Design implications of analytical and laboratory studies of permanent abandonment plugs

We analyze analytically and numerically the stress distribution along the p lug-rock interface and within an axially loaded plug emplaced in a borehole in rock. Experiments show that the interface strength increases with decre asing 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 le vels 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 diversi ons, a ratio of 8.0 may not be necessary or justified. For permanent abando nment 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 deter ioration. We recommend in situ experiments on larger diameter plugs to asse ss the validity of the proposed size effect extrapolation obtained in this study.