification channel). Based on the monitoring results, the influences of different operating parameters (gasification agent components and flow rates) and gasification channel types on coal fracturing activity and gasification area expansion are studied. Then, the crack distribution model constructed by moment tensor analysis is used to quantitatively analyze the crack position, type, and direction at the source of the AE during gasification, so as to study and predict the damage and destruction of coal during gasification. The study results show that: there is a strong correlation between the actual temperature change in the gasifier and the AE events, and an increase in temperature in the range of 0−750 ℃ will lead to an increase in the AE events. The located AE sources in the experiment can accurately represent the position of coal fractures, and the expansion range of the gasification area can be predicted by monitoring the overall distribution of the AE sources. The gasification area of the coaxial hole model mainly expands to both sides, while the gasification area of the V-shaped connecting hole model mainly expands along the direction of airflow. Higher oxygen concentration and gas flow at temperatures below the critical oxidation zone temperature (900 ℃) significantly promote the expansion of the gasification zone. The crack distribution model based on the moment tensor analysis can effectively help to study the fracture law of coal during underground coal gasification. The results of crack distribution model show that the tensile failure dominates the destruction of the coal mass during gasification. The overall direction of crack aggregation is close to the stratification direction of the coal seam. In addition, the results of crack location distribution show that the crack initiation and propagation activities are active in the reduction zone.