Foams
are used as divergent fluids for conformance control in enhanced
oil recovery (EOR) operations. It is created by mixing a surfactant
and a gas in situ in high-permeability reservoirs (e.g., surfactant
alternating gas or SAG). Foams exhibit instability issues at reservoir
conditions with a highly complex pore network, high pressure, high
temperature, and high salinity. Here, we examine the stability and
efficacy of foams formed with and without the addition of carbon nanoparticles
(or nanodots). Carbon particles have demonstrated stability, mobility,
and scalability for harsh reservoir environment use and application
as a tracer technology. In this study, we investigate the feasibility
of using inexpensive carbon dots as “foam boosters.”
The experiments involved using different levels of brine salinities
(ranging from seawater to formation connate water), different concentrations
of the nanodots (ranging from 5 to 500 ppm), different types of surfactants
(anionic, cationic, and nonionic) and gases (CO2, N2, and air), and different levels of temperature (ranging from
27 to 100 °C) to target representative conditions of Saudi Arabian
reservoirs. In addition, we examined both foam structure, such as
the gas–liquid interface, and liquid film lamella to better
understand the mechanisms contributing to foamability and foam stability.
The study highlights and unravels the complex interrelationship of
the different influencing components on the stability of foam with
the addition of the carbon particles. The bulk and porous media stabilities
of the foams are analyzed using a static foam analyzer and an HP/HT
core-flood system, respectively. Foam stability is assessed in terms
of type and amount of modified/functionalized carbon particles, surfactant,
and gas. We observed that the bulk foam containing only trace amounts
(5–10 ppm) of carbon nanodots shows improved stability in a
high-salinity medium. The particles improved the foam stability maximum
by 70% and more than doubled the foam half-life for some foams. Confocal
microscopy images of the foam structure of systems containing an increased
concentration of carbon particles reveal an increased thickness of
the lamellae and a decreased average bubble size. This is a clear
indication of the enhanced foam stability. Carbon dots decreased the
drainage rate of the lamellae and delayed the bubble rupture point
or coalescence. The particles improved the foam stability by preferentially
positioning itself in the lamella and preventing liquid drainage and
film thinning.
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