Abstract
After long-term water injection development, most of the oilfields in China have entered the stage of high-water cut, which has reached up to 90%. Due to the strong heterogeneity of the reservoir, more than 50% of the oil remains underground in most oilfields. Therefore, how to predict the distribution and content of remaining oil quickly and accurately in heterogeneous reservoir has become the key of EOR. In this paper, a new effective water-flooding unit model is established based on a three-dimensional flow function, which can characterize the influence of vertical heterogeneity on flow and the streamline distribution. In addition, two shape functions are defined in the model to characterize the oil–water two-phase flow characteristics in an injection-production unit. The results show that the streamline in the lower part of the positive rhythm reservoir is denser, which leads to the formation of dominant seepage channel with ease. However, for the reverse rhythm reservoir, dominant seepage channel forms in the upper part of the reservoir. Besides, for the two types of reservoirs, the greater the permeability difference is, the faster the water cut increases. Furthermore, under the same rhythm condition, the positive rhythm reservoir reaches 90% water cut half a year earlier than the anti-rhythm reservoir. This study provides new insight and guidance for the development of remaining oil in rhythmic reservoirs.
Highlights
At present, the main oil fields in China have entered the late stage of development after a long period of exploitation
A theoretical model of the three-dimensional EWU is established through the heterogeneous shape function and the principle of superposition of hydrodynamic potential; it is applied to the reservoir under the pros and cons of rhythm
The three-dimensional EWU theoretical model is based on the two-dimensional EWU theory (Nan et al 2016), because the flow function is defined by the incompressible fluid under two-dimensional plane conditions
Summary
The main oil fields in China have entered the late stage of development after a long period of exploitation. The flow units in the reservoir are divided according to the EWU theory through the streamline distribution and velocity distribution and obtain the saturation change at different times on the streamline. The lower streamlines of the oil reservoirs in area II are relatively sparse, indicating that the flow velocity is slow and the degree of oilfield development is low. It can be seen that permeability is a more important factor, which determines the degree of reservoir exploitation Perforation is used to increase the water injection pressure in the low-permeability area at the bottom to expand the water flooding area; on the other hand, the top area is properly sealed to slow the formation
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