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Summary The technology of steam-assisted gravity drainage (SAGD) with a dual horizontal well pair has been widely adopted in thermal recovery for heavy oil in recent years. However, the close distance between injector and producer makes it easy to cause steam breakthrough, which means lower thermal efficiency as well as higher investment. It is generally acknowledged that there is a vapor-liquid interface between the injector and producer. A suitable liquid level is desired to prevent steam from being produced directly; otherwise, an overly high liquid level would influence oil productivity or even submerge the injector. The existence of a liquid level generates a temperature difference (i.e., subcool) between two wells. Subcool has widely been used to characterize the liquid level in research, yet it is inaccurate. Further studies are still needed on how to maintain a suitable and stable liquid level in SAGD development. In addition to the heat-loss model and geometric features of the steam chamber (SC), mass conservation, energy conservation, and gravity-drainage theory are used to develop a multistage mathematical model for liquid-level characterization during the SAGD process. The new model is validated against both field data and simulation results. On the basis of this model, an optimal production/injection ratio (PIR) at different times could be calculated to maintain a stable liquid level above the producer, avoiding steam channeling accordingly. Besides, the model can also be used to predict optimal steam-injection rate under constant-pressure injection. Other SAGD dynamic performance predictions, such as SC expansion speed, could also be derived from this model. In addition, recommendations for liquid-level adjustment are offered on the basis of field conditions.

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