Energy principle of simulation experiments on coal and gas outburst

Coal and gas outburst is an extremely destructive gas dynamic disaster in mines. Since such dynamic disasters are usually difficult to observe directly, physical similarity simulation of coal and gas outburst has become an important method for collecting observational data on outburst and studying outburst mechanisms. However, the energy principles of outburst similarity simulation experiments still lack systematic research. A review of outburst simulation experiments over the past 70 years revealed that experimental coals generally have a higher porosity (10%–40%, median 21.2%), which significantly increases the expansion energy of the initial free gas stored in the coal pores (by several times to several tens of times). To further elucidate the energy release characteristics of outburst simulation experiments, a series of outburst simulation experiments were conducted using a true triaxial coal and gas outburst simulation system, based on similarity criteria. Combined with observational data and energy analysis, it was found that gas expansion energy is the predominant component of outburst energy, accounting for 87.50%–95.31% of the total energy, of which the contribution of initial free gas accounts for 1/3 to 2/3. This demonstrates that the essence of laboratory simulation experiments is to represent the outburst process as a dynamic process driven by high-pressure gas in coal. Due to the high initial free gas expansion energy resulting from the high porosity of experimental coal, simulation experiments can be induced without relying on stress conditions, and even by non-adsorptive gases (such as He) under low pressure (about 0.45 MPa). In contrast to actual field outburst, simulation experiments often exhibit simultaneous release of gas expansion energy and stress energy, lacking interaction between stress and gas, thus making it difficult to reproduce the excitation process of outburst. In the design of future experimental apparatuses, matching the porosity of experimental coal to that of the original coal seam (usually 1%–11%) is key to whether outburst similarity simulation can achieve breakthrough progress in similarity.