|Fig. 1: Petrobras' oil platform in the Tupi basin. The extraction of pre-salt oil effectively started in May 2009. Source: Wikimedia Commons|
The year 2011 was marked by a great growth in energy consumption, the strongest since 1973 (+5.6%). Global oil consumption grew by 2.7 millions barrels per day (b/d), reaching a record level of 87.4 million b/d. Global oil production, on the other hand, increased by only 1.8 million b/d and was not able to match the growth in consumption.  These numbers give raise concerns about the future of oil as a major energy source.
The pre-salt denomination is used to designate geologic layers that were formed before salt layer accumulated above it. Once the oil found above the salt layers started to run out, prospection for oil in pre-salt layers gained strength. In the end of 2007, a great reserve of oil and natural gas was found in the pre-salt layer extending for 800km offshore between the brazilian states of Espirito Santo and Santa Catarina. It is believed that the greatest reserves of pre-salt oil are found from the Northeast to South of Brazil, in the Gulf of Mexico and in Africa's West coast. 
The brazilian pre-salt cluster structure was created around 160 million years ago, from the separation of the continental superstructure Gondwana into the American and African continents. The rifting created the conditions necessary for the deposition of sediments and, as sea water filled the space between them, a low energy and high salinity environment, propitious to the development of bacteria colonies, was formed. The secretion of these bacteria, allied with the precipitation of carbonate salts, created nuclei for the formation of microbialites (carbonate rocks), on which oil accumulated. 
Most oil has pre-salt origin, but in some cases the salt slips and opens way to the oil, which them accumulates in the post-salt rocks. Even though they have the same origin, the two oils have differences. In the post-salt case, bacteria may consume the lighter part of the oil, from which gasoline and diesel are extracted. For the pre-salt oil, a high reservoir of rocks such as coquinas and vulcaniclastics, allied with a higher temperature - above 80 degrees Celsius (given the greater depths), creates a condition that sterilizes the oil and preserves its qualities. 
The exploration of pre-salt layers is a very challenging enterprise. Some of the main difficulties include: ultra-deep water (greater than 2,000 meters), deep carbonate reservoirs (greater than 5,000 meters), spread over very large areas, with high gas-oil ratio (e.g. GOR grater than 200 in Tupi area) , CO2 content (8-12% in Tupi), high pressure and low temperature, laying below a thick salt layer (more than 2,000 meters of salt), location around 300km from the coast and often severe oceanic conditions. 
These challenges require substantial development in offshore extraction capabilities, including well technologies and gas processing and exporting technologies (minimizing CO2 emissions to the atmosphere).  Other challenges to the exploration of pre-salt layers include the hard interpretation of diagnostic seismic signals due to the thickness of salt layers and the lack of knowledge of how materials perform and last under high pressures. In the Laboratory for Non-Destructive Testing, Corrosion and Soldering, for instance, four COPPE (UFRJ - Brazil) professors are working alongside 30 Petrobras engineers to submit steel to corrosive hydrogen sulfide gas at extreme pressures. 
Other challenge is studying the geometry of rocks to optimize well positioning. Moreover, when it is perforated, the salt can exert tension and block the well, therefore it is necessary to use a steel coating. Besides, when it leaves the rocks, the petroleum is extremely hot and can create precipitation in the flexible extraction lines, which are in contact with the cold sea water. Therefore, research is being done to find chemicals to inhibit precipitations and keep the lines in a low temperature. 
The exact amount of oil in the Brazilian pre-salt layer is not yet known, but estimates suggest that Brazilian known reserves should at least double with this discovery. In the year 2007, the proven reserves of oil in Brazil were around 13.920 billion of barrels and estimates suggest that up to around 100 billion barrels are likely to exist. According to British Petroleum 2011 statistics, Brazil's known oil reserves figured around 12.9 billion barrels in the year 2009 (representing 1% of the total share), and since then more minor discoveries have been consistently made.  This would raise Brazilian oil stock to be among the greatest in the world, behind only Saudi Arabia, Iran, Kuwait and United Arab Emirates. Because of its higher quality, even though it is around twice as expensive to extract oil from pre-salt layers, it still has a higher market value than ordinary oil.  Technological and logistical adaptations developed by the Brazilian company Petrobras have the aim of reaching the production of 1 million barrels per day by the year 2017.
With the discovery of oil and natural gas in its offshore pre-salt layer, Brazil has the potential to become one of the biggest producers of oil and its derivatives. However, due to the many challenges the extraction process involves, a lot of investment has been and will have to be made in offshore technology in order for this potential to become reality.
© Idel Waisberg. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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