"We don't have technological problems, and we're in control of all aspects of the project," Petrobras' Jose Sergio Gabrielli told the local Estado news agency. "We just don't know how nature is going to work or, that is, the reservoirs."

Among other things, the tests will help determine the optimal number of wells needed to pump oil from the field, Gabrielli said.

"Today, each well costs about $60 million, so it's one thing to drill 3000 wells and another to drill 300 wells," Gabrielli said.

Petrobras will start the 15-month long test at Tupi on Friday, with the test ship pumping about 30,000 barrels of oil per day from the field, said a Dow Jones Newswire report.

Brazilian President Luiz Inacio Lula da Silva is expected to celebrate the event personally.

Tupi is in the Santos basin's subsalt layer off the coast of Rio de Janeiro and Sao Paulo states.

Development of the reserves is expected to be pricey and complicated, with the reservoirs under 2000 metres of water and a further 5000 metres below sand, rocks and salt.

In November 2007, Petrobras estimated Tupi had recoverable reserves of between 5 billion and 8 billion barrels of oil equivalent.

In late 2010, Petrobras will start a pilot production programme at Tupi, installing the "Angra dos Reis" floating production, storage and offloading vessel at the field.

The FPSO will be capable of producing 100,000 barrels of oil per day, as well as 4 million cubic metres of natural gas per day.

The vessel will also have storage for 1.6 million barrels of crude oil.

According to Gabrielli, the natural gas will be linked to a platform at the massive Mexilhao natural gas field via a 216-kilometre pipeline.

In addition to Tupi, Petrobras plans long-term tests at the nearby Iara and Guara fields between 2010 and 2011, Gabrielli said.

Published: 27 April 2009 22:37 GMT  | Last updated: 27 April 2009 22:37 GMT