Enhanced oil recovery by CO2 injection is an effective method for recovering additional oil beyond waterflooding. In recent years it has garnered a lot of attention for two primary reasons: (a) the stable high price of oil and (b) environmental aspects of CO2 sequestration. Its use has been increasing steadily over the past few years. In many respects it is a win-win situation with CO2 sequestration and additional, incremental oil produced. However, the CO2-EOR process is handicapped, especially in thick reservoirs, by CO2 gravity override. Due to density differences between the injected CO2 and resident fluids in the reservoir, the lighter CO2 tends to rise to the top of the reservoir thereby bypassing some of the remaining oil. This results in poor sweep efficiency and conformance. Different techniques have been used to overcome the CO2 gravity override by either increasing its density, viscosity, or reducing its relative permeability. This paper investigates the use of gelling CO2-water emulsions, stabilized by silica nano-particles, to control the mobility of CO2.
The stability of nano-particles was first investigated using iso-octane (iC8) as a proxy for CO2. The stability of these emulsions, or foams, was investigated as a function of nano-particles concentration, type, hydrophilicity degree, and also as a function of iC8/water ratio. The silica nano-particles concentration ranged from 0.5 to 2 wt%, and iC8 phase volume ranged between 50 and 90%. Stability experiments were conducted at room temperature and up to 17 hours using both hydrophobic and hydrophilic colloidal silica nano-particles. Following the screening studies with iC8, rheological measurements were made using CO2 at 200°F and 1,800 psi at different (CO2/water) ratios and nano-particles concentrations. Compared to pure liquid CO2, high emulsion viscosities from 1.1 to nearly 2.5 cP were achieved. These values represent almost a 100-fold increase over pure sc-CO2 viscosity. Additionally, in some cases rigid gels were observed with time following emulsion generation. The CO2-water-nanoparticle emulsions were generally stable. This work provides the rheological results of the emulsion systems as a function of time, nano-particles concentration and CO2 phase volume. The high viscosity CO2/water emulsions have the capability to enhance CO2 mobility, act as a diverting agent during CO2-EOR, and improve sweep efficiency.