Coreflooding without flooding: Buoyancy-based multiphase-flow core analysis for H₂/CO₂ storage sites

Achieving large-scale underground hydrogen storage and carbon-dioxide sequestration is central to the energy transition and climate-neutrality goals. Reliable prediction of multiphase flow in geological formations is essential for the design and safety of such systems and largely relies on accurate estimation of fluid-rock properties. Conventional coreflooding approaches have been widely used to determine permeability and relative permeability for forced-displacement flow; however, they face challenges in directly characterizing the gravity-capillary flow regime. Furthermore, small-scale sub-core heterogeneity should be considered in the property estimation studies and gravity-capillary driven flow should be a focal point, as it prevails in H₂/CO₂ storage far from wells or after injection and production has been terminated, leaving the fluids to migrate due to buoyancy and capillary forces. We present a new buoyancy-based method for estimating three-dimensional permeability (k(x,y,z)) and relative permeability curves (k_r) of core samples at the sub-core scale, without imposing external flow. The approach focuses on gas-water redistribution in a sealed vertical core due to gravity and capillary forces. The method inverts transient and equilibrium saturation fields obtained during the flow using imaging to recover both k(x,y,z) and k_r. Synthetic tests on numerical simulations of H₂-water flow are conducted and show that the permeability field is reconstructed with an error below 4% for almost all cases. k_r curves are also accurately recovered using the new method, with some errors observed for highly nonlinear curves. Parametric analyses show that the method is generally robust and accurate, providing insight on the unique gravity-capillary driven core-flow. The new approach provides complementary information to conventional coreflooding and could support more reliable characterization of geological hydrogen and CO₂ storage sites.