Molecular Simulation Of Crude Oil Cracking Into Gas And Its Preliminary Application

Thorough understanding of the gas formation processes of crude oil cracking in reservoirs is important for predicting the type of hydrocarbon resources,evaluating the thermal stability of oil reservoirs,and guiding deep oil and gas exploration.Traditional thermal simulation experiments and chemical kinetic methods cannot accurately describe the intermediate process of cracking reactions,and cannot deeply understand the microscopic mechanisms of crude oil cracking at the atomic/molecular level.In this thesis,the ReaxFF reactive molecular dynamics（ReaxFF MD）simulation technology has been applied to the study of crude oil cracking reaction processes and mechanisms.Based on MD simulation results of typical kerogen hydrocarbon generation,the geological maturation scale of the molecular simulation temperature was primarily established for the first time.At the micro-molecular level,the main products distribution,kinetic characteristics and reaction mechanism during the cracking reaction of typical hydrocarbon molecules in crude oil,mixtures of saturated hydrocarbon and asphaltene,and complex crude oil macromolecules were systematically analyzed.Combined with thermal simulation experiments and geological analysis,the influence of chemical composition of oils on the stability of oils was clarified,and the lower limit of maturity for the stable preservation of liquid hydrocarbons was deduced.Molecular dynamics simulation provided a new technical method for studying the micro-geochemical characteristics of crude oil cracking reactions.The molecular dynamics（MD）simulations based on Reactive Force Field（ReaxFF）were carried out for three typical（type I,II and III）kerogen molecules at temperatures ranging from 1200 K to 2600 K.The results show that the H/C and O/C atomic ratios of kerogen molecules decreased with increasing simulation temperatures,and the evolutionary paths of the three kerogen fit well with those shown in the van Krevelen diagram.The activation energies of pyrolysis for these three typical kerogen were 61.1kcal/mol,53.5 kcal/mol and 52.8 kcal/mol,respectively,consistent with those obtained from both laboratory simulations and basin modeling.By comparison with the relationships between measured vitrinite reflectance（R_o）values and the H/C atomic ratios of kerogen samples reported in the literature,a quantitative relationship is established between the R_o values of kerogen and simulation temperatures（T）,which is similar to the Easy%R_o used in laboratory thermal simulations.It lays a good foundation for the further study of hydrocarbon generation of kerogen and crude oil cracking using the MD simulation method.ReaxFF MD simulations were performed at 2000 K～3000 K for five representative hydrocarbon molecules（n-tetradecane,2-methyltridecane,drimane,phenanthrene,and dibenzofuran）in crude oil.The differences in distribution of pyrolysis products,evolutionary characteristics of compositions,kinetic behaviors,and reaction mechanisms of different types of hydrocarbon molecules,such as linear alkanes,branched alkanes,cycloalkanes,aromatic hydrocarbons and hetero-aromatic hydrocarbons,were analyzed.The chain alkane cracking is dominated by chain breaking and dehydrogenation reactions,mainly producing small molecules（C₁–C₄）alkanes and olefins as well as hydrogen molecules,reaching the peak of gas production at 2500 K.Compared with n-alkanes,the branched alkanes produce more methane molecules due to the influence of methyl substituents.The cracking of cycloalkanes differs from that of chain alkanes in that ring-opening cleavage occurs during high temperature cracking to form small molecular fragments as well as condensation reactions to form large molecular condensation product molecules higher than their own carbon number.The aromatic hydrocarbon cleavage is more difficult than that of the chain alkanes,and the significant reaction of phenanthrene occurs at 2750 K.The dehydrogenation and condensation reactions are repeatedly carried out at higher temperatures to produce polycyclic aromatic hydrocarbons until coking,and the formation of large molecular fragments(C₆₂₂H₄₁₁)with oriented arrangements of aromatic layers is observed at 3000 K.Due to the weak bond energy of the C–O bond in the structure of the dibenzofuran molecule,its cleavage starts at a lower temperature than that of the phenanthrene molecule,and the reaction occurs at 2250 K with the formation of CO,H₂O,H₂ and small molecule gases in the products.Using the molecular dynamics simulation method,n-tetradecane was used as the n-alkane representative and mixed with asphaltene molecules in different proportions to carry out the ReaxFF MD simulations.The results show that the increase of asphaltene content facilitates the cracking of n-alkanes and causes the formation of short-chain hydrocarbons.The golden tube thermal simulation experiments of confined system with five sets of two-component models with different ratios of saturated hydrocarbons and asphaltenes also exhibits that the gas hydrocarbon yield increases with the increase of asphaltene content.Therefore,the results of both MD simulation and thermal simulation experiment show that the high mature light oil is more resistant to cracking and has relatively high thermal stability under the same conditions.The relatively low content of asphaltene in high mature light oil and condensate oil of the Tarim Basin is one of the reasons for the stable preservation of the Tarim Basin ultra-deep oil reservoirs.Consequently,in addition to thermal history,hydrocarbon generation history and reservoir secondary alteration,the chemical composition and properties of crude oil itself should be considered in the study of oil reservoir stability.According to the chemical composition of crude oil,the typical compounds of different types in crude oil were considered comprehensively,and the database of molecular structure models of crude oil was established.Then a calling program of the molecular structure of crude oil based on the oil molecular model was compiled by using Fortran language.Based on the data of physical properties and group components of the crude oil samples from the Harahatang area of the Tarim Basin,combined with the chromatography-mass spectrometry data of saturated hydrocarbons and aromatic hydrocarbons,149 representative molecules including saturated hydrocarbons,aromatic hydrocarbons,non-hydrocarbons and asphaltene group components were selected in a comprehensive consideration.Then a crude oil molecular model was established,which was generally consistent with the chemical composition and properties of crude oil samples.A three-dimensional model(C₂₆₂₉H₄₉₇₁O₇N₃S₈)of the complex crude oil macromolecule containing 7618 atoms was established for the first time.The equilibrium density of the crude oil molecular model is basically consistent with the measured density of crude oil samples,indicating the molecular dynamics simulation results are reliable.Through the ReaxFF MD simulation for the oil molecular model at different temperatures,it is determined that the simulation temperature for the disappearance of liquid hydrocarbon does not exceed 2750 K.The geological extrapolation by the established MD maturation scale clarified that the maturity corresponding to the lower limit of liquid hydrocarbon stable preservation is about R_o=2.1%～2.4%.It is basically in agreement with the values obtained from the geological deduction of the kinetic parameters of crude oil based on the thermal simulation experiment.This study provides theoretical support for predicting deep hydrocarbon resources from the molecular level in terms of micro-mechanism.