Study On The Formation Mechanism Of H₂S By Thermochemical Sulfate Reduction During Heavy Oil Thermal Recovery

As a harmful gas in natural gas,hydrogen sulfide is considered mainly caused by ther-mochemical sulfate reduction reaction now.While the thermal recovery conditions for the heavy oil provide the required heat and the environment for the reaction of the associated gas or the crude oil with the sulfate in the rock mineral.Studying the formation mechanism of hy-drogen sulfide in the process of thermal recovery of heavy oil in TSR,which is important for the prediction of hydrogen sulfide,the control work and the safety of oil production.In this paper,the basic properties of various substances involved in the thermochemical reaction under steam injection are tested by modern test technology,such as XRD,XPS pho-toelectron spectroscopy.And the sulphates corresponding to the five kinds of metal elements such as sodium,calcium,magnesium,aluminum and iron have been identified as the sulfate type involved in TSR.The results of sulfur-containing compounds in heavy oil have showed that the types of sulfur-containing organic compounds that could be involved in TSR are alkyl sulfur and thiophene sulfur.Through thermal simulation experiments between crude oil and sulphate,the effects of reaction conditions on the formation of hydrogen sulfide during TSR reaction have been stud-ied.It has been determined that the main products of TSR are hydrogen sulfide,gelatinous asphalt,metal oxide/metal sulfide,hydrocarbon gases,and carbon dioxide.The concentration of hydrogen sulfide,which is generated by different sulphates take part in TSR with crude oil is:aluminum sulfate>ferric sulfate>magnesium sulfate>calcium sulfate>sodium sulfate.But the extent of TSR reaction by ferric sufate is the greatest.The degree of TSR reaction is positively correlated with the reaction temperature and reaction time.The TSR experiments of the model compounds have shown that the three types of model compounds are easier to carry out TSR to generate H₂S,compared with crude oil under the same conditions,and thiols>thiophenes>alkanes.The conversion of inorganic sulfur to or-ganic sulfur has been demonstrated by the detection of sulfide.Simultaneously,it has been pointed out that the higher the content of sulfide in crude oil,especially the labile sulfur com-pounds,the greater the degree of TSR,the higher the amount of hydrogen sulfide generated.Finally,the reaction processes of n-hexadecane and tetrahydrothiophene in TSR have been discussed.The reaction produces more easily decomposable organic sulfur intermediates,re-sulting in more H₂S.Finally,the kinetic parameters of the reaction system generating H₂S have been obtained by the dynamic experiment between crude oil/model compound and magnesium sulfate.Based on the kinetic model,the apparent activation energy of H₂S formed by the reaction of Liaohe heavy oil with magnesium sulfate,n-hexadecane with magnesium sulfate and tetrahy-drothiophene with magnesium sulfate were 78.23 kJ/mol,47.49 kJ/mol and 25.50 kJ/mol re-spectively.