UPDATE 2-Petrobras posts first loss in 13 years; result shocks

* Petrobras posts first quarterly loss since 1999

* Company blames weaker real, well charges for loss

* Chief Executive Foster vows to revive profit

* None of the analysts in poll predicted shortfall (New throughout; adds analyst comment, additional results, comment from CEO Foster throughout)

By Jeb Blount and Leila Coimbra

RIO DE JANEIRO, Aug 3 (Reuters) - Brazil's state-led oil giant Petrobras on Friday reported its first quarterly loss in more than 13 years as a weaker Brazilian currency raised debt costs and the company faced charges related to recently drilled dry and uncommercial wells.

Petrobras lost 1.35 billion reais ($665 million) in the three months ending June 30 compared with a net profit of 10.9 billion reais a year earlier, according to a securities filing. All nine analysts surveyed by Reuters expected the company to post net income of 3.69 billion reais.

It was the first quarterly loss at Petrobras since the first quarter of 1999 when Brazil devalued its currency by about a third. The loss caught investors off guard and may lead to steeper declines in the company's share price and a revision of output and earnings predictions for this year among analysts.

Preferred shares of Brazil's largest company have shed 5 percent this year and investors were surprised in June when the company said major increases in output were unlikely before 2015 despite $237 billion in planned spending. The 2012-2016 plan is world's largest corporate investment program.

"The loss number is catching all of us by surprise," said Dany Rappaport, who oversees 250 million reais in assets for InvestPort in Sao Paulo. "As management recognizes some back-dated problems in the company's downstream units, analysts and investors might have to start rethinking our prospects for the company."

In addition to the currency and well costs, Petrobras earnings were hampered by rising capital spending, falling oil and gas output and the long refusal of the federal government, the company's controlling shareholder, to let the company raise fuel prices in line with increases in market prices for crude.

Brazil's real was an average 18 percent weaker in the second quarter than it was a year earlier. Brent crude oil , a benchmark for world prices, was 7 percent lower. Output, despite $237 billion of planned spending under the world's largest corporate investment plan, fell 1.1 percent.

"We are working to recover our profitability," Chief Executive Maria das Graças Foster said the filing. "We have been reiterating our commitment to bring our domestic fuel prices in line with world prices for five months."

Petrobras raised its wholesale fuel price for the first time in six months in late June, too late to help a second quarter where the company lost 7.03 billion reais on its refining division.

The company lost money even as sales rose. Net revenue, or total sales minus sales taxes, was 68 billion reais in the quarter, 10 percent more than a year earlier. The poll predicted revenue of 67 billion reais.

Earnings before interest, taxes, depreciation and amortization, or EBITDA, a measure of the company's ability to generate cash from operations and to pay debt, was 10.6 billion reais, 34 percent less than a year earlier.

The analysts polled by Reuters expected EBITDA of 15.3 billion reais.

($1 = 2.03 reais) (Reporting by Jeb Blount and Leila Coimbra; Additional reporting by Guillermo Parra-Bernal in Sao Paulo; Editing by Marguerita Choy and David Gregorio)