To clarify the dynamic evolution characteristics of reservoir flow units during water injection development, the upper member of the Neogene Guantao formation in Block M of Gudao Oilfield is taken as a case study. Based on logging data, water injection profile test data, subwell data, and production performance data, among others, the flow zone index (FZI static) was proposed as the static evaluation parameter of the flow unit. The relationship between cumulative water injection (W_T) and FZI change () was fitted. Hence, the caused by water injection is combined with the static parameter of flow unit evaluation (FZI static) as the dynamic parameter of flow unit evaluation (FZI dynamic). The comprehensively evaluated reality of flow units in different periods is characterized by formula . The study shows that as the division standard of the flow unit, the FZI has a good correlation with the test results of the water absorption profile, such as water absorption intensity and relative water injection volume. Using the FZI as the static parameter of flow unit evaluation, four types of flow units were divided as follows: type I flow unit, ; type II flow unit, ; type III flow unit, ; type IV flow unit, . The reservoir porosity and permeability characteristics of different flow units are highly correlated. Moreover, the relative permeability curve confirms that different flow units have different seepage capacities. Though, comparing characteristic reservoir parameters in different periods, the reservoir’s physical parameters became more conducive to fluid flow with the water injection development. The increase in the same water injection rate for type I and II flow units was greater than that for type III and IV flow units. Furthermore, when type I flow units were continuously distributed in a large area, high water consumption bands were formed, absorbing most of the water injection in the water injection wells. Hence, the waterflood efficiency was low. The change in different flow units was mainly controlled by the injection production well pattern and W_T. Combined with the relative change characteristics of interlayer flow units, the changes can be divided into increasing type change and decreasing type changes. Finally, according to the distribution characteristics of different flow units, oil saturation, and water flooding results, strategies for tapping the potential of the remaining oil from three aspects (plane, interlayer, and inner layer) were formulated.

Improving the oil production from nonmain oil layers is crucial for the complex fault block oilfields during the late stage of high water cut development to ensure stable production and capacity expansion. However, the developing characterization of the nonmain layers and their remaining oil distribution is still inadequate. Developing an efficient mode of development remains a challenge. This paper presents a detailed characterization of the sedimentary microfacies and architecture of a nonmain reservoir based on core and logging data analysis. A flexible injection-production development strategy model was designed and applied in the Hetan oilfield, located in the southeast of Zhanhua Depression of Bohai Bay Basin as an example. The results show that the nonmain oil layer of the Hetan oilfield is composed of estuary bar microfacies sand bodies. Deposits of various shapes are formed on the side or in front of the main body of the estuary dam, which constitutes the nonmain oil reservoir. The lithology is fine, the sand body thickness is small, and the plane heterogeneity is strong. The distribution is banded, potato-shaped, and fragmented. The nonmain oil layer has a small oil-bearing area of 0.1-1.0km² and a high oil saturation of over 58. The Hetan oilfield has a significant amount of remaining oil enrichment and considerable potential for production digging. To optimize production, a flexible injection and production adjustment scheme is proposed, which includes designing multitarget horizontal wells and utilizing various methods such as optimizing well spacing, coupled injection, and cycle injection. Field tests conducted in the area have shown an increase in the recovery factor of nonmain oil layers from 16.7 to 28.5. This indicates that identifying and characterizing nonmain oil layers through detailed analysis of sedimentary microfacies and reservoir architecture is useful in the late-stage development of complex fault block reservoirs to maintain sustainable and efficient oilfield development.