# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 4 Nanocatalysts  
In heterogeneous catalysis processes, a desirable property is to present the maximum surface possible between the catalyst and the reactants in order to improve the reaction. This is the main reason why nanotechnology stepped into this field. A nanocatalyst is then defined as a substance with catalytic properties which have at least one nanoscale dimension, thus enhancing its catalytic capabilities [35; 132]. Nanocatalysts can be classified into four different classes: nanoparticulate, nanoporous, nanocrystalline, and supramolecular catalysts [133]. A different classification sorts them as: mineral, water-soluble, oil-soluble, and dispersed [134]. Nanoparticles and nanoemulsions present improved capabilities to displace oil, but they face a problem when they are injected into heavy oil reservoirs. The high mobility ratios lead to the formation of channels or fingering. One solution may be injecting nanocatalysts to trigger in-situ chemical reactions to upgrade the oil by lowering the asphaltene and resin contents--a concept sometimes referred to as \"underground refinery\" (Figure 30) [22; 35; 36; 37; 38; 37]. In order to assess the effectiveness of the upgrading process, some parameters should be evaluated. Two of the most significant parameters are the H/C atomic ratio and the number of carbon atoms per molecule (Figure 31). Any upgrading process whereby the number of carbon atoms in a molecule decreases (i.e., cracking) and the H/C increases (i.e., hydrogenation of unsaturated bonds) can be assessed as an indication of heavy oil upgrading. Figure 29: Diagram describing the formation of nanoemulsions using the phase inversion temperature (PIT) method. Figure reproduced from Ref. [115]. W/O: water-in-oil. The implementation of standard thermal EOR processes in heavy oil or bitumen reservoirs requires extensive heating [137]. Steam application is costly due to, among other, the infrastructure required, the possible environmental impacts, and its operational costs [138]. This is the point where nanotechnology started to be used to improve the steam flooding, mobilizing heavy oils in the reservoir. The main advantages of using nanocatalysts to enhance traditional EOR processes are [35]: (a) improved catalytic performance [135; 139; 140], (b) the probability of contacts among reactants is increased due to the nanocatalyst dispersion, (c) the propagation of nanocatalysts and in-situ reactions cause bitumen dissolution and viscosity reduction of the recovered oil, and (d) successful in-situ refining processes reduce the downstream operating costs as well as the environmental impact associated with the production of bitumen (e.g., solid waste by-products, greenhouse gas (GHG) emissions, SO\({}_{\rm x}\)/NO\({}_{\rm x}\) production, and fresh water consumption). However, there are still some topics  
Figure 31: Hydrogen-to-carbon ratio for several petroleum cuts. Figure adapted from Ref. [136]. Figure 30: Schematic representation of in-situ heavy oil recovery and upgrading, coupling ultradispersed nanocatalyst and the steam-assisted gravity drainage (SAGD) process. Light oil is then produced at the surface and the heavy molecules, solids, and minerals remain underground. Figure reproduced from Refs. [35; 103]. which need to be addressed. The economic aspect is one of them: further research is needed to improve the catalysis process. However, an even more serious topic is the catalysts' efficiency in the rock matrix. The activity of the catalysts depends, among other things, on the temperature field homogeneity in the porous medium. When the superheated water is injected into the formation, the oil surface temperature will be high, yet the temperature will gradually decrease as the distance from the injection point increases, causing the catalyst to lose its activity. The development of nanocatalysts capable of dealing with this gradient is also deemed necessary. The first reported application of using metals in an upgrading process in oil by aquathermolysis was made by [PERSON] [141].