The Study On Carbon Dioxide Diffusion In Alkane Based On Pressur E Decay And MRI

CO2injection into oil reservoirs, as a high-efficient method of Enhance Oil Recovery (EOR), has attracted more and more attention in the oil industry. It can both enhance oil recovery and realize CO2geological sequestration. The efficiency of CO2-EOR greatly depends on the dissolution capacity of CO2in oil and the property of oil after CO2dissolving in. For reducing oil viscosity and increasing the volume of oil on a greater level to gain optimal efficiency of oil recovery, it needs enough time to make oil saturated with CO2. The gas diffusion coefficient is a very key factor to determine the gas dissolving capacity and dissolution velocity. Thus, it is necessary to study the mechanism of CO2diffusion in oil reservoirs. In this thesis, the CO2diffusion process has been studied in pure dodecane, tetradecane and octodecane and oil-saturated porous media using pressure decay method integrated Magnetic Resonance Imaging (MRI). The pressure decay method was applied to determined diffusion coefficient, while MRI was to the visualization study of diffusion process.Through analyzing the experiment data of CO2-pure alkane diffusion system using the method mentioned above, we have found, under the experiment temperature condition, the CO2diffusion coefficient in these three kind of pure alkane is in a linear increase relationship with the initial system pressure within the range of experiment test pressure. In addition, the diffusion coefficients calculated using the double-exponential method are slightly higher than those using the slope method, because the former method has taken the incubation period data into account. And the system equilibrium pressure increases linearly with the initial gas phase pressure within the range of experiment test pressure.This thesis has built a novel mathematical method for analyzing the images of the CO2diffusion process in pure alkane obtained from MRI. Under experiment conditions, the volume expansion velocity of liquid phase decreases with CO2resolving in continuously, and increases with the initial gas phase pressure. Additionally, the time of liquid phase volume reaching equilibrium state will cut down if the initial pressure increases. Moreover, the expansion ratio of oil phase increases linearly with initial gas phase pressure within the range of experiment test pressure, and decreases with the molecular mass of the oil phase increasing.This thesis has analyzed the CO2diffusion process in oil-saturated porous media using improved pressure decay method and finds that, under the experiment temperature condition, the CO2diffusion coefficient increases linearly with the initial system pressure within the scope of the test pressure in oil-saturated porous media. The CO2diffusion coefficient in porous media system is lower than that in pure alkane system. In addition, the system equilibrium pressure increases linearly with the initial gas phase pressure within the range of test pressure.This thesis has found, under the experiment temperature condition, the oil phase expansion ratio of the CO2diffusion system in oil-saturated increases linearly with the initial pressure, but slightly lower than that in pure alkane.This thesis has successfully caught the migration process of the CO2diffusion leading edge in oil-saturated porous media. Due to the change of the CO2concentration difference between the leading edge and the oil without CO2dissolving in, the migration velocity of CO2diffusion leading edge decreases. In addition, if the initial system pressure increases, the time of the leading edge reaching the bottom of the diffusion cell will become shorter.