Hydrocarbons are very often associated with salt structures. The oil and gas industry is often required to drill along and through long salt sections to reach and recover hydrocarbons. The unique physical properties of salt require special techniques to ensure borehole stability and adequate casing design. This paper assumed that the mechanical behavior of salt is regulated by the magnitude of mean stress and octahedral shear stress and under the influence of different stress conditions,the deformation of rock salt can be represented by three domains,i.e. compression domain,volume unchanged domain,and dilatancy domain,which are separated by a stress dependent boundary. In the compression domain,the volume of salt decreases until all microcracks are closed,with only elastic deformation and pure creep; in the volume unchanged domain the deformation is considered steady incompressible flow controlled by pure creep; and in the dilatancy domain the volume of salt increases during deformation due to micro-cracking,causing damage and accelerating "creep" until failure. This paper presents a hypothesis that the borehole is stable only when the magnitude of octahedral shear stress is below the dilatancy boundary. It gives the design method for determining drilling fluids density,and calculates the closure rate of borehole with the recommended drilling fluids density. If the closure rate of the borehole is less than 0.1%,the drilling fluids density window can be used during drilling through extremely thick salt formations.
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