**1. Introduction**

High energy demand has intensified research into the "underground gas storage" (UGS) discipline [1]. As a result, both long-term (seasonal) and short-term (weekly) gas storage projects have gained the attention of UGS researchers [1]. An idea in this context is to store methane produced by "power-to-gas (PtG)" technology from excessive electricity produced by renewable sources, such as solar and wind. The stored gas can then be reused for power generation whenever needed. Thus, UGS is used as a kind of "battery" for surplus green energy.

Short-term storage cycles may cause some geomechanical issues in the porous reservoir. The pore fluid pressure in the reservoir fluctuates due to intensive gas injection and withdrawal phases. These fluctuations in fluid pressure change the effective stresses in the reservoir and may also change the in-situ stress state outside the reservoir area [2]. These changes have implications for geomechanical phenomena related to fault stability, caprock integrity, and surface deformation. Numerical modelling, e.g., 3D geomechanical modelling, provides a platform to integrate lithological and mechanical heterogeneities and investigate stress state changes during injection-production cycles of high frequency.

This study concentrates on a geomechanical assessment of a former gas field in the Bavarian Molasse Basin east of Munich (Germany) for which a hypothetical transformation into an UGS site is investigated. Various scenarios with variable short-term (weekly)

**Citation:** Zain-Ul-Abedin, M.; Henk, A. Thermal-Hydraulic-Mechanical (THM) Modelling of Short-Term Gas Storage in a Depleted Gas Reservoir—A Case Study from South Germany. *Energies* **2023**, *16*, 3389. https://doi.org/10.3390/en16083389

Academic Editors: Luis Hernández-Callejo, Jesús Armando Aguilar Jiménez and Carlos Meza Benavides

Received: 27 February 2023 Revised: 5 April 2023 Accepted: 10 April 2023 Published: 12 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

schedules to test cases for gas storage and withdrawal are considered to evaluate stresses (e.g., effective stress) and deformation due to pressure changes with high-frequency injection/production cycles. German data for excess electricity from renewable energy sources (such as solar and wind) throughout the calendar year 2017 are also considered in two cases to address the issue of irregular schedules in gas supply (via PtG) and energy demand.

The modelling results provide information about the stress state within and around the reservoir because of the production and injection of each selected time step, and they are compared with the stress states at the depletion and replenishment stages. The production history of the reservoir (porous media) is categorically useful to determine stress paths within and around the reservoir and wellbore periphery, as well as caprock integrity. Further, the modelling results provide information about ground surface subsidence during peak depletion and replenishment time steps, which can be useful to minimize geomechanical risks to any gas storage facility, not only for methane or CO2 but also for hydrogen. Finally, fault reactivation analyses are also incorporated to obtain a safe gas injection rate for safe storage capacity.

The most popular gas storage method is underground gas storage among others, such as liquefied natural gas (LNG), storage tanks, and pipeline storage [3,4]. Two important issues are associated with UGS. First, it relies on gas imports due to increased demand for power generation (gas-to-power) and other domestic usages. Second, any damage to infrastructure could lead to higher gas prices or disruption of supply, with unpredictable, costly consequences for customers [1]. To address these challenges, various research has suggested using depleted gas/oil porous reservoirs or aquifers as UGS, with which significant volumes can be strategically stored [1,5].

One of the main advantages of depleted gas reservoirs is that they allow convenient and cost-effective gas storage, as they have suitable permeability properties and pore connections [6,7]. Some vital data, such as geological and geophysical characteristics, petrophysical properties, storage capacity, pressure, and the production history of depleted gas reservoirs, have been thoroughly studied and well recorded during the development phase of the reservoir field, enabling numerical modelling to simulate the injection-production process of underground gas reservoirs [3,6,8].

Water encroachment occurs in the porous reservoir vertically and laterally during the development phase of the gas reservoir. This water invasion causes fewer porous spaces in the reservoir, and different fluid distribution areas are formed in the longitudinal and traverse planes of the reservoir. During the injection and production operations of gas storage, the gas-water interface moves downward when gas is injected and moves upward when gas is produced [9]. Furthermore, as the foreign gas is injected into the reservoir, which has a different temperature than the reservoir's actual temperature, these thermal changes in the reservoir also cause some thermal stress changes in the reservoir, which is not the case in the reservoir exploitation phase. Therefore, it is necessary to conduct a systematic modelling study of the changes in the thermal-hydraulic-mechanical properties of UGS due to repeated water intrusion and multiphase seepage rules during high-speed injection production operations. The state-of-the-art thermal-hydro-mechanical (THM) modelling provides a platform to analyse and investigate all these issues related to porous UGS systems discussed above. THM models are typically derived from a wide range of geological, geophysical, and engineering data, including field measurements, core tests, well logs, drilling, and production data. After calibration, these THM models can be used for ground surface subsidence, thermal stress changes, maximum safe storage capacity, and maximum threshold pressure, avoiding fault reactivation and maintaining caprock integrity [10–12].

This modelling case study is a THM assessment of a former depleted gas reservoir field from the Molasse Basin in South Germany and has been presented in [10]. This dynamic modelling study addresses the following issues related to short-term and longterm operations of UGS: (1) the storage capacity of the reservoir; (2) thermal analysis with changing temperatures of the reservoir due to the injection of gas; (3) potential fault

reactivation analysis; and (4) stress path changes due to changes in pore pressure because of short-term production/injection cycles. These types of THM models, particularly shortterm scheduled models, have implications for PtG technology in which simulations help us to understand that excess power from renewable resources can be stored in underground gas storage reservoirs and vice versa.
