-based methods for enhanced oil recovery (EOR) in water flooded reservoirs faces various technical challenges. The lower density of CO2
compared to water causes gravity segregation and the lower zones of the reservoir will not be swept by CO2
. The low CO2
viscosity leads to viscous fingering and excessive flow in high permeability layers. The net effect can be an early CO2
breakthrough, reduced sweep efficiency and low oil recovery. Counteracting these effects can be achieved by decreasing the CO2
mobility, either by adding thickeners into CO2
or dispersing it into brine (CO2
Direct thickeners CO2
require molecules that are CO2
soluble and have groups that interact giving the increased viscosity. Efforts to develop thickeners have been ongoing through the last decades. Up to now, the best results have been obtained with a fluoroacrylate-styrene copolymeric thickener that at typical reservoir conditions is able to increase the CO2
viscosity by the order of ten using low concentrations (<1 wt. %) [1
]. However, due to costs and environmental concerns, these types of additives are unlikely to have a practical application.
The apparent viscosity of CO2
dispersed into foams may be very high, depending on the type of surfactant used. One potential problem is that many foams are sensitive to the presence of oil giving destabilisation through several mechanisms including spreading and entering phenomena [5
]. During miscible CO2
flooding, oil sensitivity may be an advantage as foam formation is desired where the oil already has been displaced, diverting the CO2
into the oil containing parts of the reservoir. However, also minute amounts of oil remaining after miscible flooding may have detrimental effects on foam propagation [8
foams can also be stabilized by nanoparticles. Nanoparticles adsorb strongly at interfaces which contribute to a higher stability. However, the mixing energy required to adsorb at interfaces is larger than traditional surfactant stabilized systems. This is an important disadvantage for oil recovery [9
]. Typically, oil field flow velocities do not exceed a few feet/day which results in low mixing energy. It may, therefore, be difficult to utilise nanoparticle stabilized foams under normal process conditions.
The use of binary mixtures of surfactant and nanoparticles may improve foamability at low flooding rates. Singh et al. [13
] showed that a mixture of anionic surfactant and fly ash could reduce CO2
mobility more than anionic surfactant alone. Cationic surfactant had the opposite effect, however. Manan et al. [14
] demonstrated that mixtures of surfactant and different types of nanoparticles improved oil recovery during CO2
flooding compared to only surfactant, the improvement depended on the type of particles used.
Patel et al. [15
] studied oil-in-brine emulsion stability of silica nanoparticles in presence of sodium dodecyl sulphate. The presence of surfactant allowed to increase suspension stability by diminishing particle flocculation. Even though they observed that nanoparticles were more effective for stabilizing oil emulsions, binary mixtures of particles and surfactant were detrimental for emulsion stability compared to only nanoparticle-stabilized emulsions. These observations are consistent with the competitive adsorption conclusion of Pichot et al. [16
A binary system with surfactant and nanoparticles will be vulnerable to the separation of the constituents during transport in porous media and beneficial system properties may thus be lost. Ideally, the foam stabilising agent should be uniform to avoid loss of performance. To find foam stabilising agents with a uniform composition having the desired properties (sufficient mobility reduction, the desired level of oil sensitivity, low loss, moderate cost, etc.) remains a challenge.
Graphene Oxide (GO) is a relatively new material which is being widely studied due to its characteristics and its relevance to a wide range of fields, such as energy materials, biosensors, catalysis and biomedicine [17
]. GO particles have shown to be surface active with a size-depending amphiphilicity and they have been reported to create very stable emulsions with organic solvents [20
]. The higher hydrophilicity of smaller particles is attributed to a higher density of –COOH on its edges and epoxy groups on its surface [21
]. Having all this under consideration, GO amphiphilicity can be tuned by variations on its size [21
] or by partially reducing the particles [22
Recently, Liu et al. [22
] have reported for the first time that partially reduced graphene oxide (rGO) can efficiently stabilize CO2
in water. Foam stability was explained by the large surface area of GO, which diminished contact between CO2
and water. Moreover, Liu et al. [23
] demonstrated that GO can be an effective demulsifier for heavy oil-in-water emulsions. This effect was attributed to the strong interactions between the GO nanosheets and the molecules of asphaltenes and resins.
GO is a candidate for application within the oil industry. However, studies for its applications in EOR has up to now not been published. In this work, we have studied the possible use of GO/rGO particles as foam stabilizing agents for CO2 EOR.
In this article dispersions of aqueous solutions and CO2 are being referred to as foams whereas dispersions of aqueous solutions and organic solvents are named emulsions.
Large GO sheets can be used effectively to disperse CO2 in brine but the particles tested were too large (4 to 30 µm) for flow through porous media. Smaller particles were considered. However, reducing particle size had a determinant effect on foamability. nGO with a particle size below 1 µm was not able to form foams. This can possibly be adjusted by partially reducing nGO to make the particles less hydrophilic but the reduction degree must be carefully considered.
CO2 in SSW foams stabilized by GO showed a time-dependent stability. This was likely the result of the competitive effect of two mechanisms, hydrogel formation and GO layer staking. Both mechanisms appeared to be triggered by the presence of divalent ions. Hydrogel formation was the faster mechanism and played a beneficial role for foam stability which initially increased reaching a maximum after two days. Contrarily, GO layer stacking was a slower process and cancelled out the stabilizing hydrogel formation mechanism giving a rigid dispersion of CO2 and brine.
Reduced graphene oxide (rGO) with 13–22% oxygen content was not able to form foams. This contradicts the results obtained by Liu et al. [22
] and may be due to a high degree of reduction of the particles used. Thus, the degree of reduction must be taken carefully into account as low contents of oxygen would increase both hydrophobicity and CO2
It can be concluded that, for the particles tested, rGO appeared to be most hydrophobic whereas nGO appeared to be the most hydrophilic particles. This is in agreement with the findings of Luo et al. [21
]. The GO appeared to have an intermediate hydrophilicity and the highest interfacial activity that enabled stabilizing CO2
Hydrogel formation in presence of divalent ions can make graphene oxide particles not suitable for EOR purposes. A dispersion of nGO in SSW could not flow through 1.2 µm cellulose nitrate filters, most likely due to hydrogel formation. Passing the filtration test was set as a requirement for carrying on with core flooding experiments. Even though GO can stabilize CO2/SSW foams, the results indicate that they are not suitable for CO2 EOR.