Study on Wellbore Stability and Instability Mechanism in Piedmont Structures

Qiang Tan1, *, Baohua Yu1, Jingen Deng1, Kai Zhao2, Jianguo Chen1
1 State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China;
2 Petroleum Engineering Institute, Xi’an Shiyou University, Xi’an, Shanxi Province, 710065, China

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© 2015 Tan et al.;

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Correspondence: * Address correspondence to this author at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China; Tel: 0086-010-89739161; Fax: 0086-010-89733911; E-mails:,


Piedmont tectonic belts are rich of oil and gas resources, however the intense tectonic stress and broken formation may cause great drilling problems in piedmont structures such as borehole collapse, lost circulation and gas cutting. Through analysis of in situ stress properties, bedding structure and mechanical characteristics, wellbore instability mechanism was expounded from rock mechanics, chemistry of drilling fluid and drilling technology. The high tectonic stress, formation strength decreasing and fluid pressure rising after mud filtrate seepage are main reasons for borehole collapse. The methods of calculating collapse and fracture pressure and determining drilling safety density window were put forward based on mechanical analysis. In order to reduce drilling problems in piedmont structures, some countermeasures should be taken from optimizing well track and casing program, using proper mud density, improving inhibitive and sealing ability of drilling fluid. Good sealing ability can reduce seepage and cut off pressure transmission, enhancing the effective support force. This is the key technology of maintaining wellbore stability in hard brittle shale in piedmont structures.

Keywords: Collapse pressure, instability mechanism, leakage pressure, piedmont structure, wellbore stability.