Study of Phase Behaviours of Reservoir Fluids with Consideration of Adsorption and Capillary Pressure
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With the development of petroleum science and technology, a systematic understanding of the fundamental phase behaviours of petroleum fluids in bulk volume has been well established. However, the effect of porous media on the phase behaviours of reservoir fluids has always been considered a deep and complicated research topic that has attracted the attention of many scholars worldwide. During the production process, with the change of pressure and temperature, the phase behaviours of reservoir fluids are then changed. Strictly speaking, the process of phase equilibrium occurs in the porous media of formation, and the effect of porous media cannot be neglected. In tight reservoirs, the fluid molecules keep close to the surface of the porous media and certain interactions exist between the fluids and reservoir media. Due to the fine particles, small pore radii, complicated pore structures, and large specific surfaces of the porous media, there always exists a large variety of surfaces between reservoir fluids and reservoir media. Thus, the surface phenomena are extremely prominent. The molecules on the surface of the media will attract the molecules of fluids as the result of molecular force and this process is presented as adsorption. Meanwhile, surface tension, wettability, and capillary pressure will strongly affect the phase behaviour of reservoir fluids. Adsorption of the gas phase on porous media is investigated as specific phase equilibrium in the thesis. Potential theory is introduced to study the adsorption of a pure gas component, which takes the advantage of prediction adsorption behaviours. It not only extends the adsorption isotherms from the lab temperature to other temperatures, but it also predicts the adsorption isotherms of a pure gas component based on the data of a different gas component. The Flory-Huggins Vacancy Solution Model (FHVSM) is introduced to study the adsorption behaviours of a gas mixture. Unlike other models, this theory gives new ideals about treating the adsorption sites on a media surface as a solution and an additional component that is then engaged in the phase equilibrium, so that it could explain the competitive adsorption phenomenon, which is proved during the adsorption process of gas mixture. Thus, except for precision predications of desorption for the depletion process, it could give simulations for some other production operations, such as CO2 or N2 injection for the replacement of hydrocarbons. The results also reveal that the FHVSM method shows a high degree of precision, which gives better predictions than the commonly used models for gas mixtures, such as the Langmuir model and multi-component potential model. The effect of capillary pressure on phase behaviour is then studied in this thesis. Usually, the fluid PVT properties are obtained from a large cell where no capillary pressure exists. The model is established by introducing a difference of gas pressure and oil pressure equal capillary pressure during dynamic phase equilibrium. This study shows the results of different phase equilibrium cases by applying the model to a retrograde condensate gas sample with consideration of the effect of surface tension, wettability, and capillary radius during the general PVT analysis process. The results reveal that the impact of capillary pressure on phase equilibrium is remarkable especially when the pore radius is in nanoscale.