Abstract: In order to explore the influence of fracture characteristics index on crack initiation pressure in vertical well and solve the problem of theoretical calculation about vertical well initiation pressure. First of all, the vertical well initiation pressure model is established based on the principles of fracture mechanics and elastic mechanics, considering the characteristics of primary fractures and fracturing fluids. Secondly, the influence of primary crack characteristics on initiation pressure is analyzed quantitatively; meanwhile, proposing the concept of "crack characteristic index control coefficient T" and deriving the mathematical expression of T. At last, sensitivity analysis of the primary crack characteristic parameters is made for the initiation pressure. It is concluded that the most sensitive factor affecting the initiation pressure is the crack width b. When b is in the range of 100-700 μm, the fracture characteristics of the formation have the practical significance for the initiation pressure, which solves the problem of the choice of fracturing target layer. The results show that: The influence factors of vertical well initiation pressure are related not only to the mechanical properties of the source rock, the stress field, the fracturing fluid properties and the well size, but also the width, length, density and roughness of the crack. Especially the small fluctuations in the width of the crack will cause vertical wells crack pressure changes. According to the actual geological prospecting data and fracturing construction data of the 5 test wells in Jiaozuo mining area, the vertical well initiation pressure is calculated and verified. It is found that the actual initiation pressure is in good agreement with the calculated results. It is shown that the theoretical model has some guiding significance for the actual control of the vertical well initiation pressure.

Key words: fractured coal, fracture characteristic index, initiation pressure, control coefficient, sensitivity analysis