Well Integrity Evaluation and Elastomer Selection for Hydrogen (H2) Storage in Salt Caverns

Abstract

Abstract The HYPOS (Hydrogen Power Storage & Solutions in central Germany) project aims to make the surplus of renewable energy usable. Hydrogen (H2) is to be produced by electrolysis and stored in existing salt caverns that have been used for natural gas storage in Germany for decades. While extensive experience is available in natural gas storage, only six H2 storage caverns exist worldwide. The challenge of H2 storage results from the different material requirements due to the special characteristics of H2 compared to natural gas. Under typical storage conditions, H2 has twice the mobility of natural gas and is characterized as highly diffusive in solid materials. As part of HYPOS, we are pioneering the evaluation of the borehole integrity of salt caverns that will be used in the future to store H2. To this end, a series of permeability measurements were carried out on rock salt, cement, and composite samples of cement-salt and casing-cement under various effective pressures. The permeability measurements of the core samples were carried out using the transient large scale two-chamber setup, a novel approach built for this purpose. Permeability down to approx. 10-24 m² (10-9 mD) can be determined on cylindrical samples in this facility. The composite sample casing-cement has the form of a hollow cylinder to simulate a section of a real borehole. Furthermore, four different sealing materials used in completions were tested in autoclaves, and their resistance to the H2 environment was evaluated. These innovative methods provide a comprehensive understanding of the challenges and potential solutions in H2 storage, contributing to the overall progress of the HYPOS project. Measurements using the two-chamber method have proven reliable for determining the effective permeability of initially water-saturated cement and rock salt. The permeability of the cement samples shows a dependency on the effective pressure. The permeability of the salt (cavern wall) is in the order of 10-23 m2. The results of the composite casing-cement show that it has approximately the same permeability as pure cement. The cement samples have a maximum permeability of approx. 3×10-20 m² under typical working cavern conditions and are therefore within the range of required technical tightness. Overall results show that the composites cement-salt and casing-cement are tight to H2 storage conditions. Elastomer materials of types 286 NBR, 299 NBR, 332 HNBR and 459 FKM have been experimented with the presence of CO2 and H2 (separately) under the pressure of 50 bar and ambient temperature of 14 °C in an autoclave for one week for CO2 and four days for H2. The results showed the resistance of these materials to H2; no reactions were observed. This study's novelty is using a full wellbore size section (1:1), and the built facility for this purpose enables working on a broad range of pressure and temperature conditions. This study enables us to evaluate the suitability of currently available and newly mined salt caverns for hydrogen storage to accelerate the energy transition to a H2 economy.