The target heavy oil reservoir was developed by horizontal wells with large well spacing (250 m ~ 450 m) during the early huff and puff stage. However, more than 70% of the reserves was left in the reservoir after stimulation development because of its small heating radius, and required further enhanced oil recovery through subsequent steam flooding for inter-well reserves extraction.

Different from traditional vertical well, a special hot-water condensate zone developed at the reservoir bottom due to vertical radial flow during horizontal well steam flooding was demonstrated in our previous work. Inhibiting the development of hot-water condensate zone became the design key for horizontal well steam flooding. Based on laboratory physical experiments and numerical simulation method with 13 years of huff and puff development history match, the first offshore steam flooding scheme was studied to optimize and obtain the main heat injection parameters. Finally, field practice was carried out 30 months ago to verify project design.

Results showed that, compared with mature vertical well steam flooding designed for suppressing the overlap in top steam zone, the horizontal steam flooding was designed to suppress channeling flow caused by weak stability of hot-water displacement oil front in bottom condensate zone. Therefore, the heat injection well was designed in the low structure position for horizontal well steam flooding, while designed heat injection well for vertical well steam flooding located in the high structure position. Meanwhile, to ensure effective expansion of steam zone, the production/injection ratio was optimized as high as 1.4 to 1.6 for horizontal well steam flooding, rather than low production/injection ratio of 1.2 to 1.4 for conventional vertical well. Also, it was demonstrated that foam can effectively prevent steam channeling under high oil saturation conditions, especially in the high superheated temperature. So the profile control timing for horizontal well was advanced in the early thermal connection stage, instead of the late steam breakthrough phase for vertical well. The field practice has been carried out for 30 months, the daily oil production of well group increased from 180 m3 to 250 m3, and the instantaneous oil/gas ratio was developed as high as 0.8 to 1.0.

It was suggested to pay special attention for the additional hot-water condensate zone at the reservoir bottom for horizontal well steam flooding. The proven development strategy, inhibiting condensate zones at reservoir bottom and promoting steam zones at reservoir top, customized for horizontal well can also be applied in offshore similar thin heavy oil reservoirs with large well spacing.

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