Breakout Analysis for Anisotropic Rocks (BAAR): MATLAB-Based Code to Study Failure Zone Development Around Boreholes in Anisotropic Shales
- A. Pilacik (Polish Geological Institute - National Research Institute) | M. Adamuszek (Polish Geological Institute - National Research Institute) | M. Dabrowski (Polish Geological Institute - National Research Institute)
- Document ID
- American Rock Mechanics Association
- 51st U.S. Rock Mechanics/Geomechanics Symposium, 25-28 June, San Francisco, California, USA
- Publication Date
- Document Type
- Conference Paper
- 2017. American Rock Mechanics Association
- 1 in the last 30 days
- 92 since 2007
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ABSTRACT: Stress distribution around the borehole and the resulting initiation and propagation of technologically induced compressive and tensile failure zones are strongly affected by the elastic anisotropy of the rock medium. We develop the Breakout Analysis for Anisotropic Rocks (BAAR) code in MATLAB to study failure zones around the borehole in anisotropic rocks. BAAR allows for a two-dimensional analysis of the mechanical and thermal effects around a pressurized borehole in an anisotropic thermo-poroelastic rock medium subjected to variable far-field loads. The implemented failure criteria are used to assess the stability of the borehole wall and to establish the optimal pressure of the drilling fluid. BAAR can be additionally used for the estimation of principal stress magnitudes based on the measurements of breakouts and tensile fractures, the estimation of strengthening effect of intermediate stress on the rock medium around the borehole, the study of the heat and pore pressure distribution in the rock medium and its effect on the stress field. We present an example of the stability analysis for the transversely isotropic Sasino and Paslęk shale formations.
Borehole stability analysis is crucial to the success of drilling, stimulation, and production processes. Borehole wall is a surface of discontinuity, on which stresses concentrate. High stress concentration may lead to compressive failure of the borehole wall i.e. initiation of the breakout zone, which can cause technical issues, and in extreme cases loss of the borehole. Borehole stability can be ensured mechanically by controlling borehole-fluid pressure and to a lesser extent by modifying its temperature and chemistry.
We develop Breakout Analysis for Anisotropic Rocks (BAAR) code in MATLAB to study stress distribution around the borehole and its effects on borehole stability. The analysis is performed for the plane strain state in the plane normal to the borehole axis. We derive an analytical solution for the stress concentration around a circular hole in an orthotropic elastic medium, which is based on the complex variable method by Mushkhelishvili (Lekhnitskii 1968, Savin, 1968). The analytical solution provides both the stress and displacement fields. The thermal stress component is calculated based on a series solution to the transient heat conduction problem in polar coordinates (Nowacki, 1970, Carslaw and Jaeger, 2011). Borehole stability analysis in BAAR can be conducted using several failure criteria such as Mohr-Coulomb, Drucker-Prager, Lade-Duncan, or Wiebols-Cook (Colmenares and Zoback, 2002; Zhang et al., 2010), facilitating a comprehensive approach to the problem using all available data. We present several examples of BAAR applications, including the borehole stability analysis for transversely isotropic shale formations of the Lower Palaeozoic Baltic Basin.
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