Pore-scale insights for steady-state two-phase flow in natural porous sandstone from pore connectivity characterization

Abstract

Multiphase fluid transport in the subsurface natural porous sandstone governs numerous energetic industrial, and environmental activities. In this work, a nanometer-millimeter pore connectivity quantification is compiled by integration of multiple scale pore structure characterization techniques involving casting thin section (CTS), scanning electron microscope (SEM), X-ray computed tomography (X-μCT), Nuclear magnetic resonance (NMR), pressure-controlled porosimetry (PCP), and rate-controlled porosimetry (RCP), whereby the pore connected pattern, connective ratio, and connected full-range pore size distribution (CPSD) are obtained, upon which the controlling mechanisms for the steady-state two-phase flow (STPF) physics are further explored by incooperating physical displacement experiment. Connectivity evaluation indicates that high permeable sandstone shares a reticular connection network with scale-invariant connected ratio stays at around 0.60, low-permeability sandstone exhibits branch-like pattern with the ratio ranging from 0.53 to 0.60, while tight sandstone is characterized by local chain-like pattern with an average ratio of 0.31. Deviated Darcy linear and power-law flows present successively in the fractional non-wetting phase flow in STPF with reducing connectivity degree. Wetting phase mobility, dynamics of multiphase interaction, dynamic expansion of non-wetting phase flow path interpreted based on the CPSD, incorporating DLVO theory, augmented Young-Laplace equation, and effective hydraulic radius model, explain the pore-scale controls for the flow physics. The CPSD determines multiphase fluid mobility potential and dynamics of multiphase interaction, controlling the accessibility and expansion of flow pathway of non-wetting phase. Preferential non-wetting phase flow path expansions in the outer layer and inner layer of bound water film zone held by interfacial force and capillary force, and accompanying flow resistances in the connected pores <1000 nm control the flow regime primarily. The pores of 30–50 nm in the flow paths are responsible for threshold pressure gradient (TPG), pressure disorders, and snap-offs during non-wetting phase intrusion, resulting in power-law deviations for the fractional flow of non-wetting phase. A dynamic fractional non-wetting phase flux prediction model is proposed by modifying fractal-based Hagen-Poiseuille equation considering flow physics, pore heterogeneity, and critical percolation length scale variations along with connected flow path expansion.