Residual oil distribution characteristic of fractured-cavity carbonate reservoir after water flooding and enhanced oil recovery by N2 flooding of fractured-cavity carbonate reservoir

Abstract

The 4th block Ordovician fractured-cavity carbonate reservoir of Tahe oil-field consists of high-density fractures and large caves which are well developed. The oil is stored in fractures and caves, rather than carbonate rock matrix. In other words, the carbonate rock matrix has no oil storage capability and permeability is extremely low. The fractured-cavity carbonate reservoir possesses the features of severe heterogeneity, complex structure of reservoir space. During the development, many shortcomings such as short period of stable production, early water breakthrough, fast increasing of water cut, high natural decline rate and low degree of development are exposed gradually. The well which cannot achieve effectiveness by water injection was selected to carry out N2 flooding experiments in Tahe oil-field. By the end of September 2012, the total injection volume of high pressure N2 was 700 m3 and the incremental oil production was 2600 t. However the displacement mechanism was still poorly understood. In this paper, a two-dimensional visual fractured-cavity carbonate reservoir model was developed based on the distribution feature, space structure and connection type of fractures and caves in reservoir space. Four main types of residual oil i.e., attic oil, locked oil, bypassed oil and residual oil trapped in the filled area were determined from the water flooding experiments. Also, their formation mechanism was investigated. In addition, the recovering mechanism of residual oil was clarified on the basis of N2 flooding experiments. The results show that N2 flooding can significantly increase the oil recovery by more than 40% after water flooding, and it mainly mobilized attic oil and partial locked oil. The entire experiment process was recorded by video, therefore the displacing process of residual oil can be observed directly. These results in this study indicate that N2 flooding has a broad application prospect in fractured-cavity carbonate reservoir.