Investigations of the Fracture Heterogeneity Effect on Imbibition Process during Water Flooding in Buried-Hill Oil Reservoirs, Liaodong Bay

The Archean metamorphic buried-hill oil reservoirs are characterized by the development of natural fractures in different scales, which makes them an important type of reservoir in the Liaodong Bay, China. The heterogeneity of fractures between vertical members could lead to significant differences in multiphase flow. In this study, to investigate the different characteristics of the relative permeability and capillary pressure curves during imbibition, spontaneous imbibition experiments using three representative core plugs collected from the upper and lower members of semiweathered crust and inner basement zones in the BH reservoir under reservoir conditions. Then, regression of the cumulative oil production was conducted to obtain the capillary pressure and relative permeability curves base on imbibition mathematical model. Experimental results show that oil production rate depends on the density of microfractures. The oil production rate of the sample JZS2 from the lower member is larger than the sample JZS1 from the upper member as the former developed more microfractures, which provides more channels for oil migration. Therefore, irreducible water saturation and residual oil saturation from the relative permeability curves are smaller for the sample JZS2. While for sample JZS3 with almost no microfractures developed, the oil production rate is much smaller as oil inside the matrix has to migrate longer distances. Hence, its two-phase region is the smallest among the three samples. Based on the regressed relative permeability and capillary pressure curves, numerical simulations in the BH reservoir were performed. History match results show that water-cut can be well matched by employing the three sets of relative permeability and capillary pressure curves instead of one. Three stages classified based on the oil production contribution ratios from the fracture system. Produced oil was mainly contributed from the fracture systems in Stage I, and gradually decreased in Stage II because less oil was left in the fracture, and water coning occurred subsequently. In Stage III, oil was produced from the matrix systems mainly. Overall, this study illustrated the effect of fracture heterogeneity on the imbibition process during water flooding and provided insights into designing water injection strategies for displacing remaining oil in fractured oil reservoirs.