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This paper presents a new challenge under study in Brazil: the offshore storage of natural gas and CO2in salt (halite) caverns opened by solution mining in shallow and ultradeep water. The salt rock mechanics in Brazil started with the underground mining of sylvinite (NaCl.KCL) overlying tachyhydrite (CaCl2.MgCl2.12H2O) and research initiated in the 1970s (Costa 1984), to enable the mining of this ore. Applying a numerical simulation of drilling and excavation, the exploration and operation of mines were possible for more than 35 years. The same technology has also been used in the stability analysis of several salt caverns opened by solution mining of halite (NaCl). With the discovery of the giant pre-salt reservoirs in Brazil, underlying about 2000 m of stratified salt rocks, with the presence of tachyhydrite, all the knowledge acquired in the conventional and solution mining designs was ready to be used. More than 200 pre-salt wells have been designed and drilled successfully using the same methodology and computer codes. These approaches are proposed here as a solution to the presented challenge.

This paper presents a proposal for an experimental salt cavern in offshore ultra-deep water for CO2abatement, including the instrumentation plan and well conceptual design evaluated for carbon capture and storage (CCS) application. These studies are based on applied computational mechanics associated with field experimentation that has contributed to the technical feasibility of the underground potash mine at the State of Sergipe in Brazil. This knowhow allowed the stability analysis of several salt caverns for brine production at the State of Alagoas in Brazil and to the drilling through stratified thick layers of salt of the pre-salt reservoirs in Santos Basin. Now, this knowledge has been applied in the design of onshore and offshore salt caverns opened by dissolution for storage of natural gas and CO2. The geomechanical study, through the application of computational mechanics, of offshore giant salt caverns of 450 m high by 150 m in diameter, shows that one cavern can store about 4 billion Sm3or 7.2 million tons of CO2. Before the construction of the giant cavern, which will be the first gas storage offshore in the world, it has been decided to develop an experimental one, with smaller size, to obtained field parameters. The experimental cavern will allow the calibration of parameters to be used in the structural integrity analysis of the cavern and well for storage of natural gas which is rich in CO2under high pressure.