Experimental Study On Miscibility And Diffusion Of Gas And Oil In Porous Media

The greenhouse effect caused by the high CO2 concentration in atmosphere has a great effect on the global ecological environment system, CO2 injection to enhance oil recovery (CO2-EOR) is one valid CO2 emission reduction project at present. To ensure the security of geological storage process and the efficiency of CO2-EOR, it is necessary to investigate the minimum miscibility pressure of injected CO2 and residual oil. In the process of gas injection, CO2 dissolves in the crude oil in a form of molecular diffusion. The diffusion coefficient plays a significant role in risk assessments of project and prediction of the whole performance. In this work, we used X-ray computerized tomography (CT) scanner to visualize the phase state change of gas and oil as pressure change and the process of CO2 diffusion under the conditions of constant temperature and constant pressure.We proposed a new experimental method to observe the phase state change of gas and oil and to estimate the minimum miscibility pressure (MMP) based on the X-ray CT visualization technology. The new CT measurement method had been verified by comparing the experimental results with previous studies. At the same time, we observed the volume change of oil phase and the contact angle change of two phases during the phase state change process. It showed the volume of liquid phase had a good exponential growth relationship with pressure and the contact angle of two phases enlarged as pressure increased.Based on the X-ray CT visualization technology, the experimental system to study the phase state change and oil and gas miscibility in porous media had been set up. With this system, we investigated the miscible process of gas and oil in porous media. Then we analyzed the influence of porous media pore throat radius on miscibility. The results showed that as the pore radius of the porous media decreased, the effect of capillarity on in-situ fluid phase behavior in porous media increased. This led to the increase of interfacial tension of liquid phase in porous media. The oil and gas miscible pressure in porous media increased ultimately.We proposed an experimental method to measure the diffusion coefficients of gas in oil phase under the conditions of constant temperature and pressure using the CT visualization system. By analyzing the CT images to obtain the CO2 normalized concentration along the diffusion direction, CO2 diffusion coefficients along the diffusion distance at each time could be figured out using a non-iterative finite volume method. The results showed that the diffusion coefficients gradually declined along the diffusion distance at each time, and the diffusion coefficients declined in a good exponential relationship with the increase of diffusion time at each experiment. It also showed that the diffusion coefficients increased as experimental pressure rose. Combining this measurement method with threshold segmentation, we measured the effective diffusion coefficient of gas in oil-saturated porous media to investigate the effect of porous media. The results indicated that the porous media delayed the rate of CO2 diffusion. Compared with the CO2 diffusion coefficient in pure oil solution, the CO2 effective diffusion coefficient in oil-saturated porous media was smaller.