# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 2 Depleted/Depleting Hydrocarbon Reservoirs (DHRs) for H\({}_{2}\) Generation  
Oil and gas reservoirs typically become depleted within a few decades, with the exact timeframe depending on factors like a reservoir's size, its characteristics, and the rate at which it is being depleted. Natural depletion refers to the process of producing all of the recoverable hydrocarbons using the energy of the reservoir. However, due to thermodynamic and petrophysical limitations, a portion of the hydrocarbons initially present in the reservoir cannot be produced and remain there; in depleted oil reservoirs (DORs), 50% to 90% of the original oil in place (OOIP) remains unproduced. In contrast, primarily due to the higher compressibility of the gas, depleted gas reservoirs (DGRs) have a significantly lower percentage of unproduced resources, ranging from 10% to 40%. Enhanced oil recovery (EOR) methods are employed to extract a portion (5% to 20%) of the remaining oil in the reservoir by injecting chemicals and/or energy from the surface, thereby improving production efficiency. In fact, there would still be a reasonable amount of oil left in the reservoir. In gas reservoirs, the methods used to recover the remaining gas after natural depletion are often considered not economically viable because of the additional costs of separating the natural gas (NG) from the injected gas. Within the context of optimizing and decarbonizing the remaining energy resources, the further utilization of depleted hydrocarbon reservoirs (DHRs) is a topic frequently discussed in the literature. A common proposal is to use the \"emptied\" reservoir volumes for CO\({}_{2}\) storage in an effort to reduce the carbon footprint of various industries. This type of geologic carbon storage (GCS) has evolved into its own industry. The technology has been discussed and developed for many decades, gradually reaching an industrially applicable level. Reportedly, a CO\({}_{2}\) storage capacity of approximately 75 million metric tons per year (Mt/year) will be achieved by 2024 in current and under-construction GCS projects, including the reuse of DHRs [9]. The global geological CO\({}_{2}\) storage resources in DHRs are estimated at up to 1000 gigatons (Gt) of CO\({}_{2}\) ([PERSON] et al. 2023) [10]. The related literature discusses alternative solutions for optimizing the use of DHRs in energy transition efforts. One proposed solution appears to offer a twofold benefit: producing H\({}_{2}\) in situ using the remaining hydrocarbons and leaving the co-generated CO\({}_{2}\) in the reservoir. Table 1 summarizes the three technologies discussed in the literature that are applied in this context, along with relevant references for each. The principles of these technologies are similar to those used for H\({}_{2}\) production from hydrocarbons in surface installations. A common feature among all these technologies is the injection of an energy source from the surface, allowing sufficient time for in situ reactions to produce gases including H\({}_{2}\) and CO\({}_{2}\). The lighter H\({}_{2}\) is then separated from heavier components such as CO\({}_{2}\) either in situ or through separation techniques at the surface or downhole. The H\({}_{2}\) is then transferred to the place of use. The schematic overview of generating H\({}_{2}\) from DHRs is depicted in Figure 1.  
A substrate to feed the microorganisms in the reservoir is injected from the surface or the reservoir is heated with injected energy to initiate a steam flooding or in situ combustion process. The injection triggers the proposed reactions in the reservoir, resulting in H\({}_{2}\) and