The window between pore pressure and fracture gradient is usually the most critical factor in well design. The width of this window dictates well control and loss circulation prevention. Most notably, in deep offshore basins this window becomes narrower which makes it more challenging to control the ECD. Similar scenario happens in highly deviated and/or horizontal borehole in which fracture gradient significantly decrease as deviation angle increases.
To mitigate the small tolerance between pore pressure and fracture gradients an engineering practice referred to as "wellbore strengthening" is conducted to increase the fracture gradient. The method relies on propping and/or sealing the fractures with specially designed materials. Different competing theories exist for wellbore strengthening mechanisms. These can be categorized into two groups. The first group believes that strengthening happens as a result of increasing wellbore hoop stress when a wedge inserted into the fracture while the second group emphasis on fracture tip isolation with suitable materials and enhancing fracture propagation pressure. The existing numerical models and lab experiments done so far did not fully replicate the same operational phenomenon to understand the true mechanism.
The main objective of this study was to compare the existing wellbore strengthening models using both numerical simulations and analytical solution for different field cases. Results of finite-element analysis for fracture initiation, propagation, sealing and stress changes after each step are provided. Our numerical results indicate that wellbore hoop stress cannot be higher than its ideal case in the fractured zone by plugging and/or sealing fractures. However, strengthening the wellbore and increasing the fracture gradient can be done by enhancing fracture propagation pressure.