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Methane production
Corrosion
Microbial activity
Sulfate reduction
Well performance testing
Slug withdrawal
Abstract:
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.
