Reliable estimation of in-situ stresses around salt bodies is critical for planning safe and economic drilling programs. A better understanding of in-situ stresses before drilling can be achieved using geomechanical models that account for the complex interaction between creeping salt and the surrounding sediments. The present day stresses around salt bodies are investigated with finite element models using appropriate constitutive models for salt and its surrounding sediments. Salt is modeled using a visco-elastic creep law which captures the relaxation of deviatoric stresses. The response of the surrounding materials is assumed to be rate-independent and is modeled using elasticity or with a Mohr-Coulomb elasto-plastic constitutive law. Several finite element models were developed to identify major factors that control in-situ stress. The models reveal that one of the major controls is the connectivity of the salt body to its source. It is shown that when the salt body is connected to its source, the density contrast between the salt and surrounding sediments is the most important controlling factor. The stresses in completely isolated salt bodies are mainly controlled by the far-field stresses, the salt pressure, and the relative depth and geometry of the salt body.
Significant amount of exploration, development, and production is taking place near salt bodies around the world, including the deepwater Gulf of Mexico (GoM), Middle East, North Sea, offshore Brazil, North Caspian Sea, and North and West Africa. Kashagan field, located in the northern Caspian Sea (Kazakhstan), was the largest field discovery (2000) since Prudhoe Bay in Alaska more than 30 years ago. The hydrocarbon deposits lie about 4,500 m below the seabed, under a thick body of salt which tends to close over drill holes. Another significant hydrocarbon discovery is Brazil's pre-salt Tupi field in 2006 [1], which is the largest hydrocarbon discovery since Kashagan. In the GoM, several important deepwater targets are beneath 2,000 meters of salt and at well depths in excess of 7,000 meters [2, 3]. When subjected to differential loading, salt flows and produces highly irregular and complex geometries. At geological time scales, deformation of salt is dominated by isochoric creep under deviatoric stresses and with time its stresses relax to an isotropic state; it is a pseudofluid [4-9]. The presence of salt bodies modifies the insitu stress field because salt cannot sustain shear stresses. Salt creeps significantly more than other sedimentary rocks. Most sedimentary rocks are predominantly rate-independent frictional materials that strengthen with depth due to the increased confining pressure whereas salt weakens with depth as creep rate increases with rising temperatures. Also, the unit weight of halite remains nearly constant to depths of interest (about 8 km), while most sediments experience porosity reduction and associated increase in unit weight with increased depth [11]. These contrasts in the mechanical behavior together with salt's inability to sustain differential loading, are responsible for stress anomalies near and within salt bodies (i.e., changes in stress magnitude and principal stress rotations). Drilling wells in deepwater subsalt environments is expensive and technically challenging.
