Abstract: To overcome the difficulty in quantifying heat-transfer laws and temperature distribution at the steam-chamber front during SAGD in heavy-oil reservoirs with top water,based on a SAGD heat-transfer model and according to the change in crude-oil flowability before and after the heavy-oil temperature inflection point,the oil zone outside the steam-chamber front was divided into two regions:mobile oil and immobile oil.Based on the one-dimensional transient heat-conduction equation and moving boundary conditions,a new model was established for calculating temperature distribution at the steam-chamber front during SAGD,and a calculation method was further developed for the critical oil-zone thickness between the steam chamber and the top-water layer as well as the nitrogen insulation-layer thickness.Results show that when Du 84 Block in the Liaohe Oilfield has been producing with SAGD(steam assisted gravity drainage)for 15 years,the critical thickness between the top of the steam chamber and the bottom boundary of the top-water layer is 17.60 m;when the nitrogen-layer thickness reaches 3.29 m,the nitrogen insulation measure can maintain the temperature at the top of the oil zone such that it remains lower than the temperature inflection point.This method can provide theoretical guidance for long-term effective development of heavy-oil reservoirs with top water and for applying SAGD-assisted fluid stimulation technologies.

Key words: heavy oil, SAGD, steam-chamber front, temperature distribution, critical reservoir thickness, nitrogen insulation-layer thickness