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ABSTRACT: The time-dependent behavior of salt rocks poses challenges to well design. Salt mobility is associated with critical wellbore closure and excessive casing loads. In the presence of cement imperfections, non-uniform loads on casing strings can lead to failure. A parametric finite-element analysis considers an uncemented annulus, a fully cemented annulus, and a partially cemented annulus with a channel covering 20%, 50% and 80% of the pipe circumference. No pressure build-up was considered. Commonly applied to Brazilian salts, a double-mechanism creep law is adopted, and strain rate uncertainties are accounted for. The stress distribution is less severe when uniform loads act on the casing surface. A partially cemented annulus results in non-uniform loads and deformation of the casing cross-section. A small volume of cement channels leads to localized regions of critical stress within the casing. As the channel size increases, the higher moment arm results in higher casing stresses, especially when two-point bending occurs at the salt-casing contact. Analysis of casing yield and failure when cement channels are present along salt sections allows for more accurate evaluation of casing integrity. 1. INTRODUCTION Well design in salt formations faces many challenges due to the time-dependent behavior of these rocks. Wellbore closure due to salt creep may lead to drill string sticking or aborted casing runs, and the correct drilling mud weight must be applied. After casing is set, salt creep can increase casing loads and compromise well integrity. When submitted to a deviatoric stress, salt exhibits a time-dependent strain response according to Figure 1. During the first stage of loading (primary creep) salt undergoes a high rate of deformation, that gradually slows down until it reaches steady state (Secondary) creep. Finally, during tertiary creep the strain rate increases until rupture (Jaeger et al, 2007; Goodman, 1989). The strain rate is influenced by many factors, such as mineralogical composition, impurities, formation temperature and the imposed differential stress (Liu et al., 2013, Fjaer et al., 2008). |
