Experimental Investigation of Nanoparticle-Enhanced In-Situ Emulsion Generation and Flow Distribution in Porous Media

Abstract

A persistent gap in the nanoparticle-enhanced oil recovery (NP-EOR) literature is the absence of a systematic framework that connects the bulk physicochemical properties of nanoparticle–surfactant formulations to their actual displacement performance in porous media, particularly when tested against viscous field crude oils. This study addresses that gap by evaluating three metal oxide nanoparticle systems - titanium dioxide (TiO2, anatase, 10–30 nm), aluminium oxide (Al2O3, alpha, ~40 nm), and nickel oxide (NiO, ~50 nm) - each formulated with sodium dodecyl sulphate (SDS, 2,000 ppm) in low-salinity brine (5,000 ppm NaCl) and tested against a real heavy crude oil (277 mPa·s, API 24.63°) from the Pre-Caspian Basin, Kazakhstan, in Upper Berea sandstone cores. A multi-technique screening programme spanning 500–2,000 ppm nanoparticle concentrations encompassed zeta potential measurement, UV–Vis spectrophotometric sedimentation monitoring, pendant-drop interfacial tension (IFT) measurement, optical microscopy with ImageJ-based droplet size quantification, and rheological characterisation of nanofluids and generated emulsions across 25–60°C; core flooding experiments in tertiary and secondary injection modes were preceded by capillary desaturation curve construction to ensure capillary-dominated displacement conditions. Colloidal stability followed the order NiO (ζ = −31.89 mV; UV–Vis retention 78% at 168 h) > TiO2 (−25.59 mV; 45%) > Al2O3 (−23.65 mV; 27%), governed by differential SDS adsorption onto positively charged nanoparticle surfaces. All formulations exhibited near-Newtonian viscosity of approximately 1 mPa·s, ruling out viscosification as a recovery mechanism. The NiO–SDS system reduced oil–water IFT from 23.60 mN/m (SDS alone) to approximately 1.2 mN/m in diluted measurements, consistent with synergistic co-adsorption producing a Pickering-type interfacial film. In tertiary core flooding, NiO nanofluid achieved 32.87% incremental recovery (total recovery factor 89.51%), with effluent analysis indicating in-situ emulsification producing fine O/W droplets with a median diameter of approximately 5 μm sustained over more than 2 pore volumes, alongside IFT reduction and moderate pore-scale flow diversion (resistance factor 1.03). Al2O3 nanofluid yielded 16.80% incremental recovery with a progressively and disproportionately increasing differential pressure over more than 7 pore volumes, a pattern more consistent with mechanical pore plugging driven by poor colloidal stability than with productive emulsification. TiO2 nanofluid produced a resistance factor of 0.28, with differential pressure falling below the waterflood baseline throughout injection, suggesting probable wettability alteration toward more water-wet conditions, though direct contact angle confirmation was not obtained. The central finding of this study is that bulk emulsion droplet size is not a reliable predictor of in-situ EOR performance: NiO, which generated the coarsest bulk emulsions at 1,000 ppm, produced the finest and most dynamically regenerative in-situ emulsions during core flooding, demonstrating that interfacial film strength and colloidal stability - rather than bulk droplet size - are the governing screening parameters for pore-scale emulsification efficacy. This work proposes a structured physicochemical screening-to-performance framework as a practical basis for pre-selecting nanoparticle formulations for EOR evaluation.