# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 5 H\({}_{2}\) Generation in DGRs
### Reservoir Engineering Aspects  
#### 5.2.1 Mode of Application  
Steam, produced at the surface with a given quality (60-80%), is injected into a reservoir at a predefined rate. Due to changing thermodynamics during the trip in the wellbore and in the reservoir, the steam becomes drier with a potential transition to a superheated state. Heat losses for heating the reservoir lead to steam condensation. The surface facilities and installations required for steam generation and injection are not different from those used in EOR operations, with a steam generator and a pump being the minimum requirements. Figure 3: Theoretical CH\({}_{4}\) conversion rates (% mol) as function of P and T (modified from [13]); the white dashed line frame shows the area of interest as studied by [13]; the white solid line frame shows the P-T range that can be realized in a DGR with steam injection. In the related literature, it is mentioned that the H\({}_{2}\) separation from other gases is performed by downhole membrane separators. Such membranes made of precious metals (e.g., palladium-vanadium) are expensive because they have only a low filter capacity [18; 19]. On the other hand, they are negatively influenced by the presence of other gases like H\({}_{2}\)S. Another proposed way of producing H\({}_{2}\) is the separation of the gases directly in the reservoir. It is claimed that the generated H\({}_{2}\) moves upward to the crest of the reservoir whereas co-generated CO\({}_{2}\) accumulates in the deeper parts of the reservoir due to gravity segregation [15]. This idea is worth discussing. The H\({}_{2}\) and CO\({}_{2}\), and probably other gases, are components of the gas mixture generated as a result of the thermal processes in the reservoir. Such a separation of the components of a gas mixture, if any, is a rather slow process. From the related fundamentals of the behavior of gas mixtures, it is known that when gases of different natures are brought in contact (this is the case when they are produced in the reservoir in a process such as that discussed), they do not arrange themselves according to their density but diffuse spontaneously, mutually, and uniformly through each other and thus persist as a mixture for any length of time [62]. Their concentrations can change based on their molecular energy as thermodynamic conditions change over time, a process that takes much longer than economic considerations. Against this theoretical consideration, it could be argued that the gases are generated in different locations and times so that they find sufficient time to separate from each other and move before complete mixing. This possibility was investigated numerically using a simplified 3D generic model of a DGR. The model of an inclined reservoir with a 1.2 \(\times\) 1.2 km lateral extent had a thickness of about 100 m while the vertical distance between the top and bottom of the reservoir was 450 m, with a grid cell number of 21 \(\times\) 21 \(\times\) 10. The reservoir initially contained CH\({}_{4}\) in a pressure-depleted state (30 bars). Gas generation was modeled using a horizontal well that penetrated through the entire width of the reservoir and was located at the mid-layer of the reservoir. To mimic non-uniform H\({}_{2}\)/CO\({}_{2}\) generation, the perforations along the well provided equal masses of 10 t/day CO\({}_{2}\) and H\({}_{2}\) in alternating sequence to the reservoir. This was implemented by modeling each perforation/well-reservoir cell connection as an individual source. The phase behavior of H\({}_{2}\)-CO\({}_{2}\)-CH\({}_{4}\) was modeled using the Peng-Robinson equation of state provided by the fluid property characterization tool of the commercial reservoir simulator used. The calculated phase behavior was also compared and calibrated with experimental data from the literature [63; 64; 65]. The H\({}_{2}\) and CO\({}_{2}\) generation rates of 10 t/day were chosen based on economic plausibility. After 10 years of gas generation, the operation ceased and the behavior of the generated gases was observed.