(213a) Highly Stable Nitrogen-in-Brine Foams with Surface-Modified Silica Nanoparticles for Enhanced Oil Recovery

Objectives/Scope

The design of nitrogen and CO₂ foams with longer lifetimes would enable more effective gas mobility control and diversion into low permeable regions to raise the sweep efficiency in enhanced oil recovery (EOR). The objectives are to show that foams formed with functionalized nanoparticles (NPs) with surfactants are more stable than foams stabilized with surfactants alone in porous media, and to explain the behavior in terms of the rheological and interfacial. The surface modification on 20 nm silica NPs, upon balancing hydrophilic versus hydrophobic grafted ligands, provides NP colloidal stability in in concentrated brine (up to 15% total-dissolved-solids) as well as nanoparticle adsorption at the interface required for strong foams. The NP surfaces are designed to form weak gels at the interface and in the continuous aqueous phase to enhance foam stability for several days. In addition, these foams are sufficiently viscoelastic to function as energized fracture fluids.

Methods, Procedures, Process

Surface modification of silica NPs were conducted with low molecular weight ligands through silylation to achieve high nanoparticle adsorption at the brine-gas interface. Furthermore, the weak gelation of NPs in the aqueous phase was characterized with the dilational and shear modulii at the interface and the bulk rheology. Core flood experiments modelled with glass beadpacks were performed to study the foam generation and apparent viscosity at reservoir conditions. The morphology and stability of the foam were characterized in terms of the bubble sizes changes over time through microscopy.

Results, Observations, Conclusions

The high salinity and the proper ratio of hydrophilic to hydrophobic ligands on the surface produced weak attractive interactions between NPs resulting in weak elastic gels as determined from the interfacial rheology and bulk shear moduli. In the core flood experiments, apparent viscosities reached 95 cP in 1-Darcy porous media at 3000 psi and temperatures up to 90°C. Upon addition of surfactants to further reduce the interfacial tension, the minimum pressure gradient for foam generation could be significantly lowered. Foam stability against coalescence and coarsening was improved by the weak gels and irreversible nanoparticle adsorption, with negligible changes in foam bubble size over 48 hours.

Novel/Additive Information

Novel functionalized nanoparticles with high colloidal stability in concentrated divalent brines and strong irreversible adsorption at the gas-brine interfaces have enabled the design of highly stable foams at harsh reservoir conditions. The development of mechanistic insight from the NP surface chemistry to the interfacial and rheological properties to the stability of the foam in porous media is of great interest for designing formulations to improve the sweep efficiency in EOR.