The well symbols are created inside PETRA, rather than being an external set created elsewhere. There is a jpg file under the "PETRA Documents" tab (across the top of the forum) that shows the available symbols. Also, for your reference, here is a link:
http://www.ihscommunity.com/energy/index.php?action=page;id=3
You might be able to graphically clip out the symbols of interest and put them in your legend.


