Analysis of Salt Creep and Well Casing Damage in High Pressure and High Temperature Environments

摘要

Drilling and completing wells through deep thick salt formations is technically challenging and costly. Saltrnmaterial flows over time whenever a stress difference, or shear stress, is induced. The rate of deformationrnprimarily depends on the stress difference, and on the temperature. Both of these factors increase withrnincreasing depth, often leading to severe loading and deformation of wells, and sometimes severe damage andrnloss of functionality. Geomechanical analysis can be applied to estimate such loading, to estimate damage risks,rnand to optimize well designs for these challenging conditions. We describe herein a process to evaluate saltrncreep and casing damage risk for high pressure and high temperature conditions typically encountered in deeprnsalt formations. Because well costs often exceed 50 million dollars each, appropriate well design and riskrnanalysis, supported by geomechanical modeling of salt and casing behavior, is critical to project economics.rnTo simulate the visco-elasto-plastic behavior of salt, we apply the time-dependent constitutive frameworkrnavailable in FLAC3D. The available formulation is modified by Terralog to account for damage accumulationrnduring primary loading, associated strength degradation, compaction-dilation transition based on the Drucker-rnPrager yield criterion, and loading-unloading response.rnWe provide an illustrative example for a deepwater Gulf of Mexico field, in which a multi-string casing-incasingrndesign was considered to resist long-term creep. Laboratory creep data was used to calibrate thernconstitutive model, which was then applied to a near wellbore scale geomechanical model that included therncasing strings, cement, mud pressure, and 10ft of surrounding salt. The simulation results indicate that the forrnthe design configurations considered, the minimum time for salt to contact the outer casing was on the order of 2rnyears for the most severe scenarios (lowest annulus pressure), and more than 20 years for the strongestrnconfigurations. Geomechanical analysis of this type provides a relatively low cost approach to quantify casingrndamage risks and to optimize casing designs for completions in high stress and high temperature environments.