Characterization of the spontaneously recharging natural hydrogen reservoirs of Bourakebougou in Mali

Abstract

In today's race to find ways to produce cheap and green hydrogen, the natural hydrogen wells in Bourakebougou offer a promising solution and are a good example of how H₂ can be produced in the natural environment. Not only has one well been successfully exploited to generate electricity for the local village, but twenty-four other exploratory boreholes have also demonstrated the presence of natural H₂ in the surrounding area. The Bourakebougou H₂ field offers a unique opportunity for geoscientists to determine the key characteristics of natural hydrogen reservoirs. This paper presents the coring, logging, and geochemistry studies that were performed to better characterize the nature of the Bourakebougou H₂-bearing reservoirs. The shallowest main reservoir, in which there is the highest content of H₂, is made of dolomitic carbonate (Neoproterozoic cap carbonate). These carbonates are largely karstified and show a high degree of heterogeneity in porosity (0.21-14.32%). Based on the analysis of the drilling imagery of the carbonated reservoirs, the accumulation of hydrogen occurs in the karst (void) representing a secondary porosity in the rock matrix. Other reservoirs, especially the deepest ones, are porous sandstone rocks with much more homogeneous porosities (4.52-6.37%) compared to the massive carbonates. For the wells analysed, the neutron tool reacted in a specific way when there is the presence of hydrogen. Hence, it stands out as being the primary tool to detect the presence of natural hydrogen beyond simple gas logging. When comparing a H₂ reservoir system to classical oil and gas reservoir systems, the results show that the hydrogen reservoir is a dynamic system that is progressively recharged in H₂-rich gas at the production timescale.

Figure 1

(a) Geological map of the study area designed and modified from the Global Geological Map of Mali (1/1 500 000), DNGM, (b) Synthetic and simplified structural section of the study made according to the cross section line in Fig. 1(a). The maps were generated using Adobe illustrator CS6 (Version 16.0.0, https://www.adobe.com/).

Figure 2

Synthetic stratigraphic log and electric logging of the Bougou#6 borehole, which is the most characteristic of the area studied in the Bourakebougou sector, from the surface to the basement. CC cap carbonate, BS banded sandstone, S massive sandstone, mc zone of intense contact metamorphism, Mag presence of magnetite, U uranium-rich level, Hem presence of hematite. The coloured lines correspond to the well tops.

Figure 3

Position map of the thirteen wells used in this study, based on Google Earth satellite imagery of the Bourakebougou area (Mali) (https://earth.google.com/web/). The map was prepared using Adobe illustrator CS6 (Version 16.0.0, https://www.adobe.com/).

Figure 4

The different data used to identify and characterize the different formations and hydrogen accumulation zones between 0 and 530 m. Track 1: The true vertical depth, Track 2: The natural gamma-ray (GN), Track 3: The density (RHOB) and neutron porosity (NPHI) in limestone matrix scale, Track 4: The formation’s nature, Track 5: concentration of H₂ from gas logging and Track 6: cores images. mc zone of intense contact metamorphism, Mag presence of magnetite, Hem presence of hematite.

Figure 5

Macroscopic characterization of a karstic carbonate reservoir, cores are from Bougou-14, Bougou-5, Bougou-4, Bougou-19, and Bougou-3.

Figure 6

Borehole imagery versus gas logging showing the presence of hydrogen gas within the karstic system.

Figure 7

Carbonate reservoir observed by optical microscopy under (a,b,d) polarized and analysed light, (c) reflected and non-analysed light and (d) reflected and analysed light. (a) micro dolomite, (b) rhombohedral dolomite, (c,d) fracture with sulphide, calcite, and antigorite, (e,f) samples with alizarin coloration under unanalysed natural light.

Figure 8

Neutron porosity comparison between a hydrogen-rich well (Bougou-6) and a hydrogen poor well (Bougou-13).

Figure 9

Synthetic diagram showing the presence of a free gas phase in the uppermost carbonate and sandstones reservoirs, with the gas mostly dissolved at depth especially in the deeper sandstone reservoirs then degassed in the rising drilling fluids. It also shows the dolerite intrusions and the marble location. The diagram was generated using Adobe illustrator CS6 (Version 16.0.0, https://www.adobe.com/).