The announcement signals a powerful political support from Bolivia's largest neighbor to embattled President Evo Morales who faces an "autonomy rebellion" from the country's four richest provinces.
The plan also marks a turning point in strained relations between the government of Bolivian President Evo Morales and Petrobras, which criticized Morales' energy nationalization last year and froze planned investments.
The two countries signed a series of energy cooperation deals during a two-day visit by Lula that are aimed to secure Brazil's supplies of natural gas and patch up ties with its poorer neighbor.
"I want to tell Petrobras's president that although there was a certain lack of trust after the changes to our energy policies, we've never sought to harm any company," Morales said in a speech at the presidential palace in La Paz.
"Every investor should recover their investment, not just recover it, but also have a right to the profits. We'll always guarantee that," said Morales, a close ally of Venezuela's fiery president Hugo Chavez.
A text of the Petrobras investment accord said the firm was planning to spend between 750 million and one billion US dollars to increase gas output.
The other energy agreements include a plan by Brazilian petrochemical firm Braskem to study a joint venture with Bolivia's state energy company YPFB to build a methane plant. Petrobras employees will also give technical and commercial training to their counterparts in YPFB.
Brazil is Bolivia's main natural gas customer and a gas shortage in Rio de Janeiro in October reminded the country how much it relies on its smaller neighbor for supplies of the fuel. Economic expansion in Brazil, Chile and Argentina has increased demand for energy across the region and Bolivia has South America's second-biggest gas reserves after Venezuela.
However, Lula's visit was not confined to energy issues. He also signed accords to give loans for farm machinery and increase cooperation between the countries' universities.
Diplomats in Brasília have said Lula wants to win back Brazil's influence in Bolivia from Venezuela's Chavez, and he sought on Monday to draw a line under the spat over the energy nationalization.
"In the end, Evo Morales and Lula didn't fight like some people wanted us to fight. We didn't become adversaries … we became comrades," Lula da Silva underlined.
The trip was well-timed for Morales, who is facing fierce resistance to his left leaning reform plans from conservative opponents. They frequently attack his close relationship with Chavez, but the more moderate Lula is a less controversial ally.
"We've come with a message of solidarity to reaffirm Brazil's willingness to contribute so that Bolivia can find its path to stability and social and economic development", said President Lula da Silva.
And at this moment "of political turbulences if I could only give you an advise Evo, without interfering in Bolivian domestic policies, I would recommend patience and more patience. The Bolivian people will find the way that leads to the consolidation of democracy", added the Brazilian president.
Morales said that this "meeting unblocks Brazilian investments in Bolivia" and reiterated that Bolivia acknowledges the regional leadership of Brazil and Lula da Silva "as my eldest and wisest brother: before as union leaders, now as presidents, looking to ensure social justice".
Both presidents exchanged their country's highest honors: Morales awarded the "Condor of the Andes" medal to Lula da Silva and the Brazilian president the "Southern Cross" to his Bolivian colleague.
An agreement was also signed for the building of two bridges along the common border of 3.000 kilometers.
Mercopress