Prediction of fracture pressure of wellbore for anisotropic formations

Key words:  rock mechanics  anisotropy  transverse isotropy  wellbore stress  fracture pressure

Abstract:

The fracture pressure of wellbore is very important for drilling and hydraulic fracturing, while the influence of rock anisotropy has been ignored in the prediction of the fracture pressure. In this study, a stress distribution model around a vertical borehole considering the influence of rock anisotropy was deduced, and a computing method was proposed. The influence of the occurrence of bedding planes, the degree of anisotropy, the in-situ stress and pore pressure on the fracture pressure was analyzed. The results indicated that the bedding plane with a small dip angle almost has little influence on the fracture pressure, while the bedding plane with a large dip angle has a significant influence. When the azimuth of the bedding plane is along with the maximum horizontal in-situ stress, the fracture pressure will reach to its lowest value, and it even can be 50％ lower than that of an isotropic case, consequently, it is vulnerable to the stability of the borehole. The higher the degree of rock anisotropy, the greater impact on the fracture pressure. In general, the fracture pressure decreases with the increasing of the ratio of horizontal stresses and pore pressure. The influence of the in-situ stress on the fracture pressure is obviously higher than that of the rock anisotropy, while the effect of the anisotropy can be larger when the ratio of the horizontal stresses is high. The present model improved the computational accuracy of the fracture pressure, and it also can provide theoretical guides for drilling and hydraulic fracturing in anisotropic formations.