* Japan firms to buy 14.7 pct stake in project from Woodside Petroleum * Firms to help in financing project, to take 1.5 mln tonnes of gas/yr * Woodside shares gain 3.7 pct; Mitsui, Mitsubishi down in weak mkt By Narayanan Somasundaram and Risa Maeda SYDNEY/TOKYO, May 1 (Reuters) - Japan's Mitsui & Co and Mitsubishi Corp will jointly buy a 14.7 percent stake in an Australian LNG project from Woodside Petroleum for $2 billion, underscoring Japan's demand for gas in the face of a drastically reduced nuclear power generating capacity. Japan, the world's largest liquefied natural gas (LNG) importer, is racing to secure gas supplies as LNG substitutes lost nuclear capacity. Its last working nuclear power station goes offline on Saturday. The country's nuclear capacity, which previously accounted for about a third of the country's electricity production, has slowly been taken offline since the Fukushima nuclear crisis last year triggered nuclear safety concerns. The crisis forced Japan to import 18 percent more LNG mostly for power generation in the past year and prompted the government to scrap a 2010 plan to increase the share of nuclear energy in power generation to over 50 percent by 2030. Should Japan maintain some nuclear power capacity, the country will probably burn as much as 20 million tonnes more LNG per year by 2020 than the roughly 70 million tonnes used in the year ended in March 2011, said Shigeki Sakamoto, a senior researcher at Japan Oil, Gas and Metals National Corp. "Even with the latest moves by Japanese utilities to secure long-term supplies, they cannot help but rely on spot contracts until 2016," Sakamoto said. Japan is set to announce a new energy policy this summer. The country's last operating reactor shuts for maintenance on Saturday and there is no fixed schedule yet for restarting units idled for regular checks or closed after last year's earthquake and tsunami. Woodside shares rose 3.7 percent to close on Tuesday at A$36.20 ($37.69), their highest since March 9, after the announcement of the deal in an Australian Securities Exchange filing. Mitsui shares closed 1.8 percent lower and Mitsubishi ended down 2.1 percent in a weak overall Tokyo market. Japanese firms have stakes in at least five other Australian LNG projects, and are said to be eyeing the stake sale in BG's Curtis gas project. Australia has $200 billion of proposed LNG export projects with plans to add more than 80 million tonnes per annum (mtpa) of LNG production before the end of the decade to become the world's leading LNG exporter, surpassing Qatar. DEAL DETAILS The Japanese companies agreed to buy the Woodside stake through a company called Japan Australia LNG (MIMI Browse) Pty Ltd that would give them a 16 percent i n terest in Woodside's East Browse and 8.1 percent in West Browse ventures. They also agreed to take 1.5 million tonnes of gas a year from the project, and offered Woodside help with getting competitive finance from Japanese banks for the Browse project, estimated by analysts to cost around A$30 billion ($31 billion). Woodside said its stake in the project will fall to 31.3 percent from 46 percent. It will remain the operator of the project. "The level of interest shown during this process reflects the strong ongoing demand for LNG from premium developments such as this," Woodside Chief Executive Peter Coleman said in the statement. A deal would finalise the auction process Woodside announced in January after receiving approaches from several companies and allows Woodside to extract early value from the project. It also adds some certainty to a project that has been dogged by a dispute among its partners -- Shell, BP , Chevron, and BHP Billiton -- over the best location to process the gas. There is also mounting external opposition to building a gas processing plant at James Price Point, which is favoured by Woodside. Last month, Australia gave partners in the Browse project off its western coast more time to reach a final investment decision, as attempts are made to end in-fighting and quell opposition from environmentalists and landowners.