# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 6 H\({}_{2}\) Generation in DORs  
### Concept  
Due to its claimed higher potential, the generation of H\({}_{2}\) is addressed more from DORs than DGRs in the literature [16, 73]. The in situ reforming of crude oils (heavier hydrocarbons according to reactions 2-6) to generate H\({}_{2}\) necessitates higher temperatures (>700 \({}^{\circ}\)C) that can only be achieved with the ISC process in the reservoir; SF cannot deliversuch high temperatures. There have been some attempts to enable heavy-oil decomposition at lower temperatures using oil-based nanoparticles in steam application [74]. In ISC, a \"fire\" is generated in the reservoir and sustained by injecting O\({}_{2}\)-rich gas/air continuously. To start the fire in the reservoir, an ignition process is required. The ISC can be applied in forward and reverse modes whereas only forward ISC (or high-pressure air injection, HPAI) is applied commercially. In forward application, the fire front advances in the reservoir and is pushing a mixture of combustion gases, steam, hot water, and mobilized oil towards the production well(s). The forward ISC is further categorized into \"dry\" and \"wet\" ISC according to the injection fluid being used to sustain the process: air for dry ISC and air and water for wet ISC. In the reverse process, also called countercurrent ISC, the front moves in a direction opposite to that of air flow [52; 75; 76]. A typical temperature profile for dry ISC with frontal advancement is shown in Figure 5, depicting the main saturation zones generated by the process. Figure 5 illustrates the advancement of the convective wave whereby points A and B mark the positions of the ISC front and the so-called convection point, respectively. Most of the heat generated during the ISC process is stored/compressed between these two points, also referred to as the burned zone. The incentive to switch to wet ISC is related to the preference to reduce this amount of heat and transfer as much heat as possible to the zone ahead of the ISC front where the oil is. Basically, ISC involves the oxidation of the crude oil (reaction 4); however, it includes a combination of reactions that take place sequentially and/or simultaneously. These reactions are generally lumped in two groups: low-temperature oxidation (LTO) and high-temperature oxidation (HTO) reactions. Based on ISC experiments, it has been concluded that most of the O\({}_{2}\) is consumed in LTO (around 250-300 \({}^{\circ}\)C), forming a coke-like residue. In the HTO mode, which generally occurs above 350 \({}^{\circ}\)C, the products of combustion are primarily carbon oxides and water. The fuel for these reactions is the immobile hydrocarbon fraction normally designated as coke formed in LTO. The process requires control over the propagation of the reaction (or combustion/heat) front, which is provided by the proper adjustment of the O\({}_{2}\) supply. This is not straightforward as the reaction front prefers the direction of higher oil saturation (fuel) whereas the injected O\({}_{2}\) tends to flow towards the zones of higher gas permeability/saturation. There is limited laboratory work that mimics the process of using ISC for H\({}_{2}\) production, and existing studies are far from reflecting real-world reservoir conditions. To date, no field-scale experiment has been published, except for some studies reporting H\({}_{2}\) production during ISC field applications. However, at least one patent exists on the topic [66]. [PERSON] et al. 's paper reports the production of H\({}_{2}\) in concentrations as high as 20% (molar basis) in the gas streams of production wells in a Marguerite Lake in situ field application in eastern Alberta [77]. They concluded that the H\({}_{2}\) is generated by the coke gasification reaction (4) followed by the WGS reaction. In their successive papers, [PERSON] and [PERSON] report experiments of in situ EOR processes also generating H\({}_{2}\)[73; 78].