*Article* **Thermal-Hydraulic-Mechanical (THM) Modelling of Short-Term Gas Storage in a Depleted Gas Reservoir—A Case Study from South Germany †**

**Muhammad Zain-Ul-Abedin \* and Andreas Henk \***

Institute of Applied Geosciences, Technical University of Darmstadt, 64287 Darmstadt, Germany

**\*** Correspondence: abedin@geo.tu-darmstadt.de (M.Z.-U.A.); henk@geo.tu-darmstadt.de (A.H.); Tel.: +49-6151-1622-347 (M.Z.-U.-A.); +49-6151-1622-344 (A.H.)

† This paper is an extended version of my PhD thesis with title "Coupled Thermal-Hydraulic-Mechanical (THM) Modelling of Underground Gas Storage—A Case from the Molasse Basin, South Germany" submitted in 2022 Technical university of Darmstadt, Darmstadt, Germany, March 2022; p. 103.

**Abstract:** This study addresses the use of former gas storage facilities as short-term storage for renewable energy through power-to-gas (PtG) technology in Germany. Three test cases with coupled thermal-hydromechanical (THM) modelling were conducted to evaluate short-term injection and production schedules. The operating rates were controlled by the upper and lower limits of the wellbore pressure. The maximum difference in pore pressure and effective stress was 0.6 MPa in all cases. Fault reactivation analysis was performed on the THM models to estimate fault stability. The critical pore pressure for safe reservoir operation was determined to be 1.25 times the original pore pressure, corresponding to a WBHP value of 20.25 MPa. The upper limit of the gas injection rate for safe storage operation was estimated to be between 100,000 and 150,000 m3/day. The thermal stresses were found to be negligible for short-term cases. The storage capacity of PtG technology was reported to be up to 1,322,400 kWh/d of renewable electricity, which can contribute to Germany becoming a greenhouse gas neutral country by 2050. The workflows and results of the study are applicable to all gas storage in a porous medium, including methane, CO2, and hydrogen.

**Keywords:** THM modelling; short term underground gas storage; dynamic modelling; "battery" for power-to-gas
