Study On The Mechanism Of Increasing Oil Recovery By N ₂ Oil In Zhaotong Reservoir Of Tahe Oilfield

Carbonate fractured-vuggy reservoir occupies a very important position in the world’s oil and gas resources. This kind of reservoir has a large scale in reserves and its reservoir properties are very complexly. TaHe Oilfield. Tarim Basin, is a typical carbonate fractured-vuggy reservoir. By conducting water injection for oil and unit water injection early. slow down the reservoir production decline. But with the deepening of the development process, the effect of water injection for oil is increasingly poor. Pure water flooding development mode is very difficult to maintain the needs of TaHe oilfield production. Gas injection, as a method of development, freauently is used in the middle and later periods of the fractured-vuggy reservoir development. Gas injection has been carried out to enhance oil recovery in Tahe oilfield as a test. and achieved good results. In order to expand the scale of the technology. the theory mechanism of gas injection for oil should be carried out urgently.In this paper, the study content comes from the Sinopec Northwest Oilfield Branch’s research topic. This paper has researched the study of the fractured-vuggy reservoir’s experiment at home and abroad and the technique of gas injection to enhance oil recovery. On the basis of the similarity theory, geological condition of S48well group and TK404well in TaHe Oilfield as the background, design visualization of large-scale physical models and high-temperature and high-pressure full-diameter core physical model. Do indoor experimental research that N2injection for oil with the preparation of models and analyze the mechanism for enhanced oil recovery in fractured unit by injecting N2. The main research works which have been done are following points.1. The visualization physics experiments of N2huff-puff with the model of TK404single well have completed. The results showed that:After the bottom water flooding, a large part of oil in the upper cave can not be produced, and become the "attic oil"; Under the action of gravity differentiation, N2occupy the upper part of the cave and form N2cap, so that the "attic oil" that in the upper part of the cave would be replaced to the middle and lower parts of the cave to be produced.2. The high temperature and high pressure experiments of N2huff-puff with the full-diameter core model of TK404single well have completed. The results showed that:The porous media in the model have been formed after filling with quartz sand, and it can slow down the gas-liquid interface’s advancing speed and delay the time of gas breakthrough; In the process of huff-puff, the more number of depressurization, the higher degree of recovery; Under the high temperature and high pressure, N2will be partly dissolved in crude oil generally after contact with crude oil, and it makes crude oil volume expand in order to displace the oil that in the little fractures/caves and a good part of the micro fractures.3. The visualization physics experiments of N2flooding with the model of S48well group have completed. The results showed that:The relative position of fractures and caves, the connectivity of caves and the coordination ratio of fractures and caves have effect on the displacement efficiency; With N2flooding, N2occupy the upper part of the cave to displace the "attic oil" effectively and the remaining oil in the great fractures/caves that difficult to use by water flooding can be produced; N2flooding improved fluid’s flow conditions in the reservoir, increased the density difference and reduced the interfacial tension; By different methods of injection-production have a greater influence on the recovery degree, choosing the appropriate injection well and production well has important significance to improve the whole reservoir recovery.