More likely said energy/oil companies would buy the patents/inventions (or just "do" the inventor(s) in) , sit on them while they screw the consumer with fossil fuels until they run out, then go cap in hand to governments with this brilliant idea that they have for free energy, then screw the consumer under the pretense that they had to lay out vast amounts of capitol to develop this free energy technology.
The basic idea in patents it that the plans are freely available from the patent office. Were any real free energy device patented, somebody would've tested it (like the mythbusters, that tested several of them). Although I don't think the US patent office accepts free energy patents anymore.
I don't think any sane person who did actually invent a working free energy device would do anything but release the plans, create a couple of working prototypes and sent them to be tested. The "secret oil industry" would have no time to react.
It is also a myth that the oil industry tries to stop the development of electric cars. In fact, Neste, an oil company, was in the forefront of electric car development in the 80's (with a company called ElCat), the problem was that nobody wanted to buy them, lead battery electric cars are just too heavy and suck. Now, with lithium-ion batteris and high cost of petrol things are starting to change.
http://www.elcat.fi/eng/index.php