China is rich in heavy oil reserves. In recent ten years, the production of heavy oil has rapidly grown through application of steam injection and the annual production reached up to 11. 9 million tons in 1993. At present, with the increase of cycles of steam injection, the effectiveness becomes poorer and the challenge of changing steam stimulation into steamflooding has to be confronted. Because of heavy oil reservoirs in China characterized with complex geological conditions, deeply buried, multi-layers and serious heterogeneity, especially some complicated conditions occurring in steam cycling, the effectiveness of steamflooding pilots has not been ideal commonly. Therefore, it is urgent to study and determine the comprenhensive conditions at which the reservoirs are feasible to be converted from steam cycling to steeamflooding at the present technological situation.

In this paper, based on three typical types of heavy oil reservoirs, systemetic studies have been conducted to investigate the effects of oil viscosity, oil formation thickness, vertical permeability difference, reproduction ratio of injected water in steam cycling and channelling path etc. on steamflooding effectiveness. Then, a practical screening criterion of steamflooding is proposed to provide technical bases for the conversion.

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