Fikri Oğuzhan ŞENGÜL
The world energy demand has significantly risen every passing day. The unstoppable energy demand leads the science world and governments to the storage of energy. Therefore, it is undeniable that hydrogen storage is a hot topic these days. Although conventional energy comprises energy requirements remarkably, renewable energy and green energy investments are increasing under the leadership of developed countries due to global warming. Hydrogen has a nonnegligible role in meeting energy needs at this stage because it can be produced anywhere without CO2 emissions. That’s why it is considered green energy. As a result, the hydrogen storage process has taken attention more in the last decade. In this study, firstly, the hydrogen properties are explained in detail. Then, the influencing parameters of underground hydrogen storage are clarified.
Today, the production of conventional energy such as fuel oil, natural gas, and coal is cheaper than renewable energy. However, the consumption of conventional energy is more harmful than renewable energy. Hence, the importance of renewable energy has increased excessively because it produces less CO2 emissions.
It is debated whether hydrogen energy is renewable or not in recent years. Some authors claim that hydrogen energy is not renewable energy since it could not be used sustainably. On the other side of the coin, others assert that if the electricity comes from renewable energy for the electrolysis process, electric current is used to split hydrogen from oxygen, the hydrogen fuel should be called renewable or green energy. In this study, we call green energy for hydrogen fuel.
Firstly, hydrogen properties are explained in detailed. Physical and chemical properties are defined briefly. Then, important parameters of underground hydrogen storage are clarified gradually. This part is divided into three; solid properties, fluid properties and solid-fluid interactions. Finally, the main conclusion and future work are given.
Understanding the physical characteristics of hydrogen helps in the operation of safe and successful storage. When compared to carbon dioxide or methane, hydrogen gas has a wide range of physical characteristics and properties. It is colorless, odorless, tasteless, combustible, and non-toxic. Furthermore, hydrogen, the most abundant chemical substance in the universe, is the lightest element. Because of the less dense, the same amount of hydrogen needs more pressure than natural gas or CO2. For example, methane is eight times denser than hydrogen at ambient temperature. This emphasizes the significance of storage capacity in a hydrogen storage operation. In addition, hydrogen has a lower viscosity. Thus, it is less likely to cause a coning problem.
Solubility is the crux of hydrogen storage in saline aquifers or depleted oil and gas reservoirs. Hydrogen has a lower solubility than methane. Therefore, lower solubility is considered a crucial benefit since less hydrogen loss that occurs during the dissolution process is expected in a hydrogen-methane-brine system. In other words, hydrogen dissolves in water; therefore, hydrogen loss occurs significantly. In addition to this, the presence of salt reduces solubility in the brine. That’s why the water-hydrogen-salts systems is the most common storage. Indeed, the rate of hydrogen dissolving depends on the pressure, temperature, and contact area. Hence, it can vary with situation.
Hydrogen has a diffusion coefficient three times greater than methane in pure water at normal pressure and temperature because hydrogen has low molecular weight. As a result, hydrogen leaks to the surface handily across overburden layers. Despite the fact that the effective diffusion coefficient is smaller than that of pure water, the size of the effective diffusion coefficient makes the situation more acceptable for leakage potential. The most frequent physical characteristics of hydrogen are shown in Figure 2.
Depending on the temperature and pressure, hydrogen particles H2 can exist in a variety of states. According to Figure 3, at -262 °C, hydrogen is a solid at low temperatures, having a density of 70.6 kg/m3. It is a gas at higher temperatures, with a density of 0.089 kg/m3 at 0 °C and a pressure of 1 bar . Because hydrogen is the smallest chemical particle known, gaseous H2 has a high penetrability; it diffuses in solids several times quicker than, for example, methane. This might pose issues during storage in salt caves. The presence of water in the pore space of the rocks improves hydrogen tightness in aquifers and depleted hydrocarbon deposits, combined with a low solubility of hydrogen in water, equal to 0.00018 mol/mol at 25°C and a pressure of 100 bar, and a low diffusion coefficient of the order of 10^(9)m2/s in pure water and 10^(–11)m2/s in water-soaked argillaceous rocks. Because salt caves are inherently dry buildings, there may be issues with hydrogen diffusion through the salt walls.
Another possible issue may arise as a consequence of chemical reactions when hydrogen comes into contact with ambient rocks. Under atmospheric pressure and in the absence of catalysts, reactions in the mineral matrix appear to occur very slowly at temperatures below 100 °C. Moreover, when a metal pipeline is exposed to hydrogen for an extended period of time, especially in high concentrations and at high pressure, its material durability might be greatly reduced. Hydrogen blistering, hydrogen-induced cracking, and hydrogen embrittlement are all terms used to describe the effect of hydrogen on the characteristics of steel alloys. Other possible issues stem from hydrogen leaking via pipe walls. In a typical polymer pipe used in natural gas distribution systems, the permeability index for hydrogen is four to five times higher than that for methane. The loss of natural gas and hydrogen mix 60:40 calculated for US pipe installations, on the other hand, would account for just 0.0002 percent of the entire quantity of gas utilized in the US. As a result, such leaks are seen as economically negligible.
3.Influencing Parameters of Underground Hydrogen Storage
There are some factors affect hydrogen storage which are solid properties, fluid properties and solid-fluid interactions.
3.1 Solid Properties
3.1.1 Absolute permeability (ka)
It is a measure of the ease with which a single fluid can flow through the reservoir rock. Permeability (k) is a measure of the ease with which a porous material will transmit fluid. Darcy law is used by determining permeability values.
As a result, injecting H2 into (or recovering H2 from) highly permeable storage sites is more energy efficient.
3.1.2 Effective porosity
The percentage of void space in a rock is known as porosity. The ratio of the total pore volume to the bulk volume is known as absolute porosity, whereas the ratio of the interconnected pore volume to the bulk volume is known as effective porosity. The connected end pores are predicted to contribute to H2 flow in UHS due to effective porosity. Hydrogen storage capacity (C) at the reservoir scale is expressed as:
3.2 Fluid Properties
3.2.1 Fluid density
Because of geothermal and hydrostatic gradients, pressure and temperature rise with formation depth. Therefore, we should observe effects of temperature and pressure on fluid density. That is, the density of H2 increases dramatically with pressure and decreases moderately with temperature.
3.2.2 Fluid Viscosity
More critically, H2 is one to two-fold smaller than H2O as viscosity value. Fluid viscosity ratio determines fluid-fluid interfacial stability and viscous fingering, particularly when combined with relative permeability ratio. The unstable displacement of a more viscous fluid by a less viscous fluid is known as viscous fingering. The fingering of an injection fluid into an in-situ fluid can have a negative impact on reservoir flow behavior and recovery.
3.2.3 Fluid-Fluid interfacial tension
In the context of fluid-rock interactions, it is especially essential since it determines capillary pressures. The interfacial tension between hydrogen and water is nearly pressure independent. However, this tension drops dramatically as the temperature rises.
3.2.4 Solubility of Hydrogen
At ambient conditions, H2 solubility in H2O was clearly lower than at UHS conditions. Consequently, H2 solubility measurements taken under ambient settings cannot be used in UHS planning. In depleted oil reservoirs, H2 loss owing to dissolution trapping is projected to be significantly less than in aquifers.
3.2.5 Hydrogen diffusivity
H2 diffusivity increases with increasing temperature and decreasing pressure drastically. H2 loss due to diffusion in deep aquifers and depleted hydrocarbon reservoirs could be significant.
3.3 Solid-Fluid Interactions
Because of the heterogeneous rock surface chemistry, mineral composition, and pore geometry, H2-rock wettability is dispersed irregularly in the subsurface. C, PC, SH2, residual H2 saturation, injectivity, and containment security are all determined by H2-rock wettability.
3.3.2 Solid-fluid interfacial tension
Young’s equation explicitly links solid-fluid interfacial tension to rock wettability. Because clean quartz does not occur in the subsurface, the interfacial tension between stearic acid aged quartz and H2 is more indicative of UHS conditions.
3.2.3 Capillary pressure and capillary forces
PC determines the amount of brine displaced (and hence H2 injected) as well as the pressure necessary for this action. As a result, PC is a significant factor in determining the feasibility of a UHS project in general. Furthermore, PC determines structural trapping and, as a result, H2 storage capacity and containment security. The buoyancy is calculated assuming that both formation brine and H2 exist as a continuous phase.
3.2.4 Relative permeability
kr is a function of Sw and the wettability of the rock. In comparison to CO2 and CH4, H2 has a relatively weak ability to move through brine saturated sandstone. The specific (geological) sandstone heterogeneity was blamed for the low krH2.
3.2.5 Mobility ratio
With a large Mobility ratio, viscous fingering occurs, resulting in low H2 sweep efficiency and ineffective formation brine displacement by H2.
In high surface area systems like coal and shale, gas adsorption-desorption is often crucial. For CO2, CH4, and N2, these mechanisms are well understood, while data on H2 is rare.
4.Conclusion and Future work
To sum up, hydrogen has unique properties and hydrogen storage relies on some important conditions. Those properties are familiar from the oil industry. However, the technique are not the same. Therefore, the energy industry has some concerns about the feasibility and economic analysis. It is easy to produce hydrogen, but crucial point is storage of hydrogen. It has high cost and unfeasibility. If we want to have green energy for zero carbon emission and energy independent, it must be feasible and profitable. For the next study, we will focus on economic aspect and hydrogen storage in geological structures.
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