Thesis by Omar Maiga: « Caractérisation géologique et géophysique 3D d’un système de réservoirs d’hydrogène naturel - Exemple du champ de Bourakèbougou, Mali » (3D geological and geophysical characterization of a system of natural hydrogen reservoirs - Case of the Bourakebougou field, Mali).
In the current drive to find ways to produce inexpensive, green hydrogen, the natural hydrogen found underground is attracting growing interest as a component of a new energy mix.
In Mali, the Bourakebougou field is the emblematic benchmark for natural underground hydrogen accumulations of this type today: 25 exploratory boreholes have demonstrated its presence in high concentrations (98 mol%), both in a geological reservoir located around one hundred meters below the surface and in other reservoirs at greater depths.
In order to better characterize these reservoirs and the hydrogen retention and preservation processes at play, PhD research was conducted at IFPEN, in partnership with the field operator Hydroma.
An in-depth study of coring and logging1 results, as well as of the geochemical data obtained, was carried out in order to better characterize the nature of Bourakebougou’s hydrogen reservoirs [1].
The main reservoir - the shallowest and with the highest concentration - is made up of dolomite carbonates (cap carbonates from the late Proterozoic glacial period). The majority of these carbonates are karstified2 and present a high degree of heterogeneity in terms of porosity (0.21-14.32%). Based on analysis of borehole imaging data from carbonate reservoirs (the shallowest main reservoir and other deeper secondary reservoirs), hydrogen accumulation primarily occurs in karstic cavities (Figure 1). Other reservoirs, particularly the deeper ones, are made up of porous sandstone rocks with much more homogeneous porosities (4.52-6.37%) than massive carbonates.
By comparing the reservoir system studied with classic oil and gas systems, it can be observed that the hydrogen reservoir is a dynamic system that gradually becomes recharged with hydrogen-rich gas at a rate similar to that of extraction. This can be explained by the fact that hydrogen, most of which is dissolved at depth in deep aquifers (as confirmed by logging data), is released as a result of the drop in pressure associated with production. It therefore recharges the upper karstic reservoir in the form of a gaseous phase, enabling a relatively constant pressure to be maintained on the scale of a few years.
The significant mobility of hydrogen molecules, due to their very small size, gives them highly diffusive properties leading to the frequent surface seeps recently recorded all over the world. It is for this reason that most of the discussions surrounding natural hydrogen exploration focus on its surface occurrences and the processes involved in its production. However, the hydrogen trapping capacity and process are the most critical issues in attempting to discover gas-phase accumulations that are both significant and conducive to profitable production. It is for this reason that in a different phase of this PhD research, a detailed characterization of cap rocks (in this case dolerite rocks) containing hydrogen in the Bourakebougou field was conducted [2].
This study revealed that the sealing capacity of dolerites is linked to the fact that they are only slightly fractured and none of the fractures are open. The role of aquifers was also highlighted as contributing to subsurface hydrogen retention. As the latter is not very soluble in water, in low pressure and temperature conditions, it becomes less and less likely to diffuse easily to the surface from the shallow depth of the main reservoir.
In conclusion, this study underlines the fact that the exploration of natural hydrogen fields should not be based only on the presence of a generation process, but also on the presence of a highly efficient trapping system.
1- Continuous physical recording of variations, as a function of depth, of a given characteristic of formations encountered during a survey.
2- A karst is a geomorphological structure resulting from the hydrochemical and hydraulic erosion of all soluble rocks.
References:
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Maiga, O., Deville, E., Laval, J. et al., 2023, Characterization of the spontaneously recharging natural hydrogen reservoirs of Bourakebougou in Mali, Nature SR 13, 11876 (2023).
>> https://doi.org/10.1038/s41598-023-38977-y
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Maiga, O., Deville, E., Laval, J. et al., 2024, Trapping processes of large volumes of natural hydrogen in the subsurface: The emblematic case of the Bourakebougou H2 field in Mali, International Journal of Hydrogen Energy 50, 640-647.
>> https://doi.org/10.1016/j.ijhydene.2023.10.131
Scientific contact: Eric Deville