Influence of Naphthenic Hydrocarbons and Polar Solvents on the Composition and Structure of Heavy-Oil Aquathermolysis Products

Estimates of the conventional proven hydrocarbon reserves show a tendency toward depletion, and in the near future they will be insufficient to meet the ever-growing global energy demand. The problem of crude oil deficiency can be solved by involving unconventional hydrocarbon resources in both recovery and refinery. However, the hydrogen deficiency of heavy oil and natural bitumen requires technical and technological solutions. Therefore, the hydrogen-donating capacity of water and solvents, particularly in reservoir conditions, is very attractive. In this paper, we study the hydrogen-donating capacity of naphthenic and polar solvents during hydrothermal treatment of heavy oil from Ashal’cha reservoir (Republic of Tatarstan, Russia). It was established that naphthenic solvents significantly influence the viscosity reduction due to the destructive hydrogenation of resins and asphaltenes. Among naphthenic solvents, decalin significantly increases the amount of evolved gases, particularly C4–C10 isomers and aromatics. However, the maximum evolved gases among all of the used solvents correspond to formic acid. The results of elemental analysis revealed that the H/C ratio rises in crude oil samples after hydrothermal treatment in the presence of cyclohexane and decalin. FT-IR spectral indices revealed an increase in crude oil aliphaticity in case of using solvents from the cyclohexane–tetralin–decalin series due to the cleavage of carbon–heteroatom bonds in aliphatic substitutes of resins and asphaltenes. Significant changes in the FT-IR spectra of crude oil are observed in the presence of tetralin.