ALBERT

Description: Reusing deep wells whether from oil- and gas or geothermal energy production, after years of inactivity can be economically beneficial for the geothermal industry. This study examines the processes that occur in an abandoned well and their impact on reservoir performance. The Groß Sch¨onebeck geothermal research well Gt GrSk 4/05 was reopened after a seven-year production stop to characterize its hydraulic and hydrochemical properties. It targets a reservoir of Rotliegend sandstone and underlying volcanic rocks at a depth of approximately 4000 m. Prior to the hydraulic test, gas samples were collected at the wellhead outlet. These samples showed a significant increase in methane content from approximately 10–15 vol.-% as measured between 2009 and 2013 to 65 vol.-% as measured currently). The objective of this study was to identify the processes responsible for the increase in methane and to assess their impact on well productivity. Well productivity was evaluated through a slug-withdrawal test, followed by the collection of two fluid samples at depths of 1500 m and 4000 m. The chemical composition of the gas and liquid samples was analyzed along with the microbial community. Isotopes of sulfur (34S) and oxygen (18O) were measured in dissolved sulfate and 13C isotopes were measured in the gas phase of hydrocarbons. The particles separated from the suspension of the collected fluid samples were analyzed by scanning electron microscopy. Compared to previous analyses of fluid samples from this well, the methane content has significantly increased and hydrogen sulfide has been detected in the gas phase. Additionally, sulfides of copper and iron were found in the black-colored suspension from the deep-fluid sample taken at 4000 m. The two samples also differ significantly in their organic carbon content, with the sample from 1500 m containing a high amount (135 mg C/L) and at the sample from 4000 m containing a relatively low amount (5 mg C/L). Remarkably, despite the extreme conditions (150 ◦C; 265 g/L TDS at 4000 m), both samples contained several bacterial and fungal genera). Isotope analysis of hydrocarbons in the gas phase indicated mixed methane production from both, thermogenic and biogenic processes. The strong negative 34S of sulfate in the liquid samples rather suggested a thermogenic process of sulfate reduction. It is assumed that several processes occur along the wellbore, including microbial steel corrosion of the well casing (which provides hydrogen, methane, organic carbon, and iron), thermal sulfate reduction, and inflow of formation gases. However, despite the observation of complex geochemical and microbial reactions in the well, the results from the slug withdrawal test indicated no significant change in well productivity (0.6 m3/h*MPa) since the last test in 2014.