Analysis of the influence of natural fracture system on hydraulic fracture propagation morphology in coal reservoir

During hydraulic fracturing in coal reservoir,the natural fracture system has a significant influence on the initiation and propagation of produced hydraulic fractures. Based on the detailed subsurface observation of 21 ground coalbed methane wells in Sihe,Chengzhuang and Xinyuan coal mines in the Qinshui Basin,the natural fracture system and hydraulic fractures are analyzed,and the relationships between them are clarified. The results show that the natural fracture system in the coal reservoir can be divided into layer fractures,exogenetic fractures,gas-expanding fractures,cleats,bedding fractures and micro fractures based on their genesis and scale. And they are classified into four levels according to their scale and conductivity. The propped hydraulic fractures observed are mainly related to the largescale first-level layer fractures and exogenetic fractures. The associated secondary natural fractures have little effect on the hydraulic fracture propagation. The main types of hydraulic fractures include vertical fractures,horizontal fractures,T-type and inverted T-type fractures,and I-type fractures. Among them,the horizontal hydraulic fractures are controlled by the fractures between the coal seam and its roof and floor,layers between the coal lithotypes,and in the tectonic coal stratification. The propagation direction of vertical hydraulic fractures is selective,which is affected by the maximum principal stress and natural fractures. The angle between the maximum principal stress direction and the dominant orientation of the natural fracture in the study area is smaller than the critical angle,therefore the vertical hydraulic fractures mainly extend along the exogenetic fractures. The combined fracture forms of T-type and inverted T-type fractures,and I-type fractures are also restricted by coal seam structures and coal body structures. In addition,considering the influence of natural fractures on coal strength,it is proposed that the direction of hydraulic fracture initiation is consistent with that of minimum value of the sum of in-situ stress and coal binding force.