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Geomechanics and Engineering
  Volume 7, Number 5, November 2014 , pages 553-567

Borehole stability analysis in oil and gas drilling in undrained condition
Jian-Guang Wei and Chuan-Liang Yan

    Borehole instability during drilling process occurs frequently when drilling through shale formation. When a borehole is drilled in shale formation, the low permeability leads to an undrained loading condition. The pore pressure in the compressed area near the borehole may be higher than the initial pore pressure. However, the excess pore pressure caused by stress concentration was not considered in traditional borehole stability models. In this study, the calculation model of excess pore pressure induced by drilling was obtained with the introduction of Henkel\'s excess pore pressure theory. Combined with Mohr-Coulumb strength criterion, the calculation model of collapse pressure of shale in undrained condition is obtained. Furthermore, the variation of excess pore pressure and effective stress on the borehole wall is analyzed, and the influence of Skempton\'s pore pressure parameter on collapse pressure is also analyzed. The excess pore pressure decreases with the increasing of drilling fluid density; the excess pore pressure and collapse pressure both increase with the increasing of Skempton\'s pore pressure parameter. The study results provide a reference for determining drilling fluid density when drilling in shale formation.
Key Words
    borehole stability; collapse pressure; Mohr-Coulumb strength criterion; undrained; drilling fluid density; excess pore pressure
(1) Jian-Guang Wei:
Institute of Petroleum Engineering of Northeast Petroleum University, Daqing City of Heilongjiang Province, 163318, China;
(2) Chuan-Liang Yan:
School of Petroleum Engineering, China University of Petroleum, Qingdao 266555, China;
(3) Chuan-Liang Yan:
State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, 102249, China.

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