# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 1 Introduction  
Renewable hydrogen plays a critical role in energy transition as it can replace fossil fuels (such as gas, coal, and oil) in industries and transportation that are challenging to decarbonize [1]. The EU report has set targets for both domestic hydrogen (H\({}_{2}\)) production and imports that align with the zero-emission goal of the International Energy Agency (IEA) by the end of 2050. The plan will be enhanced by establishing H\({}_{2}\) infrastructure for the production, transport, and trade of 20 million tons of H\({}_{2}\) by 2030 [1]. In the USA and other industrialized nations, the importance of H\({}_{2}\) in the energy sector is set to grow as these countries establish ambitious goals to increase its usage. For example, the Department of Energy (DOE) in the USA announced the \"Hydrogen Shot, 1-1-1\" initiative, aiming to reduce the cost of clean H\({}_{2}\) to 1 USD per 1 kg of H\({}_{2}\) within a decade. This target represents an 80% reduction from its current estimated average cost of 5 USD per kg [2].  
Hydrogen is the most abundant element in the universe and has the maximum energy content on a mass basis compared to any other known fuel. Using H\({}_{2}\) as an energy carrier is environmentally friendly as it does not produce any harmful emissions. The only byproducts generated are heat and water [3; 4; 5]. In today's industry, H\({}_{2}\) is predominantly produced from fossil fuels such as natural gas (48%), oil (30%, mainly consumedin refineries), and coal (18%). Several processes are used to generate H\({}_{2}\), including thermal cracking, catalytic cracking, and the most common one, steam methane reforming (SMR). All of these processes, however, produce carbon dioxide (CO\({}_{2}\)) alongside H\({}_{2}\), raising concerns about their compatibility with climate protection efforts. A modern SMR plant, for instance, produces approximately 9 kg CO\({}_{2}\) per kg of H\({}_{2}\)[5; 6]. The proportion of the CO\({}_{2}\)-neutral H\({}_{2}\) production from renewable resources (green H\({}_{2}\)), such as in water formed by electrolysis or bioprocesses, is currently small (2% of overall H\({}_{2}\) production in 2020). The current price of H\({}_{2}\) with a fossil origin, including the removal of co-produced CO\({}_{2}\), is approximately 1-3 USD/kg (SMR and other conventional technologies) [5; 7]; the cost of green H\({}_{2}\) generation technologies is as high as 10 to 20 USD [5; 8].  
In addition to enhancing the conversion efficiency of green H\({}_{2}\) production methods, researchers are also working on making fossil fuel-based H\({}_{2}\) production more environmentally friendly by reducing CO\({}_{2}\) emissions. Within this framework, these technologies are integrated with geologic carbon storage (GCS) to safely store the captured CO\({}_{2}\) in specific geological formations. GCS transforms grey H\({}_{2}\) production, where CO\({}_{2}\) is released into the atmosphere, into blue H\({}_{2}\), which is considered climate-neutral. In a subsequent step, researchers are exploring the possibility of generating H\({}_{2}\) in situ within depleted or depleting hydrocarbon reservoirs (DHRs) while permanently storing the co-generated CO\({}_{2}\) in the reservoirs to eliminate emissions.  
The objective of this paper is to provide a brief overview of the technologies that can be used to extract H\({}_{2}\) from DHRs in a variety of ways. We assess the claimed processes from a reservoir engineering perspective, emphasizing their H\({}_{2}\) generation potential and technology readiness level (TRL) for applications. We also investigate whether it is possible to leave the co-produced CO\({}_{2}\) permanently in the reservoir in order to reduce emissions. Additionally, we provide a preliminary cost analysis to compare these methods with traditional H\({}_{2}\) generation techniques, along with an evaluation of operational risks and their associated cost assessments.