Investigation on the unsteady-state two-phase fluid transport in the nano-pore system of natural tight porous media

Abstract

Fluid transport across the natural tight micro-nanometer porous media governs numerous subsurface geological and industrial activities. The unsteady-state two-phase fluid flow at the very initial stage of non-wetting phase fluid intrusion, governing the hydrocarbon accumulation and CO2 geological sequestration process, remains uncertain. A modified unsteady-state pressurization gas displacement experiment incorporating systematic pore structure description combining mercury porosimetry, nuclear magnetic resonance, and X-ray computed tomography is conducted on the tight sandstone cores to investigate the fluid flow behaviors and to uncover their pore-scale controls. The results indicate that the unsteady-state gas–water flow deviates from Darcy’s law, in which the threshold for the onset of continuous non-wetting phase fluid intrusion (ISTP) can be observed and degree of water movement and gas intrusion strongly depend on the injection pressure. Analyses under circular pore assumption, water layer distribution model based on DLVO theory, effective flow assumption, and overall pore connectivity evaluation suggest that the water movement and gas accessibility increases are dominated by the water displacement in the free water layer zone when injection pressure < ISTP, while the criticality of continuous gas intrusion is determined by the water movement in the inner layer of weakly bound water zone located in the narrow parts of the connected pathways, especially that in the pores < 40 nm, but its increment mainly occurs in the out layer of the weakly bound water zone when injection pressure > ISTP. Distinctions in the distributions and proportions of free and bound water layers in the different connected pore systems lead to the significant variations in the ISTP, water mobility, and gas accessibility of tight sandstone. The primary reasons for the differences in the static fluid mobility and flow behaviors lay in the differed structural attributes of connected pore systems. The narrowing, heterogeneity increase, and actual flow length increment of the connected pathways will raise the resistance for continuous non-wetting phase flow and lead to losses in water mobility, accessibility of non-wetting phase fluid, and flow velocity in the unsteady-state two-phase fluid flow. The high sensitivity of the dynamic fluid flow and gas accessibility to tortuosity of the connected pore system suggests the unsteady-state two-phase fluid flow behaviors are under the coupled control of pore size, complexity, and heterogeneity.