Bond strength of cement borehole plugs in salt

Abstract

This report presents results of a series of push-out tests performed on cement plugs emplaced in salt cores. Two types of cement have been tested in salt cores drilled from blocks obtained from a mine in a bedded salt formation. Experiments on rock (salt) bridges provide a reference basis. Test durations vary from minutes to two days. A rather broad range of results was obtained, depending primarily on cement type. Average interface shear strengths, assuming uniform shear stress distributions, range from about 2 to 12 MPa. Peak shear stresses at failure, calculated assuming an elastic exponential shear stress distribution along the interface, are up to more than four times higher. Considerable differences are observed within each test group (standard deviations commonly are 20% and more). Dissolutioning along the interface has been observed in a significant number of samples. It is postulated that this dissolutioning may have been enhanced by dispersion of clay inclusions within the salt. Visible dissolutioning appears to be related to reduced bond strengths. Eighteen samples have been submitted to short-term (up to 34 days) drying and rewetting (for 8 to 28 days) after they had been pushed to failure (slip). Depending upon the reconditioning history, retested strengths more » range from 18 to 105% of the initial strength, with an average value of about 60%. 79 refs., 42 figs., 19 tabs. « less