Effect of Hydrogen Interaction with Crude Oil on the Extraction of Hydrogen if Stored in Depleted Shale Reservoirs: An Experimental Study Analyzing the Outcome of Cyclic Hydrogen Injection into Oil-Saturated Core Samples

Abstract

Abstract One of the main challenges of underground hydrogen storage in porous media (UHSP) is ensuring efficient hydrogen extraction after long storage periods. Unconventional shale resources have contributed towards 65% of the US crude oil production in 2021. Thus, if we are to explore possibility of efficient UHSP in depleted oil-producing shale reservoirs, we should ascertain if hydrogen interaction with residual oil, throughout the storage period, leads to recovering undesirable amounts of oil during hydrogen extraction. Cyclic hydrogen injection can be indicative of hydrogen recovery after extraction. The process consists of saturating core samples of different major shale formations with crude oil, at in-situ conditions. Different saturations are achieved for quasi-identical samples. Oil wettability is measured before a sample is subjected to compressed hydrogen injected at a pressure high enough to infiltrate the formation, using a core-flooding system. Hydrogen is kept inside the system at constant pressure for a soaking period, emulating the storage phenomena. Finally, a hydrogen-oil mixture is extracted from the sample, allowing estimation of hydrogen recovery after oil saturation and wettability are remeasured. The main difference between UHSP and injection of gases such as CO2 and CH4 is that storage should be designed in a way that allows hydrogen extraction with high recovery rates, and with a small need for separation operations. Ideally, we would look to extract a hydrogen-oil mixture that has a hydrogen volume close to the injection volume, and that does not contain a large portion of crude oil. Results are compared for different initial oil saturations, shale formation types, soaking periods, and injection pressures. Oil saturation is measured after extraction to estimate the amounts of crude oil extracted, and of hydrogen trapped in the formation. We would assume that an oil- wet formation with low oil saturation would result in less undesirable oil being recovered with the extracted hydrogen. Results are evaluated to identify formations suitable for storage and extraction of hydrogen. If oil saturation and wettability are majorly altered after extraction, this could mean that the suggested conditions are not suitable for hydrogen storage. Scanning and Imaging could prove to be important to evaluate change in mineralogy, fluid properties, or chemical properties after cyclic hydrogen injection, if further analysis is needed. This work adds to the ongoing research in hydrogen storage as it focuses on efficient extraction of hydrogen to be potentially stored in depleted unconventional formations. It is a continuation of previous work focused on effect of hydrogen exposure on unconventional reservoir properties, which was also inspired by Reservoir Engineering. This topic can be further developed after initial experimental studies are done. Solutions can be suggested for low performing extraction cases.