Nanoparticle-stabilized supercritical CO₂ foam for mobility control in CO₂ enhanced oil recovery
Foam has been used as a mobility control technique in CO₂ flooding to improve volumetric sweep efficiency. Stabilizing CO₂ foam with nanoparticle instead of surfactant has some notable advantages. Nanoparticle-stabilized foam is very stable because a large adsorption energy is required to bring nanoparticles to the bubble interfaces. As a solid, nanoparticle can potentially withstand the high temperature in the reservoir, providing a robust foam stability for an extended period of time. The ability of nanoparticles to generate foam only above a threshold shear rate is promising as foam can be engineered to form only in the high permeability zone. These nanoparticles are hundreds of times smaller than pore throats and thus can travel in the reservoir without plugging the pore throats. Surface-modified silica nanoparticle was found to stabilize CO₂ -in-water foam at temperature up to 80 ˚C and salinity as high as 7.2 wt%. The foam was generated through the co-injection of aqueous nanoparticle dispersion and CO₂ into consolidated rock cores, primarily sandstones, with and without an induced fracture in the core. A critical shear rate for foam generation was found to exist in both matrix and fracture, however, this critical rate varied with the experiment conditions. The effects of experimental parameters on the critical shear rate and foam apparent viscosity were also investigated. Additionally, the flow distribution calculation in fractured sandstone cores revealed a diversion of flow from fracture toward matrix once foam was generated, suggesting conformance control potential in fractured reservoirs. In order to study foam rheology, high-permeability beadpack was installed upstream of the core to serve as a foam generator. This allows the foam mobility to be measured solely while being transported through the core, without the complicating effect of transient foam generation in the core. The injection of the pre-generated foam into the core at residual oil condition was found to reduce the residual oil saturation to the same level as CO₂ flood, however, with the advantage of mobility control. The 'coalescence-regeneration' mechanism of foam transport in porous media possibly allowed the foam's CO₂ to contact and mobilize the residual oil. The injection of the foam slug followed by a slug of only CO₂ was also tested, showing similar viscosification as the continuous foam injection, however, required less nanoparticles.