# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 1 Introduction
The relationship between nanotechnology and the petroleum industry is not new, but it has been developing for the past 30 years (in the case of nanocatalysts). In downstream processes such as petroleum refining, zeolites are employed to extract up to 40% more gasoline in catalytic cracking units than their predecessors [13]. In upstream operations, the first application of nanotechnology was the development of nano-enhanced materials [14; 15]. By means of nanotechnology, lighter equipment capable of withstanding harsh conditions was produced. Nanotechnology could also be used to  
Figure 1: Scheme with the most relevant oil recovery techniques, from primary and secondary recoveries (or conventional mechanisms) to tertiary or enhanced oil recovery (EOR), and possible applications of nanotechnology in EOR. Figure adapted from Ref. [11]. develop new metering techniques with nano-sensors for high-temperature, high-pressure deep wells and hostile environments; for reservoir characterization; fluid flow monitoring; new imaging and computational methods allowing better discovery, sizing, and reservoir characterization; as well as new cementing materials [15; 16; 17; 18; 19; 20; 21; 22]. Recovery mechanisms in EOR operation can also be enhanced using nanotechnology [14; 23; 24; 25; 26; 27]. Moreover, the scale of the nanomaterials makes the suitable injection into porous media relatively easy, even in low-permeability fields where, statistically, the pore throat width ranges between 100 and 10,000 nanometers. Concerning EOR operations, one of the most relevant topics at present is how the nanoparticles behave in the harsh conditions present in oilfields (i.e., high pressure, high temperature, and high salinity), and how these conditions affect the particles' recovery efficiency [28; 29; 30; 31]. Generally speaking, there are three main nanotechnologically-enhanced processes in EOR: (a) the addition of nanoparticles in the displacing agent enhancing its rheological properties, which are known as nanofluids; (b) the use of nanoparticles as stabilizing agents in the formation of nanoemulsions; and finally, (c) active nanoparticles working as nanocatalysts injected in order to perform in-situ upgrading operations in the porous medium before extraction. Nanofluids, mainly consisting of nano-sized colloidal dispersions, have successfully been developed in laboratories, and the upcoming challenge is to develop techniques for cost-efficient industrial-scale production techniques [32]. They can be designed to be compatible with reservoir fluids/rocks and be environmentally friendly [16]. Some newly developed nanofluids have shown extremely improved properties in applications such as drag reduction, binders for sand consolidation, gels, wettability alteration, and anticorrosive coatings [33; 34]. In heavy oil reservoirs where EOR thermal production processes are usually utilized, the application of nanotechnology yielded several breakthroughs. Nanocatalysts for in-situ upgrading of heavy oil, what some authors have defined as an \"underground refinery\" [35; 36; 37], constitute a novel approach to dealing with these systems.