Abstract: During heavy-oil thermal recovery,improper application of annular thermal-insulation measures in steam-injection wells can readily induce technical problems in thermal injection,such as large steam-injection heat loss,wellhead uplift,casing damage,and wellbore corrosion.Therefore,based on the completion structure of thermal-injection wells and by comprehensively applying the basic principles of heat transfer and fluid mechanics,a model for the critical annular nitrogen injection rate was constructed.The factors influencing the critical annular nitrogen injection rate were analyzed,and an optimization method for annular nitrogen-injection thermal-insulation technology in thermal-injection wells was proposed.The results show that under different well-type conditions,horizontal wells have greater annular thermal resistance than directional wells and require higher critical nitrogen injection rates.The required critical nitrogen injection rate increases with increasing pressure,steam injection rate,and steam quality.With increases in the length and thermal conductivity of insulated tubing,the required critical nitrogen injection rate decreases rapidly.In addition,increasing interval pack-off in the annulus causes the required critical nitrogen injection rate to decrease linearly.Through integrated optimization design measures,such as simplifying the model for critical annular nitrogen injection rate,designing pack-off devices,optimizing the completion string,and reducing injection pressure,the nitrogen injection rate of the annular nitrogen-injection thermal-insulation technology was reduced by more than 75%.This study provides key theoretical guidance for improving steam heat-utilization efficiency and reducing the casing-damage rate in thermal-injection wells.

Key words: steam thermal-injection well, annulus, critical nitrogen injection rate, thermal-insulation technology, optimization design