Options for gas supply diversification for the EU and Germany in the next two decades : final report / ewi Energy Research & Scenarios gGmbH (ewi ER&S), European Centre for Energy and Resource Security (EUCERS) ; principal author EWI Dr. Harald Hecking, author EWI Simon Schulte, principal author EUCERS Dr. Adnan Vatansever, author EUCERS Dr. Slawomir Raszewski