Analysis Of Heat Transfer Characteristics Of The Vacuum Insulated Tubing For Heavy Oil Thermal Recovery

At present,some of China’s main oilfields have entered the middle and late stage of exploitation,and the difficulty of extraction is increasing year by year,and it has entered the stage of three times oil recovery mainly by Enhanced Oil Recovery(EOR),and the stable and high production is facing a lot of problems.For heavy oil reservoirs,at present,steam injection is the main extraction method,which is one of the important three oil recovery methods.In the process of thermal recovery of thick oil,steam is injected into the oil reservoir through the insulated tubing.In the process of steam injection,the radial heat loss of steam occurs along the insulated tubing,and the dryness of the steam reaching the oil layer decreases,which affects the effect of heavy oil extraction.In actual production operation,the weaknesses of wellbore heat insulation mainly exists in two aspects,on the one hand,there is direct heat loss at the joint hoop of the insulated tubing,and the radial heat loss is large,which leads to the reduction of heat insulation performance of the insulated tubing,and it is necessary to improve the structure of the joint hoop to reduce the heat loss from the joint hoop;on the other hand,the support material in the insulated tubing is structurally broken after a period of time,which leads to the increase of heat loss of the insulated tubing,and it is necessary to screen for better quality support materials to reduce the heat loss of the overall heat loss of the wellbore.Therefore,after studying the effects of the insulated tubing and the hoop on the bottomhole steam parameters,it is found that the bottomhole steam pressure and temperature gradually increase with the increase of the apparent thermal conductivity of the the insulated tubing;the bottomhole steam dryness increases with the decrease of the apparent thermal conductivity of the the insulated tubing;and the unit heat loss of the wellbore decreases with the decrease of the apparent thermal conductivity of the the insulated tubing.As the apparent thermal conductivity of the joint hoop increases,the bottomhole steam pressure and temperature gradually become larger,and the dryness of the bottomhole steam gradually becomes smaller;the average heat loss and the ratio of heat dissipation of the joint hoop increase with the increase of the apparent thermal conductivity of the joint hoop.As the service time of heat-insulating tubing increases,the apparent thermal conductivity of the heat-insulating layer will gradually become larger.In the research of joint hoop,based on the conventional insulated tubing,two improved insulated tubing structures are proposed,which are insulated bushing-containing tubing at the joint hoop and direct-connected insulated tubing;heat loss analysis models are established for the three kinds of insulated tubing and the models are verified to be correct.Simulation calculations are carried out to analyze the simulation results and obtain the temperature distribution and heat loss characteristics of the three types of the insulated tubing structures.The results show that the heat loss of the insulated bushing-containing tubing at the joint hoop is the smallest,the heat loss of the direct-connected insulated tubing is moderate,and the heat loss of the conventional insulated tubing is large,and the joint hoop is recommended to take thermal insulation measures.The insulated bushing-containing tubing at the joint hoop is 75%more insulated than the conventional insulated tubing,and the direct-connected insulated tubing is 44% more insulated than the conventional insulated tubing.In the support material research,due to the thermal coupling effect,the support material is broken,the thermal conductivity of the heat insulation layer increases,and the service life of the insulated tubing is shortened;it is especially important to screen for better quality support materials to improve the defects existing in the normal operation of the insulated tubing.In this paper,three materials of glass fiber,carbon fiber and composite glass fiber are studied to establish a thermal coupling analysis model,and the model is proved to be correct.Simulation results get three kinds of support material temperature,stress and displacement changes,and the three materials for comparison and analysis.The results show that the composite glass fiber material has the longest service life,followed by glass fiber,and carbon fiber has the shortest service life.Composite fiberglass is used instead of fiberglass to increase the service life of insulated fuel lines by 45%.There is a temperature gradient in the formation around the wellbore,and the deeper the formation,the smaller the temperature difference between the inside and outside of the wellbore,and the decreasing trend of heat loss.As for the joint hoops,it is recommended to use different joint structures according to the temperature of the bottom layer,and use the insulated bushing-containing tubing at the joint hoop in the variable-temperature layer,and use the direct-connected insulated tubing in the temperature-enhancing layer.As for the support material,it is recommended to use composite fiberglass to reduce the heat loss of the insulated tubing and thus make the thick oil production bigger.