Abstract
Hydraulic fracturing has become the dominant development technology for unconventional reservoirs. The accurate prediction of rock breakdown pressure is key to fracturing design in deep formations where the wellbore shape is prone to deform into an ellipse. However, few of the presented theoretical models for breakdown pressure focus on the wellbore deformation, which may lead to inaccurate results. Therefore, a theoretical model for analyzing fracture initiation from a perforated elliptical wellbore is proposed by using the conformal mapping method. Additionally, the analytical solution for the breakdown pressure and the initiation angle at the fracture tip is obtained by using the derived stress intensity factor at the perforation tip based on the fracture mechanics and the maximum tangential stress (MTS) criterion. The model is verified by comparing the previous experimental and theoretical breakdown pressure data for perforated circular wellbores. The effects of the wellbore deformation, perforation orientation, perforation depth, and fracture toughness on fracture initiation are analyzed. It is shown through theoretical analysis that a breakdown pressure predicted without considering the wellbore deformation will be overestimated, and it is unwise to optimize the perforation orientation based on the fracture toughness of rocks. To reduce the breakdown pressure and avoid excessive fracture curvature near the wellbore, the recommended optimal perforation depth is about 1.5-2 times the size of the wellbore and it is better to control the perforation orientation that is adopted within 20 degrees. The proposed model is a new attempt to calculate the rock breakdown pressure of a perforated elliptical wellbore, which can provide some guidance for the perforation optimization in a deep formation.
NSTL
Elsevier