Abstract：Hydraulic fracturing is an important technique to increase the production of oil and gas reservoirs. The prediction of breakdown pressure accurately is the key to the success of hydraulic fracturing. The current prediction model of breakdown pressure assumes that the wellbore is circular，but the wellbore is actually elliptical according to the log data. The complex function theory and the theory of elasticity are therefore used to derive the analytical stress field around the elliptical wellbore and a prediction model for the breakdown pressure of an elliptical wellbore is established. The correctness of the model is verified through formula deduction and numerical calculation. The influence of the ratio of major axis to minor axis and the difference of in-situ stress on the breakdown pressure are analyzed. A well in western China was studied as an example. The error of breakdown pressure calculated with this model is within 4%. The stress concentration due to the influence of wellbore shape occurs in the direction of minimum horizontal stress and fractures initiate along this direction. When the ratio of major axis to minor axis is large，fractures initiate along the direction of minimum horizontal stress and fracture bending occur near the wellbore. When the ratio is small，the collapse of small area of well wall leads to drastic increasing in breakdown pressure. The equilibrium curve has an expression of . When the point of the parameter falls in zone I，the breakdown pressure is greatly influenced by the difference of in-situ stress，and the wellbore fractures initiate along the direction of maximum horizontal stress. When it falls in zone II，the breakdown pressure is greatly influenced by the shape of the wellbore，and the wellbore fractures initiate along the direction of minimum horizontal stress. When it falls on the balance line and its adjacent area，the breakdown pressure is close in different directions of the wellbore，and the multiple fractures initiate in the wellbore at the same time.
杨兆中，刘云锐，李小刚，易良平，贾 敏. 椭圆形井筒破裂压力预测模型[J]. 岩石力学与工程学报, 2018, 37(8): 1896-1904.
YANG Zhaozhong，LIU Yunrui，LI Xiaogang，YI Liangping，JIA Min. A model predicting breakdown pressure of elliptical wellbore. , 2018, 37(8): 1896-1904.
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2018, Vol. 37 
