(Adds information on executive shakeup, oilfield divestment)

By Gram Slattery

RIO DE JANEIRO, Feb 24 (Reuters) - Brazilian state-run oil firm Petrobras reported a fourth-quarter net income of 59.9 billion reais ($11.1 billion) on Wednesday, a significant boost in annual and quarterly terms, due largely to the reversal of 31 billion reais in impairments.

The decision to reverse a large part of an $11 billion impairment taken in May indicates that the company has significantly changed its underlying assumptions about the long-term price of crude, which is itself key for investment decisions in exploration and production.

Petroleo Brasileiro SA, as the company is formally known, did not divulge details of its new pricing assumptions. The firm said the impairment reversal was also due in part to changes in its 2021 to 2025 business strategy and unspecified revisions to its project portfolio.

The company's earnings before interest, taxes, depreciation and amortization (EBITDA) came in at 47 billion reais, up 41% from the previous quarter and well above the Refinitiv consensus estimate of 32.9 billion reais.

The company said EBITDA was buoyed by the reversal of 13.1 billion reais in previous costs associated with its healthcare system. Excluding that cost reversal, EBITDA came in about 1 billion reais above analysts' estimates.

Petrobras did not comment on drama gripping its executive management. On Tuesday, Petrobras' board approved a shareholders' meeting that essentially guaranteed the departure of Chief Executive Roberto Castello Branco at the behest of Brazilian President Jair Bolsonaro.

But in a securities filing on Wednesday night, the company said its board had requested that the company's CFO, Andrea Almeida, search for information that should be publicly divulged in light of Tuesday comments by Bolsonaro, in which he said there were "many things wrong" at the company.

The board had requested information from the Mines and Energy Ministry regarding Bolsonaro's comments, but had not yet heard back, the company added.

In a third securities filing, Petrobras said it had agreed to sell its Miranga cluster of onshore oilfields to a subsidiary of Brazil's PetroReconcavo for $221 million. Reuters reported in December that the Salvador-based company was mulling an initial public offering to help it purchase Petrobras assets.

($1 = 5.41 reais) (Reporting by Gram Slattery; Additional reporting by Sabrina Valle; Editing by Christopher Cushing and Grant McCool)