# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 5 H\({}_{2}\) Generation in DGRs
### Concept  
The technology implies the injection of a catalyst precursor into a DGR at the first stage. Then, the temperature in the reaction zone should be raised to a level at which catalyzed SMR and methane cracking occur based on reactions (2), (3), and (5). The endothermic reactions (2) and (5) require heat input (\(\Delta\)H\({}^{\circ}\)298K + 206.2 kJ/mol and \(\Delta\)H\({}^{\circ}\)298K + 74.9 kJ/mol, respectively) for the transformation of CH\({}_{4}\). Due to the expected lower H\({}_{2}\) yield, only a few studies have elaborated the technology. The application success depends on the conversion rate of CH\({}_{4}\) to H\({}_{2}\). Therefore metal catalysts are proposed to mostly break C-H bonds and maintain a high and sustainable activity for a long period of time [51]. Two types of monometallic Ni-based catalysts were evaluated for enhancing the process in an experimental study [13]. The first was an in situ prepared catalyst. It can be delivered into the reservoir in the form of a water solution carrying the catalyst precursor, which is then directly converted through chemical reaction into the actual catalyst in the reservoir. The second was the ex situ prepared nickel-based catalyst. This catalyst can be delivered as a suspension into the reservoir together with steam or overheated water. In the study, the theoretical CH\({}_{4}\) conversion rates (on molar basis) were first calculated for a steam/CH\({}_{4}\) ratio of 5 as given in Figure 3. Both the area of interest for this study (dashed line frame) as well as the P-T range that can be realized in a DGR with steam injection (solid line frame) are highlighted in the figure. As shown, the CH\({}_{4}\) conversion efficiency that can be reached via steam formation in a gas reservoir is quite small, being in the range of 20% at its maximum. A maximum T of 450 \({}^{\circ}\)C was achieved in the study with an optimum P, T, and steam/CH\({}_{4}\) ratio (21 at 207 bar and 450 \({}^{\circ}\)C). The maximum CH\({}_{4}\) conversion rate was 5.8% mol. The H\({}_{2}\) concentrations measured in the gas streams (max 12.9% mol) were matching, approximately, the CH\({}_{4}\) conversion rates, confirming the stoichiometry of the related SMR equation. [PERSON] et al. also reported that the product gas mixture contains light hydrocarbon gases as well. In one experiment, H\({}_{2}\)S was found to make up a significant fraction (up to 8.91% mol) of the product gases. It is most likely formed due to the decomposition of sulfur-containing components of the core (both organic and inorganic) and the interaction of the decomposition products with H\({}_{2}\) synthesized during the experiment (not microbial in origin) [13]. It is mentioned that H\({}_{2}\)S acts as a catalyst poison and can significantly reduce the activity of the catalyst even in a small amount. In another work published by the same authors [14], if a Ni-based catalyst was used, it was possible to achieve a methane-to-hydrogen conversion rate of 13.6% in the crushed reservoir cores. However, it is also mentioned that the active zone of the core had to be warmed up to high \begin{table}
\begin{tabular}{l c c} \hline
**Thermal EOR** & **Cost [USD/kg] 1** & **Ref.** \\ \hline
**H\({}_{2}\) Production** & **Cost [USD/kg] 3** & \\ \hline Blue H\({}_{2}\) with 50 EUR/t CO\({}_{2}\) 4 & 1.6–2.7 & [5,59,60] \\ Blue H\({}_{2}\) with 100 EUR/t CO\({}_{2}\) 4 & 3.2–7.2 & [5,59,60] \\ Grey H\({}_{2}\) with 100 EUR/t CO\({}_{2}\) 4 & 1.5–5.1 & [5,59,60] \\ Green H\({}_{2}\) (Electrolysis with wind/solar energy) & 1.6–12.0 & [5,59,60] \\ Green H\({}_{2}\) (Dark fermentation) & 2.0–7.5 & [22,33] \\ Green H\({}_{2}\) ( Biomass conversion) & 1.6–8.1 & [23,23] \\ \hline \end{tabular} 1
[FOOTNOTE:1]Footnote 1: Data provided by the references (without any inflation adjustment). 2  
\end{table} 
Table 3: _Cont._temperatures achievable only in the case of an effective ISC, which is only applicable in the presence of heavier hydrocarbons in situ.