Nanoparticle-stabilized microemulsions for enhanced oil recovery from heterogeneous rocks

Surfactant-stabilized microemulsions (MEs) are often used to reduce the capillary forces responsible for trapping residual oil inside rocks. Recent studies showed that the presence of nanoparticles (NPs) in ME could enhance oil recovery, however their interfacial interactions and the impact of rock characteristics is still unclear. The objective of this study was to understand the effect of microemulsions stabilized by nanoparticles (MENP) on pore-scale fluid displacement mechanisms in a heterogeneous rock such as Arkose. A novel method was developed to synthesize silicon oxide in-situ in a ME. These nanoparticles had less tendency to agglomerate compared to nanopowders and promoted the formation of Pickering emulsions. The impact of ME and MENP on oil displacement in Arkose was examined using microtomography integrated with miniature core flooding. Aged cores were subjected to flooding with different aqueous solutions to investigate the effectiveness of ME and MENP in enhancing oil recovery. We found that ME promoted oil mobilization by reducing IFT and enhancing emulsification. The ability of ME to solubilize adsorbed oil layers contributed to a wettability alteration from oil-wet to weakly water-wet. Therefore, ME could remove 20.0% of additional oil after waterflooding. The incremental oil removal with MENP compared to waterflooding (34.3%) was higher than that of ME due to the emulsification of oil into even smaller droplets where NPs and surfactants synergistically interacted at the interface. The small oil droplets could penetrate small capillary elements of the rock that were inaccessible to ME, leading to stronger wettability alteration especially in carbonate cement

Pore-scale dynamics of nanofluid-enhanced NAPL displacement in carbonate rock

This study presents a pore-scale investigation of two-phase flow dynamics during nanofluid flooding in subsurface formations containing non-aqueous phase liquids (NAPLs) such as crude oils. The goal was to gain fundamental understanding of the dominant displacement mechanisms of NAPL at different stages of nanofluid injection in a carbonate rock using x-ray microtomography integrated with a miniature core-flooding system. The nanofluid consisted of surfactant-based microemulsions with in-situ synthesized silica nanoparticles. After establishing its initial wettability state, the carbonate core sample was subjected to various pore volumes (PV) of nanofluid flooding (from 0.5 to 10) to examine the impact on NAPL flow dynamics. We found that most NAPL mobilization occurred within the first PV of injection, removing nearly 50% of NAPL from the rock. The nanofluid invaded into larger pores first due to a sharp decrease in NAPL/brine interfacial tension (from 14 to 0.5 mN/m) and contact angle (from 140 to 88°). With higher amount of nanofluid delivered into the pores through advection, over 90% of NAPL droplets were emulsified and their size decreased from 9 to 3 μm. Subsequent nanofluid injection could further remove NAPL from the smaller pores by altering the thickness of NAPL layers adsorbed on the rock. This dynamic solubilization process reached equilibrium after 5 PV of injection, leading to a reduced layer thickness (from 12 to 0.2 μm), a narrower in-situ contact angle distribution around 81°, and an additional 16% of NAPL removal

Problematic Stabilizing Films in Petroleum Emulsions: Shear Rheological Response of Viscoelastic Asphaltene Films and the Effect on Drop Coalescence

Adsorption of asphaltenes at the water-oil interface contributes to the stability of petroleum emulsions by forming a networked film that can hinder drop-drop coalescence. The interfacial microstructure can either be liquid-like or solid-like, depending on (i) initial bulk concentration of asphaltenes, (ii) interfacial aging time, and (iii) solvent aromaticity. Two techniques--interfacial shear rheology and integrated thin film drainage apparatus--provided equivalent interface aging conditions, enabling direct correlation of the interfacial rheology and droplet stability. The shear rheological properties of the asphaltene film were found to be critical to the stability of contacting drops. With a viscous dominant interfacial microstructure, the coalescence time for two drops in intimate contact was rapid, on the order of seconds. However, as the elastic contribution develops and the film microstructure begins to be dominated by elasticity, the two drops in contact do not coalescence. Such step-change transition in coalescence is thought to be related to the high shear yield stress (~10(4) Pa), which is a function of the film shear yield point and the film thickness (as measured by quartz crystal microbalance), and the increased elastic stiffness of the film that prevents mobility and rupture of the asphaltene film, which when in a solid-like state provides an energy barrier against drop coalescence

Understanding mechanisms of asphaltene adsorption from organic solvent on mica

The adsorption process of asphaltene onto molecularly smooth mica surfaces from toluene solutions of various concentrations (0.01-1 wt %) was studied using a surface forces apparatus (SFA). Adsorption of asphaltenes onto mica was found to be highly dependent on adsorption time and asphaltene concentration of the solution. The adsorption of asphaltenes led to an attractive bridging force between the mica surfaces in asphaltene solution. The adsorption process was identified as being controlled by the diffusion of asphaltenes from the bulk solution to the mica surface with a diffusion coefficient on the order of 10-10 m2/s at room temperature, depending on the asphaltene bulk concentration. This diffusion coefficient corresponds to a hydrodynamic molecular radius of approximately 0.5 nm, indicating that asphaltene diffuses to mica surfaces as individual molecules at very low concentration (e.g., 0.01 wt %). Atomic force microscopy images of the adsorbed asphaltenes on mica support the results of the SFA force measurements. The results from the SFA force measurements provide valuable insights into the molecular interactions (e.g., steric repulsion and bridging attraction as a function of distance) of asphaltenes in organic media and hence their roles in crude oil and bitumen production