**Monitoring Scheme for the Detection of Hydrogen Leakage from a Deep Underground Storage. Part 1: On-Site Validation of an Experimental Protocol via the Combined Injection of Helium and Tracers into an Aquifer**

**Stéphane Lafortune 1,\*, Philippe Gombert <sup>1</sup> , Zbigniew Pokryszka <sup>1</sup> , Elodie Lacroix 1,2 , Philippe de Donato <sup>2</sup> and Nevila Jozja <sup>3</sup>**


Received: 1 August 2020; Accepted: 28 August 2020; Published: 1 September 2020

**Abstract:** Massive underground storage of hydrogen could be a way that excess energy is produced in the future, provided that the risks of leakage of this highly flammable gas are managed. The ROSTOCK-H research project plans to simulate a sudden hydrogen leak into an aquifer and to design suitable monitoring, by injecting dissolved hydrogen in the saturated zone of an experimental site. Prior to this, an injection test of tracers and helium-saturated water was carried out to validate the future protocol related to hydrogen. Helium exhibits a comparable physical behavior but is a non-flammable gas which is preferable for a protocol optimization test. The main questions covered the gas saturation conditions of the water, the injection protocol of 5 m<sup>3</sup> of gas saturated water, and the monitoring protocol. Due to the low solubility of both helium and hydrogen, it appears that plume dilution will be more important further than 20 m downstream of the injection well and that monitoring must be done close to the well. In the piezometer located 5 m downstream the injection well, the plume peak is intended to arrive about 1 h after injection with a concentration around 1.5 mg·L −1 . Taking these results into account should make it possible to complete the next injection of hydrogen.

**Keywords:** hydrogen; underground storage; leakage; monitoring; protocol; helium; aquifer
