Rock Physics Modeling of Hydrogen-Bearing Sandstone: Implications for Natural Hydrogen Exploration and Storage
Authors: Fuad M. I. Ahmad , Zhao H. , Jaya M. S. , Jones E. A. J. 
Published: October 2023

In the context of global carbon neutrality, using hydrogen as an energy source is becoming one of the key solutions to reduce greenhouse gas emissions. At present, hydrogen is mainly generated through a variety of thermochemical and electrochemical processes such as electrolysis, methane reforming and pyrolysis (Ishaq et al., 2022). However, these methods are generally expensive and suffer from serious issues such as intensive carbon dioxide emission and high electricity consumption (Younas et al., 2022). In fact, hydrogen gas can naturally occur in the subsurface, as evidenced by numerous hydrogen seepages found worldwide (cf. Zgonnik 2020 and the references therein). Furthermore, a significant amount of natural hydrogen was accidentally found during the drilling of a water well (Bougou-1) in Mali in 1987. The latest exploration in the vicinity of the Bougou-1 has indicated that an active hydrogen system exists in the area (Prinzhofer et al., 2018). A variety of scientific research and exploration activities have been conducted across the world to understand the occurrences, generation, and accumulation mechanisms of natural hydrogen (Tian et al., 2022). Natural hydrogen exploration is on the verge of becoming a full-fledged business, resembling hydrocarbon exploration that we are familiar with.

Seismic is one of the most crucial geophysical data that is widely used to acquire a structural and stratigraphical description of the earth's subsurface and to understand complex geologic features. In addition to structural interpretation, seismic data is often used for reservoir characterization by quantitatively extracting both rock and fluid properties from the data through the solution of an inverse problem (Dvorkin et. Al., 2014). The discipline of rock physics plays an important role in seismic reservoir characterization by providing an accurate relationship between fluid and rock reservoir properties, elastic properties, and seismic responses. However, most seismic work is done for hydrocarbon exploration, and there are very few publications that demonstrate the utilization of geophysical seismic forward modeling and inversion for natural hydrogen exploration.

Hydrogen accumulation in the subsurface relies on an effective "hydrogen system" in place, which shares some basic elements with a "hydrocarbon system," such as a reservoir and seal (Prinzhofer et al., 2018). Therefore, seismic exploration is assumed to be useful for hydrogen play detection and evaluation. One of the key aspects of natural hydrogen exploration is to understand the rock physical properties of hydrogen-bearing reservoir rocks in order to perform seismic-based reservoir characterization and potential hydrogen prospect evaluation. Currently, very few, if any, research works have been conducted regarding this topic. In comparison with hydrocarbon gases, hydrogen is characterized by ultra-light density and small molecular size. There is a significant knowledge gap in our routine rock physics analysis regarding how ultra-light gases like hydrogen affect the elastic properties of gas-bearing reservoir rocks. Questions such as whether it is possible to distinguish hydrogen gas with seismic data from other reservoir fluids (e.g., hydrocarbons, CO2, and brine) still need to be answered before conducting any seismic surveys targeting hydrogen plays.