PetroWiki is a fantastic free resource from the Society of Petroleum Engineers and you can take a look at it here: http://petrowiki.org/PetroWiki
It was created from the seven volume Petroleum Engineering Handbook (PEH) and all the chapters within these seven volumes can be found in their entirety on the site with PEH before the chapter name.
The PEH pages can not be changed and form the core of the PetroWiki resource.
But you will find more than just the PEH content on the site. The SPE invite their members to add pages to the Wiki to expand on topics or give more up to date information and this means that you can find a wealth of material on lots of petroleum engineering related topics.
So to use this resource simply go to the Wiki, http://petrowiki.org/PetroWiki, search for a topic you are interested in and explore the results.
Remember the pages with PEH at the start are directly from the Petroleum Engineering Handbook and all others have been created and added to the site by SPE members.