Abstract This study addresses the thermal–hydraulic–mechanical and chemical (THMC) behaviour of a research well doublet consisting of the injection well E GrSk 3/90 and the production well Gt GrSk 4/05 A(2) in the deep geothermal reservoir of Gros Schonebeck (north of Berlin, Germany). The reservoir is located between 3815 and 4247 m below sea level in the Lower Permian of the North German Basin (NGB). Both wells were hydraulically stimulated to enhance productivity. For the production well three stimulation treatments were performed in 2007: these three treatments result in a productivity increase from 2.4 m3/(h MPa) to 14.7 m3/(h MPa). The injection well was stimulated four times in 2002/2003, resulting in a corresponding productivity increase from 0.97 m3/(h MPa) to 7.5 m3/(h MPa). The necessary infrastructure for production and subsequent injection of geothermal fluid was established in June 2011. Between June 8, 2011 and November 8, 2013, 139 individual hydraulic tests were performed with produced/injected volumes ranging from 4.4 to 2567 m3. The productivity index decreased non-linearly from 8.9 m3/(h MPa) on June 8, 2011 to 0.6 m3/(h MPa) on November 8, 2013. Five possible reasons for the productivity decrease are discussed: wellbore fill, wellbore skin, the sustainability of induced fractures, two phase flow and compartmentalisation. For all hydraulic tests, the injectivity index remains almost constant at 4.0 m3/(h MPa). During 17 of 139 hydraulic tests a sudden increase of the productivity was observed. Possible reasons for this effect are discussed: accumulation of free gas and/or fines and scales within the fracture as well as changing hydraulic properties due to changing mechanical load on the fracture.