Investigation On The Oxidation Characteristics And Coking Mechanism Of The Extra Heavy Oil And Its SARA Fractions

With the depletion of easy-to-produce light oils together with the ever-increasing energy demand,considerable attention has been given to(extra)heavy crude reserves over the past few decades.The estimation shows the heavy oil resources in the world are roughly more than two times than the conventional light oils.However,even after numerous efforts,the recovery of heavy crudes remains a challenge due to their high viscosity,their high density,and the caused poor fluidity in porous media.Of the well-recognized Enhanced Oil Recovery(EOR)technologies,in-situ combustion(ISC)has been considered as a promising thermal methods for heavy oil production.In contrast to steam injection,ISC can generate heat in-situ by burning a small portion of crude oil in place,and consequently reduce the viscosity of heavy oils.Therefore,knowledge of the combustion pathways of crude oils are highly crucial for the success of this play in oilfield.Despite the substantial works of ISC,the driving mechanisms and chemical reactions that occur in ISC remain unrevealed as a result of the complexity of hydrocarbon mixtures and their consecutive reactions.These issues cause the ISC technique extremely difficult to handle and predict in field scale application.To add further data to the existing data base and to specifically determine the SARA interactions of an extra-heavy oil,we present combustion results from TG/DSC experiments and model fitting using crude oil,individual fractions and binary systems.The TG/DSC curves have shown clear evidence of the different combustion pathways of the SARA fractions with temperature.The alterations of peak temperature,heat release,temperature interval,activation energy(Ea),etc.,of the tested binary mixtures revealed the interactions between SARA fractions during non isothermal combustion.Asphaltenes tended to reacted with the products of saturates and aromatics LTO(low temperature oxidation)reactions,while resins did not alter the combustion pathway of asphaltenes due to their similar structures.Saturates and resins can reduce the Ea of asphaltenes HTO(high temperature oxidation)reactions as a result of the boosted coke formation.In contrast,aromatics inhibited the LTO and HTO combustions,as revealed by the increased Ea of these two stages.In this study,our attention was also placed on asphaltenes,which are thought to be the key part for sustaining the continuity of the combustion front.The structure-property alterations particularly the kinetics of combustion and pyrolysis of Tahe asphaltene fraction caused by a low temperature oxidation process were comprehensively examined.The results showed that after that oxidation 10.3 5wt%of the asphaltenes was coked.SEM observations indicated that the surface of the oxidation products especially the coke was fairly rough caused by air attacks and reactions,which benefited the subsequent combustion.It was also found that the structure of products was further compacted and condensated after the oxidation.Consistent with other reports,the distinguished regions were clearly identified on TG/DSC curves.TG data and activation energy calculated by Arrhenius model revealed the differences of the reaction sites between combustion and pyrolysis.The formed coke in the static oxidation presented the highest reaction activity and exothermic effect.The results derived from this work are of significance for assessing the feasibility of ISC in this targeted heavy reservoir.