Key phenomenon affecting asset loss are (1) stranding of H₂/CH₄ working gas within the storage formation, (2) microbial conversion of H₂ to less-valuable forms of energy, such as CH₄, (3) diffuse leakage through the caprock, (4) leakage through fault zones, and (5) leakage through wellbores with compromised integrity.

The stranding of H₂/CH₄ working gas is caused by relative-permeability and capillary-pressure hysteresis (Figure 1), which occurs because it takes more pressure for the non-wetting phase (gas) to displace the wetting phase (brine) during the drainage process than it does for the wetting phase to displace the non-wetting phase during the imbibition process. During withdrawal of H₂/CH₄ working gas, pressure is reduced and the pore space that had been occupied by the non-wetting phase is replaced by the wetting phase (brine). This process acts like a check-valve that reduces the number of pathways between the stored H₂/CH₄ working gas and the withdrawal wells, which traps (strands) pockets of the working gas.

Figure 1. Hysteretic capillary-pressure curves (a) for gas-water systems. Hysteretic relative-permeability curves (b) for water-gas systems.

Diffuse leakage through pores in the caprock is limited by two key effects. The first effect results from low permeability of caprock formations, such shale or claystone. The second effect is capillary (or residual) trapping, where the non-wetting phase (H₂/CH₄ gas) is rendered immobile in the pore space, as disconnected ganglia, surrounded by the wetting phase (brine), are controlled by fluid properties and interfacial physics at the pore scale. Because the caprock is saturated with brine, the low-permeability sealing effect of the caprock is further assisted by the capillary-entry pressure, which must be exceeded in order for the non-wetting phase (H₂/CH₄ gas) to be able to displace the wetting phase (brine).

Faults can act as a barrier or a conduit for both liquid- and gas-phase flow. Pressure buildup in the storage formation is an important driving force for upward flow of brine and gas. Buoyancy can play an important role for upward gas migration. Barometric pumping may also induce some vertical gas migration from shallow collector zones to the ground surface. The driving force for buoyant gas flow depends on the H₂/CH₄ mixture, temperature, and pressure. Studies of NG leakage from UGS reservoirs indicate that discrete pathways, such as faults, fault zones, and wellbores with a loss of well integrity are needed for significant leakage to occur. Hysteretic NG-leakage behavior, driven by geomechanical coupling has been observed during injection/withdrawal cycles. During the injection season, overpressure can be sufficient to open up fractures and to drive the upward flow of NG in the fault zone. When pressure drops during the withdrawal season, the fractures close and the lost NG cannot not be pulled back to the storage formation due to a one-way “check-valve” effect, which causes the stranded NG to accumulate above the caprock horizon.