Research and development of 3D large-scale physical simulation experimental system for coal and gas co-extraction and its application

In order to overcome the shortcomings of experimental research methods for coal and gas co-extraction model,a set of 3D large-scale physical simulation experimental system for coal and gas co-extraction was developed. The system adopts modular design and highly integrates the machine,electricity,liquid and gas functions in one. It is main- ly composed of seven sub-units including large-scale box body (3. 0 m×2. 5 m×1. 8 m) and base,automatic hydraulic mining,flexible loading,automatic ventilation,gas extraction,gas injection,and integrated data acquisition and control. Following the geometric similarity ratio of 1 ∶ 100,the loading unit can simulate a maximum mining depth of 2105 m, the mining unit can simulate a mining height of 0-12 m and a face retreat distance of 200 m. The ventilation unit can simulate various ventilation modes such as U-type,U+L-type,Y-type with different flow quantities. The gas extraction unit can simulate gas drainage using different kinds of methods and a combination of them,such as a high-level gas drainage roadway,cross measure borehole and surface borehole. The gas injection unit adopts independent injection mode to realize gas emission at different positions and different gas emission rates. Comprehensive data acquisition and control unit is able to obtain the characterization parameters such as overburden fissures,strata pressure,gas migration, gas extraction and automatic control of the whole experimental system. Therefore,the experimental system can simulate coal seam mining process under different ventilation,gas emission and extraction conditions,making it possible to sim- ultaneously study the overburden fracture evolution,mine pressure distribution,pressure relieved gas migration,gas ex- traction and other scientific issues during coal mining operation. The system was used to simulate the mining process at the 302 working face of a mine in Shanxi province,China. The laws of overburden breakage and fracture evolution were obtained,such as,the first weighting of the main roof was 45 m,the periodical weighting interval is 20 m,and the over- burden failure shows a 3D elliptical parabolic pattern. The dynamic variation of stress was obtained,indicating the peak stress moves forward continuously with the stress concentration coefficient variation between 2. 11 and 2. 63,and the peak stress occurrence at about 6-11 m ahead of the working face. In terms of the storage and distribution of pressure relieved gas,it was found that the gas concentration increased rapidly at 76-120 m behind face in the goaf and became stable beyond 120 m. The gas concentration increased gradually from the lower section to the higher section of the frac- tured zone which was about 5-60 m above the extraction level,reaching about 65% -68% at the top of fractured zone. The experimental results show that the system can well carry out the model experimental study of the whole process of coal and gas co-extraction in working face.