| Petroleum is a kind of fluid mineral buried in deep strata, compared with other mineral resources, oil recovery rate is low. After primary oil recovery by natural energy, water flooding, as an effective measure to maintain reservoir pressure and accelerate the rate of oil production, is widely used in secondary oil recovery of oilfields. As water resources are widespread distributed and the price is low, water flooding has significant practical advantages. The vast majority of oilfields in operation of our country have been using water flooding as development way. With the development of oilfield, tertiary oil recovery has been developed with diverse forms. The purpose of secondary and tertiary oil recovery both focus on taking remaining oil in the reservoir out as much as possible economically and efficiently. Although the effect of tertiary oil recovery is obvious, application of tertiary oil recovery method is subject to fluctuations of crude oil prices, and there are some limits in specific application with a lot of extra capital investment. If new ways of water flooding development with no additional funds but effective oil recovery can be found, the existing water flooding development technology system will be further improved, and oilfield development effect will be improved.Geological reserves of about2/3were left underground in secondary oil recovery by water flooding development in our country, even after acidification, fracturing and other stimulation treatment, the overall recovery rate is still not high, only improving the performance of fluid flow near well zone. Through comparative study of oil and gas accumulation and water flooding process, it can be found that oil-water front presents episodic movement during oil and gas accumulation process. Oil-water aggregation process contact rules and water advance and retreat orderly during the process of oil phase from dispersion to accumulation, so that the accumulation of crude oil effect is good and the sweep efficiency is high, and water drive oil process can make the original oil saturation reservoir above2/3. While, water flooding is a process of injected water making oil dispersed. Because of oil-water viscosity, the fingering phenomenon is serious, the oil-water contact surface is irregular uneven, and the oil phase continuity is poor, making a low sweep efficiency. Meanwhile, the existing water flooding development model ignores the distribution state of underground oil displacement, water injected mostly moves along the direction of maximum pressure gradient between injection well and production well, thus forming the mainstream line and separating the patches of crude oil. Furthermore, because of low viscosity of the injected water, fingering phenomenon is obvious, so that the distribution state of underground fluid will become more complex and the oil phase continuity is further damaged, resulting in a large number of high cutting dispersed oil left underground. Studies suggested that by changing the injection-production mode to oil-water front forming a episodic development, the sweep efficiency of water flooding would be expanded and oil recovery would be enhanced. Unlike the drill of infill adjustment wells, episodic flooding is to maintain the continuity of the oil phase, while the former would only make the oil dispersed more serious. Numerical simulation results of single unit showed that under the same water injection total amount and development time, oil recovery of vertical well episodic flooding of seven spot and nine spot well patterns could both be enhanced for about4.5percentage points.For anisotropic reservoirs, vertical well episodic flooding of six spot well pattern and vertical-horizontal wells mixed well pattern were studied on basis of five spot well pattern episodic flooding research. The numerical simulation results validated that the two schemes above were able to improve oil recovery of the simulation block3-4percent under the same water injection total amount and development time.According to the research results, episodic flooding presented some superiority. The theory took the oil distribution state underground into account, implementation of the program aimed to maintain the distribution state of underground oil and water and using efficiency of injected water was improved. In addition, episodic flooding technology solely relies on the adjustment of injection-production pattern with low technical difficulty, and there is no additional investment without the use of additives. Numerical simulation results showed that episodic flooding well patterns had the following advantages: Firstly, oil recovery of episodic flooding well patterns were higher than that of the conventional well patterns under the same water volume injected. This means the development mode has higher sweep efficiency, better injection water use efficiency, which can effectively save water resources. So it has an important significance for inadequate water supply block to improve the development effect of water flooding. Secondly, the development time would be shorter for episodic flooding to reach the same degree of oil recovery, thus greatly shorten the payback period and reservoir production time and improve the economic benefits of oil field development. Thirdly, well numbers in work during the whole process of episodic flooding development were less than that of conventional well pattern development, which could save the high cost of drilling infill wells in "dead area" and daily maintenance costs of the injection and production wells. For the last, the whole process is no chemical agent injection and no additional cost of production, so the produced water treatment is easier and will have less impact on environment. It has outstanding economic benefit and social benefit as it is conducive to environmental protection and conservation oriented development of the oilfield,.Research showed that episodic water flooding method is an effective way to improve the development effect with economic efficiency, and it is worthy of further in-depth research, thus improving the technology theory system of episodic flooding and guiding the field application in the future. |
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