Description of the material. Stable CO2/water (C/W) foams at high temperatures and salinities have been achieved with substituted amines in limestone, sandstone and glass bead packs with permeabilities from 1 to 78 Darcy. Foams were formed upon injection of the CO2 soluble surfactant in the CO2 phase and would be beneficial for improving sweep efficiency in EOR process.Application. Despite significant interest in CO2 foams for EOR, very few studies have reported stable foams at high temperatures (120 °C) and high salinities, which are often encountered in the Middle East and elsewhere. The foams provide mobility control and stabilize the displacement front in CO2flooded zones to improve sweep efficiency.Results, Observations, Conclusions. The amine surfactants are switchable between the nonionic and cationic states with pH or the nature of the solvent. They exhibit nonionic behavior when introduced in the CO2 phase, which favors injectivity, and cationic in the presence of concentrated brine with dissolved CO2. The hydrophilic/lipophilic balance of the amines was tuned by modification of the amine head group or tail length to design strong foams. It was important to increase the basicity of surfactants to enhance the solvation in the aqueous phase over a pH range of 4 to 7. These surfactants were effective in lowering the interfacial tension between water and CO2 at high temperature and salinity. They generated viscous C/W foams in limestone, sandstone and glass bead packs at 120 °C in the presence of 22% TDS brine when surfactants were injected from either the aqueous or CO2 phase. At pH below 6, these surfactants exhibited low oil/water partition coefficients on the order of 0.1 which suggests that these surfactants will have minimal retardation due to partitioning into oil in the EOR process.Significance of Subject Matter. These surfactants stabilized C/W foam at high temperature and salinity, and partitioned to the water phase over dodecane phase for efficient surfactant utilization. The high solubility in CO2 is beneficial for the surfactant to be available along CO2 flow pathways in a reservoir to minimize viscous fingering and gravity override.
Foams used for mobility control in CO2 flooding, and for more secure sequestration of anthropogenic CO2, can be stabilized with nanoparticles, instead of surfactants, bringing some important advantages. The solid nature of the nanoparticles in stabilized foams allows them to withstand the high-temperature reservoir conditions for extended periods of time. They also have more robust stability because of the large adsorption energy required to bring the nanoparticles to the bubble interface.
Silica nanoparticle-stabilized CO2-in-brine foams were generated by the co-injection of CO2 and aqueous nanoparticle dispersion through beadpacks, and through unfractured and fractured sandstone cores. Foam flow in rock matrix and fracture, both through Boise and Berea sandstones, was investigated. The apparent viscosity measured from foam flow in various porous media was also compared with that measured in a capillary tube, installed downstream of beadpacks and cores.
The domain of foam stability and the apparent foam viscosity in beadpacks was first investigated with focus on how the surface wettability of nanoparticles affects the foam generation. A variety of silica nanoparticles without any surface coating and with different coatings were tested, and the concept of hydrophilic/CO2-philic balance (HCB) was found to be very useful in designing surface coatings that provide foams with robust stability. Opaque, white CO2-in-water foams (bubble diameter < 100 µm) were generated with either polyethyleneglycol-coated silica or methylsilyl-modified silica nanoparticles with CO2 densities between 0.2 and 0.9 g/cc. The synergistic interactions at the surface of nanoparticles (bare colloidal silica) and surfactant (caprylamidopropyl betaine) in generating stable CO2 foams were also investigated.
The common and distinct requirements to generate stable CO2 foams with 5-nm silica nanoparticles, in rock matrices and in fractures, were characterized by running foam generation experiments in Boise and Berea sandstone cores. The threshold shear rates for foam generation in matrix and in fracture, both in Boise and Berea sandstones, were characterized. The ability of nanoparticles to generate foams only above a threshold shear rate is advantageous, because high shear rates are associated with high permeability zones and fractures. Reducing CO2 mobility in these zones with foam diverts CO2 into lower permeability regions that still contain unswept oil.