Evaluation of New Drilling Approach for Widening Operational Window: Implications for Wellbore Strengthening


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.

Meeting Name

SPE Production and Operations Symposium (2011: Mar. 21-23, Oklahoma City, OK)


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Analytical Solutions; Critical Factors; Deviation Angles; Engineering Practices; Fracture Gradient; Fracture Initiation; Fracture Propagation Pressure; Fractured Zones; Hoop Stress; Loss Circulation; Numerical Models; Numerical Results; Offshore Basin; Second Group; Strengthening Mechanisms; Strengthening Model; Stress Changes; Well Control; Well Design; Wellbore; Boreholes; Computer Simulation; Finite Element Method; Loss Prevention; Oil Field Equipment; Petroleum Engineering; Pore Pressure; Residual Stresses; Strengthening (Metal); Well Drilling; Offshore Oil Wells

International Standard Book Number (ISBN)


Document Type

Article - Conference proceedings

Document Version


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© 2011 Society of Petroleum Engineers (SPE), All rights reserved.

Publication Date

01 Mar 2011