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Abstract
Drilling and completing wells through deep thick salt formations is technically challenging and costly. Salt material flows over time whenever a stress difference, or shear stress, is induced. The rate of deformation primarily depends on the stress difference, and on the temperature. Both of these factors increase with increasing depth, often leading to severe loading and deformation of wells, and sometimes severe damage and loss of functionality. Geomechanical analysis can be applied to estimate such loading, to estimate damage risks, and to optimize well designs for these challenging conditions. We describe herein a process to evaluate salt creep and casing damage risk for high pressure and high temperature conditions typically encountered in deep salt formations. Because well costs often exceed 50 million dollars each, appropriate well design and risk analysis, supported by geomechanical modeling of salt and casing behavior, is critical to project economics.

To simulate the visco-elasto-plastic behavior of salt, we apply the time-dependent constitutive framework available in FLAC3D. The available formulation is modified by Terralog to account for damage accumulation during primary loading, associated strength degradation, compaction-dilation transition based on the Drucker-Prager yield criterion, and loading-unloading response.

We provide an illustrative example for a deepwater Gulf of Mexico field, in which a multi-string casing-in-casing design was considered to resist long-term creep. Laboratory creep data was used to calibrate the constitutive model, which was then applied to a near wellbore scale geomechanical model that included the casing strings, cement, mud pressure, and 10ft of surrounding salt. The simulation results indicate that the for the design configurations considered, the minimum time for salt to contact the outer casing was on the order of 2 years for the most severe scenarios (lowest annulus pressure), and more than 20 years for the strongest configurations. Geomechanical analysis of this type provides a relatively low cost approach to quantify casing damage risks and to optimize casing designs for completions in high stress and high temperature environments.

Introduction
Increasing exploration and production from deep regions around the world require drilling through and completing wells in thick salt formations, leading to very high well costs. Appropriate designs are required to withstand creep induced loads during drilling and after completion. The introduction of a wellbore in a salt formation changes the existing local stress field, inducing a stress difference between the borehole and surrounding salt, and resulting time-dependent (creep) loading on well casings. Production of hot fluids from subsalt formations adds additional thermal strains to the wellbore and thermally induced creep acceleration.