# Hydrogen from Depleted/Depleting Hydrocarbon Reservoirs: A Reservoir Engineering Perspective
## 3 Microbial H\({}_{2}\)-Bio-H\({}_{2}\) Generation in DORs
### Concept
Based on available data and considering that molecular oxygen (O\({}_{2}\)) is rapidly removed by microbial and/or chemical processes, it is suggested that only strict anaerobic bacteria can be considered truly indigenous to oil reservoirs [37]. This suggestion is supported by many studies that confirm the widespread presence of anaerobic biospheres in oil reservoirs [38; 39]. The concept of generating bio-H\({}_{2}\) involves using the reservoir as a bioreactor, with various metabolic pathways including fermentation, anaerobic hydrocarbon degradation, acetogenesis, methanogenesis, and other anaerobic respiration processes such as nitrate or iron reduction, as well as the reduction of oxidized sulfur species, in addition to DF. Although the idea is relatively new, the production of H\({}_{2}\) in a reservoir due to fermentative microorganism reactions is known from the literature related to microbial enhanced oil recovery (MEOR) [40; 41]. Recent studies have reported experiments mimicking the process in the reservoir, suggesting field trials [11; 12]. The H\({}_{2}\) yields obtained in these studies are also provided in Table 2. As shown in the table, using oil as a standalone carbon source is not promising in terms of H\({}_{2}\) generation. However, in a study, when 1 g/L of glucose and 25 mg/L of a surfactant were added to improve the interaction of oil with aqueous phases (thus making hydrocarbons more accessible to microorganisms in the aqueous phase), H\({}_{2}\) generation was enhanced compared to the case in which oil was the only substrate [11]. The following results can be derived from the table and related references. Many different microorganisms are capable of initiating DF. In reservoirs where the temperature is less than 80 \({}^{\circ}\)C, it is common to find one or a community of such microorganisms. Their physiology and mechanisms of action are well studied in the related industry. However, the microbiology of reservoirs is field-specific and should be analyzed to determine their potential for H\({}_{2}\) generation. H\({}_{2}\)/sucrose yields slightly higher than 4 (in mol basis), as reported in a few references, are due to the use of other carbon sources and additives such as yeast. However, the addition or use of low-quality carbohydrates (such as molasses and starch) decrease the H\({}_{2}\) yield. Using in situ hydrocarbons as standalone carbon sources is not effective for generating bio-H\({}_{2}\). However, adding low concentrations of glucose and surfactants in the aqueous phase can increase the yield. It should be noted that the yield is still lower than that obtained with the maximum possible carbohydrate concentration. Aquifers containing beneficial bacteria (e.g., Enterobacteriaceaeae and Thermoanareobacterialriumas discussed previously) and thermophysical conditions can also be used for bio-H\({}_{2}\) generation with comparable yields. The high pressure in the reservoir reduces microbiological activity, thus decreasing H\({}_{2}\) generation. Unfortunately, there is not enough conclusive dedicated dynamic data (corefloods) supporting the H\({}_{2}\) generation in a DOR; however, data from MEOR studies confirm H\({}_{2}\) generation in porous media, although not as much as the results of batch experiments as indicated in Table 2[42].