The joint project CLEAN was a Research & Development (R&D) action accompanying a pilot Enhanced Gas Recovery (EGR) project designed by GDF SUEZ E&P Deutschland GmbH (GDF SUEZ) in cooperation with Vattenfall Europe. Within the framework of this project a total of 16 German research and industry partners participated.The project was set up as pilot to investigate the processes relevant to EGR by the injection of CO2 into a subfield of the almost depleted Altmark natural gas field (Germany). Despite the setback that permission for active injection was not granted by the mining authority during the period of the project, important results fostering the understanding of processes linked with EGR were achieved.The CLEAN results provide the technological, logistic and conceptual prerequisites for implementing a CO2-based EGR project in the Altmark and provide a benchmark for similar projects in the world.