Research On The Coking Mechanism Of Heavy Oil And The Combustion Zone Propagation Characteristics During In-situ Combustion

In-situ combustion(ISC)shows great application potential in heavy oil production because of its high recovery rate.The thermal conversion of heavy oil into coke and the sustainable propagation of the combustion zone are important parts of the ISC process.In this paper,the thermal conversion characteristics,coking mechanism and kinetic model of low-asphaltene heavy oil of China were studied.Based on the numerical simulation of ISC,the propagation characteristics of the combustion zone were analyzed,and a determination method of the propagation state of the combustion zone was constructed,which provided guidance for the development and application of ISC.First,a fixed-bed experimental system used under atmospheric pressure and a pressurized reactor experimental system were established.The coking experiments of heavy oil were carried out under several thermal conversion conditions.The coke formation path were found to be different in an inert atmosphere and an oxidizing atmosphere.In an inert atmosphere,the pyrolysis of heavy oil starts at 350°C,and the produced free radicals are converted into coke through polymerization and aromatization reactions.In an oxidizing atmosphere,the oxygen addition reaction of heavy oil starts at200°C,and coke is formed through the polymerization and oxidative cracking of oxygen-containing functional groups.Thermal conversion conditions present little influence on the pyrolysis of heavy oil,but show significant influences on the oxidation of heavy oil.The heating process,O₂ concentration and pressure mainly affect the coke formation by changing the rate and progress of each reaction during heavy oil oxidation.Steam affects the coke yield of heavy oil by increasing the coke oxidation rate.Afterwards,the coking characteristics of the fractions of heavy oil and their relationship with heavy oil coking characteristics were studied.Among the main fractions of low-asphaltene heavy oil,the coke yields of saturates,aromatics,and resins increase sequentially,while the coke oxidation activities decrease sequentially.The coke yields of the same fraction from different heavy oils are basically the same after removing the influence of evaporation,but the coke oxidation activities of the same fraction from different oils are different.The coke yield of heavy oil can be estimated based on the weighted average of its fraction content,the coke yield of each fraction after removing the influence of evaporation and the evaporation weight loss of each fraction.Then heavy oil coking models in two atmospheres and a coke oxidation model were proposed.The heavy oil coking model in the inert atmosphere includes the pyrolysis of each fraction to form coke precursors,and the coke precursor polymerization to form coke.The coking model in the oxidizing atmosphere includes the low temperature oxidation of each fraction to form coke precursors,the coke precursor polymerization to coke,and the low temperature oxidation of coke.Based on the multi-heating rate thermal analysis experiments and the thermal conversion experiments in the fixed-bed reaction system,the kinetic analysis of heavy oil coking and coke oxidation was carried out.Combined with the evaporation rate equation,a kinetic model of heavy oil coking and coke oxidation was constructed.Experimental verification showed that the model could be used to describe the heavy oil coking and coke oxidation processes of different heavy oils under complex conversion conditions.Finally,the combustion zone propagation characteristics were studied and a method for determining the propagation state was established.Based on one-dimensional ISC numerical simulations under various gas injection parameters,reservoir conditions and oil properties,the combustion zone propagation includes three states:sustainable state,extinguished risk state and extinguished state.The temperature of the combustion zone shows good correlation with the propagation state.Based on the dimensionless analysis of the energy conservation equation,the temperature of the combustion zone is mainly controlled by the dimensionless numbers HD and CW.A phase diagram for determining the combustion zone temperature based on the HD and CW numbers was established,which could be used to judge the propagation state of the combustion zone under various operating conditions.