Iris Publishers- Open access Journal of Civil & Structural Engineering | Hybrid Subsea CCS System-CO2 Storage in Offshore
Ultra Deep-Water Salt Caverns
Authored by Alvaro M Costa
Computational geomechanics is the field of computational mechanics where geo-engineering systems are analyzed using the
prinCurrently there is a demand for CCS of large quantities of CO2 associated with CH4
in the pre-salt offshore oil fields in Brazil.
The pre-salt reservoirs have as caprock 2000 meters of continuous rock
salt. Rock salt is a special geomaterial. It has negligible
permeability and porosity, is able to support very high stresses,
develops the self-healing effect, fractures will heal only with time
and a good geomechanical project can design very large openings in the
salt body. In Brazil the rock mechanics and computing
modeling related to underground excavations in salt rock started in the
years 1970´s with very complex challenges, starting with
the project of an underground mining of sylvinite (potash ore) overlying
tachyhydrite, a very weak salt rock, solving the challenges
of the solution mining of salt caverns, for brine production, in bedded
stratified halite with intercalations of shales, development of
special geomechanical projects of oil wells for drilling through very
thick stratified salt rock barrier and finally the application of
salt caverns opened by solution mining for natural gas storage and CCS
of CO2. Salt caverns onshore, opened by solution mining have
been used since the years 1950´s to store hydrocarbons and contaminants.
A good example of a very large underground storage is
the Strategic Petroleum Reserve in the United States, storing since 1974
around 800 million barrels of crude oil. Today there are
more than 4000 caverns opened by solution mining for the storage of oil
and gas. This article introduces a new concept called the
Hybrid Subsea CCS System, which performs all the offshore CO2
separation process with subsequent storage in underground salt
caverns offshore. Today much is said on the concept of “Subsea Factory”
in the global oil industry, which is every day closer to become
a reality. Many operators have been applying high R&D investments in
bringing down to the sea floor equipment from the deck of
the production platforms like: multiphase pumps, oil separators,
electrical transformers, gas compressors, among other pieces
of equipment, reducing the size of the floating platforms and in some
sites, becomes the only solution available due to the water
depth, which cause a great limitation in the elevation of the
hydrocarbon through conventional and even with non-conventional
riser systems. The main challenge of doing a complete Subsea “factory”
or develop the “Subsea CCS System” is the storage. Shell,
USP (University of São Paulo) and FAPESP (Sao Paulo Research Foundation)
are developing a R&D project of a hybrid system using
partially the subsea factory technology with the high structural
performance of salt rock caverns, opened by solution mining, in
ultra-deep water. This hybrid system is expected to do at the same time
the separation between the natural gas and CO2, Carbon
Capture and Storage of CO2 and allowing the monetization of
the separated natural gas. This paper describes the conceptual design
of this hybrid system showing the steps from the drilling phase of the
wells, the process of opening the salt caverns by solution
mining, the subsea equipment required, the basic logistic system to
servo control this equipment and the geomechanical project of
salt caverns to store Natural Gas and CO2 in salt domes in ultra-deep water.
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