# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 4 Thermal H\({}_{2}\) Generation from Hydrocarbons: Theoretical Basis
### Pyrolysis  
Pyrolysis (thermal cracking) is a thermal decomposition process occurring in the absence of oxygen, which converts light hydrocarbons into elemental carbon and H\({}_{2}\):  
\[\mathrm{C_{n}H_{m}\leftrightarrow nC+m/2H_{2}\ \Delta H>0} \tag{5}\]If the solid carbon (coke) is generated as depicted in Equation (5), it can be considered sequestrated in the reservoir. At the same time, side reactions also occur that consume generated H\({}_{2}\). These reactions mostly include methane-forming reactions like reverse CO\({}_{2}\) reforming:  
\[2\text{CO}+2\text{H}_{2}\leftrightarrow\text{CH}_{4}+\text{CO}_{2}\;\Delta\text{ H}=59.0\;\text{kcal/mol} \tag{6}\]  
Methane pyrolysis is widely reported in the literature as not producing CO\({}_{2}\) because all the carbon is in solid form. In practice, NG is used as a source for industrial processes instead of pure CH\({}_{4}\); hence, other compounds such as CO\({}_{2}\), H\({}_{2}\)O, and higher hydrocarbons are involved in addition to CH\({}_{4}\). These compounds also react during the pyrolysis process, which strongly affects product selectivity and conversion. Various catalysts are known for this process, ranging from metallic catalysts to carbonaceous catalysts. The reactions (2) through (5) generate H\({}_{2}\). The H\({}_{2}\) yield decreases with an increase in hydrocarbon molecular weight, i.e., when the ratio of hydrogen and carbon (H/C) in chemical compounds decreases. Thus, yield decreases in the following sequence: methane -> lighter oil -> heavier oil -> coal. From a thermodynamic standpoint, depending on the conditions, reactions (5) and (6) can both consume H\({}_{2}\) to generate CH\({}_{4}\). This implies that the interplay of these reactions cannot result in an excess of H\({}_{2}\) a priori. The efficiency of H\({}_{2}\) conversion increases with temperature and the use of catalysts. These processes are all at commercial TRL and applied as mature technologies to produce H\({}_{2}\) from fossil fuels at an industrial scale, with reforming and pyrolysis being the most used in fulfilling the actual H\({}_{2}\) demand. The application of these processes in the reservoir to generate H\({}_{2}\) from the remaining hydrocarbons require the direct injection of heat or air/oxygen to burn the hydrocarbons in situ. Assisted by the EOR background, steam/overheated water injection into a reservoir (up to 350 \({}^{\circ}\)C) or the in situ combustion (ISC) of hydrocarbons (up to 700 \({}^{\circ}\)C for oil combustion) are claimed to be technical solutions. However, the question remains whether they can convert the remaining hydrocarbons to H\({}_{2}\) economically. The air requirement (AR) is one of the main parameters driving the efficiency of ISC and is defined as the injected air (or O\({}_{2}\)-rich gas) amount per oil volume recovered (or burned in the reservoir) as a result of the operation. This also determines the compression capacity, therefore influencing the overall project economics significantly. Although reservoir temperatures during steam flooding (SF) thermal EOR are generally lower than in ISC, it is also argued that steam injection can generate H\({}_{2}\) by activating some of the above-given reactions like steam reforming and WGS. One of the economic and technical determinants of SF is the cumulative steam/oil ratio (SOR), which corresponds to the steam required to recover a unit volume of oil. This is generally expressed as water volume converted to steam divided by the incremental oil recovery, both given with the same unit. This falls within the range of 0.1 to 8; the lower the SOR is, the more efficient the SF in a given reservoir is. To expand on the overview of thermal EOR methods that can be used for H\({}_{2}\) generation, some economic features are provided in Table 3 based on the literature data. The same table also includes the costs of H\({}_{2}\) generation with conventional technologies, allowing for a better comparison.