Study On The Mechanism Of Karst Horizontal Well Initiation And Pressure Crack Network Formation

Shale gas development has become a focus all over the world.Horizontal well fracturing technology is very important for efficient shale gas development.It is important to consider initiation pressure and the mechanism of the formation of networking fracture in the shale horizontal well fracturing. There are various natural fractures in shale gas reservoir and the existence of natural fractures have a significant impact on fracture initiation and the formation of networking fracture. As to urgent problems in shale gas development, it is very important to study fracture initiation mechanism, the formation of networking fracture and various factors which have a significant impact on the fracturing design and final gas recovery.This paper mainly completed the work as follows:(1)Based on the theory of elastic mechanics and principle of coordinate transformation, considering the influence of the insitu component and the bottom hole pressure,the author has established the stress distribution model of shaft wall rock and perforation wall in horizontal well through the analysis of the stress distribution around shaft wall rock and perforation wall.(2)Considering the intersection of natural fracture and perforation hole, it is necessary to study fracture initiation mode in theory. According to the stress distribution of shaft and perforation in the wall and the influence of near wellbore natural fracture, the author has established initiation pressure calculation model under different initiation model.Based on the research, the author selected an example for analysis.The paper has analyzed the influence of perforation depth, wellbore azimuth angle, perforation azimuth, natural fracture strike, dip angle, natural fracture cohesion and internal friction angle on initiation pressure. Taking W2 well for instance and according to the model, the author has calculated initiation pressure of W2 well and demonstrated the reliability of the model.(3) According to the reason of network fracture, the paper has analyzed the requirement of network fracture formation from the geological and engineering factors.On the basis of classical intersection criterion, based on the theory of rock mechanics and fracture mechanics, the author has established the criterion of intersection between hydraulic fracture and natural fracture and selected a case to study influencing factors under different failure modes.(4)Based on the two-dimensional plane strain geometric model, the author has made the qualitative analysis on the relationship between induced stress and fracture steering.Based on the initial fracture induced stress model, calculating the differential stress in the horizontal direction, the paper has established steering radius mode. By controlling the single variable method, the paper has made quantitative research on the initial difference of horizontal stress, poisson's ratio and net pressure in fracture and then made an analysis of favorable factors about complex network fracture.