Design implications of analytical and laboratory studies of permanent abandonment plugs


Akgun H. , Daemen J.

CANADIAN GEOTECHNICAL JOURNAL, vol.36, no.1, pp.21-38, 1999 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 36 Issue: 1
  • Publication Date: 1999
  • Doi Number: 10.1139/t98-089
  • Title of Journal : CANADIAN GEOTECHNICAL JOURNAL
  • Page Numbers: pp.21-38
  • Keywords: borehole plugging (sealing), shaft seal design, plug-rock interface strength, axial strength, bond strength, cement grout, ROCK, PERFORMANCE, DEFORMATION

Abstract

We analyze analytically and numerically the stress distribution along the plug-rock interface and within an axially loaded plug emplaced in a borehole in rock. Experiments show that the interface strength increases with decreasing plug radius and with increasing plug length. Axial strength decreases as a power law of plug radius. An increase in the modulus ratio (ratio of plug modulus to rock modulus) increases the interface strength, until it levels off at a ratio of about 5.0. For short plugs, the tensile stresses may reach a magnitude significant to be of concern for long-term stability of the plug and of the host rock. Our studies suggest designing friction plugs with a length to radius ratio of at least 8.0. In practice, especially for short-term performance, e.g., emergency flood control or temporary diversions, a ratio of 8.0 may not be necessary or justified. For permanent abandonment plugs, it is essential to reduce the tensile stresses in the plug and in the host rock to a level that will minimize the risk of long-term deterioration. We recommend in situ experiments on larger diameter plugs to assess the validity of the proposed size effect extrapolation obtained in this study.