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Title: Improvement of Carbon Dioxide EOR in Water-Wet Reservoirs by Using Active Carbonated Water
Author: Shu, Guanli
Advisor: Dong, Mingzhe
Chen, Shengnan
Keywords: Engineering--Petroleum
Abstract: CO2 injection for enhanced oil recovery has been widely and successfully used in many oil fields. The drawbacks of conventional CO2 injection are high mobility and gravity segregation, which lead to low sweep efficiency in the reservoir. Particularly in strongly water-wet reservoirs, a significant trapping of oil by water occurs after CO2 flood due to water blocking effect. To overcome the effect of water blocking, carbonated water flood is considered. In this thesis, a new injection strategy is proposed to recover the trapped residual oil in water-wet reservoirs. After waterflood, a slug of carbonated water is pre-flushed before CO2 flood, followed by an extended waterflood. A series of parallel tests are performed to compare the recovery efficiencies of tertiary floods. Additionally, the slug size of carbonated water is investigated and optimized to achieve the maximum economic value. In accordance with experimental results from parallel tests, it is recognized that molecular diffusion plays important role in recovering the trapped oil. In order to study the mass transfer process of dissolved CO2 from carbonated water into oil phases, a theoretical model is developed. The diffusion process is governed by diffusion coefficients of CO2 in water and oil phases. To determine these two diffusivities, an experimental method is proposed. In the experiment, two phases are placed in a closed diffusion cell immersed in a water bath at a constant temperature. To avoid natural convection, the water phase locates at the bottom and oil phase lies at the top. Combined with a developed theoretical model, pressure changes that occur in the cell are recorded and analyzed to investigate the mass transfer process. By means of the theoretical model and experimental method, effects of operational parameters on diffusion coefficients are studied. To explain the reasons for enhanced oil recovery by pre-flushing carbonated water before the CO2 flood, three types of experiments are involved in investigating the mechanisms. The derived theoretical model and experimental method can be applied to study the mass transfer process and to determine diffusion coefficients for any liquid-liquid system.
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