Monitoring of stress change in coal seam caused by directional hydraulic fracturing in working face with strong roof and its evolution

Abstract：In order to study the mechanism and evaluate the effect of hydraulic fracturing,the hollow inclusion cells were adopted for stress monitoring of coal mass nearby the fracturing borehole before and after hydraulic fracturing as well as coal mass ahead of working face after hydraulic fracturing.Through the monitoring data analysis,stress evolution laws and conclusions are as follows:① The principle stress increment,inclination and azimuth in coal mass nearby the fracturing borehole change suddenly after hydraulic fracturing.The change extent is potentially related to distance between monitoring borehole and fracturing borehole,water pressure,fracture propagation and mechanical property of coal mass;After fracturing,principle stress increments are nearly recovered to the level lower than that before fracturing,therefore,stress state can be changed by hydraulic fracturing.② Due to the effect of hydraulic fracturing and mining,vertical stress in coal mass increases and horizontal stress decreases,the change extent of principle stress increments caused by mining where inclination and azimuth are almost invariable is much more less than that caused by hydraulic fracturing.Roof weighting intensity can be reduced by hydraulic fracturing;consequently,the effect of abutment pressure on working face is weakened.③ The directional hydraulic fracturing in the roof of a fully mechanized working face in Wangtaipu Coal Mine is implemented.Afterwards,strength and integrity of roof along with effect of mining are weakened;roof caving is in time with the advance of working face,which demonstrates that the effect of hydraulic fracturing conducted in roof goes well.