Depleted gas fields storage

Depleted natural gas reservoirs are underground geological structures that naturally contained hydrocarbons and, once depleted, can be used to store gas. Depleted reservoirs consist of porous, permeable sedimentary rocks located underneath an impermeable cap rock and sealed on all sides by impermeable rocks. Depending on the reservoir size and allowable pressures (in some cases a few hundred bars), it may be possible to store up to several billion cubic meters of gas. The injection and withdrawal rates of porous structures are limited by the permeability of the rock, being generally more adequate for balancing seasonal fluctuations, due to their large storage capacity, and less for short-term variations. The proportion of cushion gas in pore storages is typically 50-60% of their total gas capacity, higher when compared to salt caverns. Hydrogen's higher compressibility factor, diffusivity, and lower viscosity should be further evaluated, as it may be more difficult to contain than natural gas. Hydrogen is also more reactive than natural gas, and in the presence of sulphate-reducing bacteria reacts with sulphate-containing minerals to produce hydrogen sulphide, a contaminant, also leading to hydrogen losses. It also reacts with CO2 and carbon-containing minerals in the presence of methanogenic bacteria to produce methane. Therefore, further validation and testing, both in the laboratory and in a real subsurface environment, is required to verify and quantify seal and reservoir integrity, dynamic flow processes (important for fast-cycling injection and withdrawal performance), hydrogen recoverability, geochemical reactivity and potential hydrogen consumption and conversion by microorganisms. Advantages to depleted gas fields as hydrogen storage are that they are larger in volume than salt caverns, and their geology is already well understood from being in operation for natural gas. Compared to the development of new salt caverns, they already have a well infrastructure for natural gas, some of which can be potentially retrofitted or repurposed for hydrogen.