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Geological Models in ODP Stage Horizontal Well Design—A Case Study of Huanghekou Sag (Neogene), Bohai Bay Basin, China


Although horizontal wells become widely used to enhance oil recovery in high water-cut stage pools, there are few research on drilling well design during ODP (Overall Development Program) stage due to limited knowledge of the target reservoir and relative larger investment, especially in offshore oilfields. A wealth of previous work focused on reservoir internal interlayer and its impacts on potential remaining oil area. However, reservoir geological modeling turns out to be the critical factor in ODP stage, which will reveal important reservoir characteristics such as reservoir distribution, variation of thickness and so on. Shallow-lacustrine deltaic reservoir is becoming one of the most important exploration & development targets in the recent years, especially in Huanghekou Sag (Neogene), Bohai Bay Basin, China. And most oilfields in this area are in ODP stage. In this work, we take BZ28-2S oilfield as an example to analysis the impacts of geological modeling on ODP stage horizontal well design and provide a guideline to reduce well-drilling risks respectively. According to the geological and geophysical work, two types of sandstone reservoir with obviously different distribution patterns are recognized. The first type of reservoir is characterized by channel-dominated sandstone with relatively limited distribution area and poorer continuity while the other type is dominated by bar or bar assemblages with wider distribution area and better continuity. Using geological models, we combine the data sets of horizontal well length, azimuth and target spot to avoid longer mudstone section in the drilling process of channel-dominated reservoir. And for the other type of reservoir, the location of horizontal section landing points is the key factor of well design in order to get a high ratio of producing reservoir. 50 horizontal wells were implemented efficiently, which take 88% of wells in BZ28-2S oilfield. The statistic data shows that 96% of the horizontal section length is oil-bearing layer with high output, which meet our design goal. This paper highlights the role of geological models and workflow that aid in reducing risks during Shallow-lacustrine deltaic reservoir ODP stage horizontal well-drilling. This geological model has been applied to several oilfields in Bohai Bay basin including QHD32-6, BZ19-4, BZ34-1N and BZ26-3. Producing results manifest that high recovery rate of horizontal wells is greatly improved.