Characteristics of Modern Geostress and Removability of No. 15 Coal Reservoir, Yangquan Mining Area, China

Abstract

Based on injection pressure drop test data and acoustic emission test data of 13 records for samples from the Wenjiazhuang and Sijiazhuang mines in the Yangquan mining area, the vertical distribution characteristics of modern in situ stress and its influence on permeability in the Yangquan mining area were analyzed, and the exploitation of the No. 15 coal reservoir was studied. The results show that the Yangquan mining area is a low-stress area within burial depths of 400–760 m, moderate-stress area within 760–950 m, and high-stress area within 950–1300 m. The Yangquan mining area hosts a strike–slip fault stress field within burial depths of 400–900 m and a normal fault stress field within burial depths of 900–1300 m, and the permeability showed a “decrease–increase–decrease” trend with an increase in burial depth. The pressure coefficient K of the modern in situ stress field in the Yangquan mining area decreased with increasing burial depth, and its relationship formula with burial depth was 68.25/H + 0.2846 ≤ K ≤ 397.63/H + 0.6061. The permeability of No. 15 coal seam had a strong logarithmic relationship with horizontal principal stress, a strong exponential relationship with horizontal principal stress difference, and a strong power function relationship with effective stress. With an increase in stress and in stress difference, permeability decreased rapidly. The results of the acoustic emission tests of the No. 15 coal were analyzed. The in situ stress type of the Sijiazhuang mine and Wenjiazhuang mine is the normal fault stress field type, which is consistent with the well test analysis results. The high ring counts in the horizontal direction of coal seam 15 lasted for a long time, which was conducive to the generation of several network-forming fractures in the process of fracturing. The No. 15 coal reservoir is located in strike–slip fault stress field and normal fault stress field areas, and the fractures generated by hydraulic fracturing expanded along the vertical direction and the maximum horizontal principal stress direction, which was favorable for the fracturing transformation of coal reservoirs and the production and development of coalbed methane.