A model has been formulated for estimating the oil production during the mature phase of a steam-injection process. This model assumes that complete steam overlay has occurred and that subsequent gravity drainage of oil is accelerated because of the viscosity reduction in the heated-oil phase. The predicted oil-production rate declines approximately linearly at early times, depending on the initial oil level in the reservoir and at a gradually decreasing rate toward zero at long times. The model shows good agreement with various field cases.
In most steamfloods, the injected steam tends to override the oil layer, resulting in early steam breakthrough at the producers. Except for very stratified reservoirs or the presence of thief zones, the development of complete steam overlay characterizes the mature steamflood. Oil production takes place largely by gravity drainage at the producers. For this situation, frontal displacement models are of limited usefulness in predicting oil production.
Miller and Leung and Kumar et al. have proposed oil-production models for the mature steamflood, and van Lookeren and Neuman have proposed methods of calculating the overlying steam-zone size. A model by Hsu includes gravity drainage and an improved calculation of the steam-zone shape. Chen and Sylvester evaluated the Miller and Leung model and two frontal advance models. The Miller and Leung model estimates oil-production rate on the basis of acceleration resulting from oil-viscosity reduction. A model proposed by Chhina et al. uses a conical shape for the steam/oil interface at injectors and producers, with a horizontal interface in the intervening region. Oil production is based on changes in these regions and is the result of pressure drive, steam drag, and gravity drainage. Butler considered gravity drainage to horizontal wells.