Inherent Errors in Current Core-Flooding Relative Permeability Data for Modelling Underground Hydrogen Storage

In the design and optimisation of an underground hydrogen (H2) storage facility in an aquifer or other reservoir system, the H2/water relative permeabilities (RP) are the most critical two-phase data for input to numerical simulation. In this paper, we present a critical analysis of the published experimental H2/water RP functions in the literature. We present fine-grid simulations of H2 displacing water (denoted H2 à water) using three of the most widely cited RP datasets (Yekta et al., 2018; Boon and Hajibeygi, 2022; Lysyy et al., 2022) in a mildly heterogeneous permeability field, at a field length scale of ~100m. Since the viscosity ratio between water and H2 is (mw/ mH2) ≈ 70, then at some length scale above a few metres, it is inevitable that the system must show immiscible viscous fingering of the H2 into the water phase. Indeed, the emergence of viscous fingering at some length scale is a “sense check” that the input data used in the simulations are correct, especially the H2/water relative permeability functions. In fact, none of the three published H2/water RP curves leads to viscous fingering. Instead, they all show stabilised flood fronts. The reasons for this are due to shortcomings of the (conventional) gas/liquid experimental methods used to obtain the RP functions. These methods yield RP functions at the wrong force balance between the capillary, gravity and viscous forces. For fingering to emerge, it is necessary to derive the viscous dominated RP functions. An alternative, more physically appropriate set of viscous dominated “fingering RP functions” is proposed and applied. When applied at the core scale, these new RP functions show fully dispersed flow due to capillary dispersion, as they do when applied in a vertical (downwards) gravity stable displacement. However, the viscous fingering emerges naturally in horizontal flow as the length scale of the system increases and viscous (and gravity segregation) forces become dominant.