Asymmetric characteristics of “three-field” in overburden of inclined coal seam groups and target extraction mechanism

Abstract

The three-field (stress, displacement and fracture field) evolution laws of the inclined coal seam group are complex, which are important for the transport and storage of pressure-relief gas. In order to investigate the three-field evolution law of inclined coal seam group, the study carried out a similar simulation experiment of multiple mining in inclined coal seam group with the 1930 coal mine in Xinjiang as an object. The collapse pattern of the overlying strata was analyzed, the stress evolution characteristics of overlying strata was obtained, the overlying strata displacement distribution and movement direction characteristics were analyzed, and the characteristics of mining-induced fracture distribution were elucidated. The influence of the three-field evolution law on gas migration was further investigated, and a directional borehole gas extraction field test was carried out for veri-fication. The study results shown that, the rectangular ladder platform of mining-induced fracture shown obvious asymmetric characteristics under multiple mining of inclined coal seam group. The overlying strata stress of the lower side was more variable, and the pressure-relief effect was more obvious with increasing mining frequency, while the overlying strata stress of the higher side was less variable. Combined with the gravity-dip effect, the overlying strata of the higher side was more prone to be damaged, and the collapse order was preferred with asymmetric characteristics. The displacement distribution of overlying strata was asymmetric, with significant displacement on the high side and large changes in the movement direction. The frequency of mining-induced fractures in the high side fractured area was significantly higher than that in the low side. The high side fracture area had more fracture distribution and fracture aperture was bigger. The mining-induced fractures shown the asymmetric characteristics of “high expansion-low compression”. The multiple mining made the asymmetric characteristics of three-field more significant. In addition, there was a “slow decreasing-fast decreasing” in the overlying strata penetration. Based on the relationship between three-field evolution characteristics and gas mi-gration, the mechanism of preferential gas extraction targeting was revealed. Combined with the experimental results, the process of determining the target area for gas extraction in the fractured zone based on the three-field evolution law was constructed. The gas extraction effect in the field was great, which ensured the safe and efficient recovery of the working face. The results of this study provide a theoretical reference for the accurate extraction of pressure-relief gas in the inclined coal seam group, aiming to improve the gas extraction from the inclined coal seam group, prevent gas overlimit in the upper corner, and achieve safe and efficient mining of the inclined coal seam group.

Keywords

• inclined coal seam groups
• overburden movement
• mining-induced fractures
• asymmetric characteristics
• gas extraction