# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 4 Nanocatalysts
### Chemistry and Kinetics of the Process  
In EOR steam injection applications, the heat is employed for visbreaking (i.e., thermal cracking) the heavy oil/bitumen--this means that not only physical but also chemical processes take place [154; 155; 156]. These chemical reactions between the rock, steam, and oil cause what is called aquathermolysis. The hydrolysis of aliphatic sulfur linkages and the cracking of the large complex molecules present in the crude oil are the main characteristics of these reactions [138]. The result of these reactions is a decrease in the content of heavy products, such as bitumens/asphaltenes/resins, lowering the viscosity, the molecular weight, and the sulfur content and increasing H/C ratio, as well as saturated and aromatic hydrocarbons [142; 154]. The presence of nanocatalysts in the porous media is considered to enhance the oil recovery by at least two principal mechanisms [35; 36]: the in-situ upgrading of heavy oils, and the wettability alteration of a rock formation by coating with nanoparticles (see **Nanofluids**). The presence of nanocatalysts in heavy oil/bitumen formations generates a change in the way the components react, creating new paths for reactions to take place. A number of researchers proposed different kinetics models for the hydrocracking of heavy oils and bitumen [157; 158; 159], as well as the influence of nanocatalysts in the reaction kinetics [135; 160; 161]. Nevertheless, the structure of molecules such as resins and asphaltenes makes the kinetics complex, and the grouped lumping concept and the definition of \"pseudo-components\" represents a good approach for this study (Figure 36). Figure 35: Comparison of oil recoveries with and without nickel particles considering different total energy inputs. Figure reproduced from Ref. [145]. Several experiments showed that aquathermolysis reactions took place via the production of reactive species from organosulfur compounds which later polymerized or reacted with water, yielding smaller fragments [155; 156]. These components participated in subsequent reactions generating others, such as hydrogen sulfide, hydrogen and carbon oxides. Based on the results obtained, an overall reaction scheme was proposed [154; 162; 163; 164]:  
\[ \begin{array}{c}\mathit{Bitumen}\ \xrightarrow{\mathit{Steam,200/300}^{\circ} \mathrm{C}}CH_{4}+H_{2}S+CO+CO_{2}+H_{2}+\mathrm{Hydrocarbons}_{\mathrm{C \geq 2}}.\end{array}  \tag{5}\]  
Furthermore, the kinetics above the aquathermolysis region were also studied [159]. The evolution of chemical reactions in the upgrading of several heavy oil cuts over the temperature range from 360 to 420 \({}^{\circ}\)C was discussed, rendering the following reaction sequence for thermal cracking:  
\[ \begin{array}{c}\mathit{Asphaltenes}\longrightarrow\mathit{Coke}+HO+LO+H_{ 2}+CH_{4}+CO+CO_{2}+C_{2}H_{6}+...\\ \qquad\qquad\qquad\qquad+\mathrm{Hydrocarbons}_{\mathrm{C\geq 3}}+H_{2}S\\ HO\longrightarrow\mathit{Coke}+LO+CH_{4}+C_{2}H_{6}+C_{3+}\\ LO\longrightarrow\mathit{Coke}+CH_{4}+C_{2}H_{6}+C_{3+},\end{array}  \tag{6}\]  
where \(HO\) and \(LO\) represent heavy and light oil, respectively. This reaction model yielded poor matches for the generation of carbon oxides, though provided a reasonable match for H\({}_{2}\)S production. Then, the process starts with the cleavage of \(C\)-\(S\) bonds present in bitumen/asphaltenes, releasing hydrogen sulfide (H\({}_{2}\)S). The reduction in the oil viscosity requires the decomposition of big and complex molecules such as asphaltene (Figure 37) or resins (visbreaking process). Aquathermolysis starts in the mentioned bonds since their dissociation energy is the lowest in the molecules. Figure 36: Detailed lumped kinetic reaction schemes for hydrocracking, proposed by [PERSON] (**left**) and [PERSON] (**right**). Figures adapted from refs. [157; 158]. This hydrolysis is achieved via the water gas shift reaction (WGSR).