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Abstract The initiation pressure and propagation pressure are essential to hydraulic fracturing operation. The initiation and propagation of an arbitrarily inclined borehole is considered more complicated compared with a horizontal or vertical borehole. By means of maximum tensile stress criterion，initiation pressure and orientation are analyzed and dimensionless initiation pressure(pb/?v) varying with borehole azimuth and inclination is obtained. With regard to hydraulic fracturing treatment carried out in the roof of Wangtaipu coal mine，initiation pressure and corresponding location are calculated. Finite element method is employed to explain the excessive propagation pressure during the fracturing process. The results are that：(1) When ?h = ?H，initiation pressure keeps ever-reduced as borehole rotates gradually from vertical direction to horizontal direction and maintains constant as borehole rotates from ?h direction to ?H direction. (2) Change rule of initiation pressure is closely related to in-situ stress regime with increase of ?H/?h or ?H/?v. (3) During the process of borehole rotating from vertical to horizon，initiation pressure performs uniquely subjected to normal faulting stress regime，striking slip faulting regime and reversing faulting stress regime，respectively. (4) The horizontal borehole direction requiring minimum initiation pressure exists for corresponding in-situ stress regime as it rotates from ?h direction to ?H direction. (5) Initiation pressure increases dramatically with the growth of rock tensile strength as it is close to the magnitude of in-situ stress. (6) The fracture turning during propagation would generate high pressure. (7) The magnitude，direction and type of in-situ stress are the groundwork for borehole design of hydraulic fracturing operation.
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Received: 10 September 2012
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2013, Vol. 32 
