# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 1 Introduction  
It is undeniable that oil has radically changed how we live and how our economy works in a way that no kind of fuel in the past has ever done. The changes generated in both our society and economy over the past 200 years are more than evident. However, oil is not a renewable energy source and many authors have speculated about the future of the oil industry [1]. It is obvious that the global economy is not ready to replace traditional fossil fuel-based energy sources. This implies two possible outcomes: either we start exploiting unconventional oil sources, increasing the price of refined products; or we make better use of conventional oil reservoirs. Enhanced oil recovery (EOR) aims at developing the latter, thus factually bridging the transition towards the use of renewable energy sources. Throughout the operation of an oilfield, three different stages can be distinguished. The first stage (i.e., primary recovery) is based on the production of oil by natural drive mechanisms. Then, the secondary recovery stage takes place. A fluid (i.e., water or an immiscible gas) is injected in order to displace the oil to producing wells. These two recovery stages can then account for 50/55% of the initial oil in place (IOIP). Then, the tertiary stage or EOR begins. The main objective of all EOR processes is to diminish the oil saturation below the residual oil saturation (\(S_{or}\)). This is defined as the saturation during a waterflooding at which the oil production becomes discontinuous and the capillary forces immobilize the remaining oil [2]. EOR aims at modifying the physical and/or chemical properties in the fluids and/or the reservoir, and can be classified into three main branches (Figure 1). Even though nanotechnology is not an EOR technique per se, the unique features found at the nanoscale allow the performance of current methods to be improved. The use of nanotechnology in chemical EOR has recently been reported in the form of polymer nano-composites (PNPs or polymer-coated nanoparticles) [3; 4; 5] and silica nanoparticles [6; 7; 8; 9]. The aim of this work is to describe the use of nanotechnology for EOR purposes by focusing on the physical characteristics of the nanoscale. The reader interested in other EOR techniques which have already been applied is kindly referred to more specific literature for this application [10]. Nanotechnology-assisted EOR processes rely on one (or several) of the following factors: mobility control using viscosity-increasing solutions, altering the rock wettability, reducing the interfacial tension (IFT), and lowering the oil viscosity by means of nanocatalysts. The efficiency depends on how the process behaves at both the macro- and microscale. The microscopic efficiency considers the displacement of the fluids at a poral scale and measures how effective the displacing agent is in mobilizing oil in place (i.e., where the rock and oil make contact). At this level of magnitude, the geometry of the pores, viscous forces, capillary pressure, rock wettability, and rheological behavior (including viscoelasticity) play a major role. The macroscopic efficiency is the relationship between the connected reservoir volume being swept by the injected fluid(s) and the volume of oil originally in the reservoir. The parameters affecting this factor are: the rock heterogeneity and its anisotropy, the absolute permeability, the mobility ratio (\(M\)), and the gravitational segregation. The mobility ratio is a relationship between two fluids present in a porous medium that allows determination of how easily one will flow with respect to the other through the reservoir. The sweeping efficiency increases as \(M\) decreases, so it is considered as an indicator of the stability and efficiency of the displacement process. As the mobility increases (\(M\gg 1\)), the flow becomes unstable due to the difference in the flowing capabilities [12]. Thus, a large viscosity difference between both fluids causes the fingering of water through the more viscous oil.