A major challenge in drilling in the Pre-salt area, in Brazil, arises from the special structural salt behavior, when compared to other geomaterials, since it develops high creep strain rates under high levels of deviatoric stresses and temperatures. The salt or evaporitic rocks, formed by the sea water evaporation, have different chemical compositions. In the Pre-salt area the more important types are: halite, carnallite and tachyhydrite. The tachyhydrite, for the same state variables, deviatoric stress and temperature, develops creep strain rates up to one hundred times higher than halite. Many operational problems, such as stuck pipe and casing collapse, have been reported when intercalation of these rocks within a thick layer is found. The challenge of designing excavations near tachyhydrite, began with the development of an underground mine to extract sylvinite ore in Northeast Brazil. The research that began in the 70s, to enable the mining of this ore overlying this rock, triggered one of the largest R&D projects in rock mechanics, including computing modeling, laboratory and field tests. For the design of the pre-salt wells this previous experience was used and additional triaxial creep tests were performed using a new rock mechanics laboratory. Field tests and computer modeling improvement were used to overcome the challenge of the Pre-salt drilling. This article describes the lessons learned on the geomechanical salt behavior and its application in subsalt wells design. In addition, it is presented the developed methodology validation, through comparison between computing modeling results with measurements carried out in experimental panels, in the potash mine, and with measurements obtained in an experimental well drilled for the purpose of calibrating and optimizing directional drilling in salt layers. These parameters and methodology have been used to support the design of the wells drilled in the Pre-Salt giant oil fields in Brazil with very successful results.
Rock mechanics applied to salt rock started in Brazil with the discovery of potash reserves, sylvinite ore (NaCl.KCl) in the State of Sergipe, during oil exploration by PETROBRAS in the 1960s. The sylvinite ore body is located at the depth varying from 450m to 640m from surface. This reserve can be, in a simplified way, divided in two layers, the upper sylvinite and the lower sylvinite separated by a halite layer, Fig.1. In some regions of the reserve, the lower sylvinite overlies the rock tachyhydrite (CaCl2·2MgCl2·12H2O), which is very weak in comparison to halite and can develop creep strain rate, two orders of magnitude faster than halite, for the same state variables, temperature and stress. Due to this geotechnical challenge it was decided to start the development of the mine by the upper sylvinite and the mining basic project of the lower sylvinite would be done after an extensive research of rock mechanics, including laboratory and field tests and computer modeling [17]. The shaft excavation of the mine started in the early 1980s. Due to the very successful rock mechanics research done, the upper and the lower sylvinite layers are currently being mined.
