| Carbon dioxide tertiary or secondary oil recovery becomes increasingly significant to the petroleum industry. CO2flooding can not only enhance or improve oil recovery clearly but also considerably reduce greenhouse gas emission. Thus, research on the CO2flooding is of great theoretical and realistic significance.In this study, combination of polymer flooding and CO2flooding and supercritical CO2flooding were studied using magnetic resonance imaging (MRI) and X-ray computer tomography (CT) techniques, respectively. In the study of combination of polymer flooding and CO2flooding, crude oil saturation and oil distribution were recorded successively using MRI technique. Through analysis of MRI data, the fundamental characteristics of the flooding process, such as the piston-like displacement front, the phenomenon of channeling and fingering, homogenization of the oil saturation distribution with CO2injection, and the distribution of oil in porous media, can be accurately detected. For polymer displacement, with the increasing concentration of polymer solution, the visicosity of polymer solution increased, which improved mobility ratio and reduced the phenomenon of channeling and fingering, the final oil recovery increased obviously. In addition, Comparing with alone applying polymer flooding process, following supercritical CO2displacement could enhance oil recovery evidently and make up for the shortage of polymer displacement and homogenizes the oil saturation distribution in the bead-pack core. Due to the higher oil/CO2interfacial tension than the oil/polymer interfacial tension, when CO2was injected in porous media, the CO2was the first to sweep the regions where interfacial tension was low and the residual oil saturation was high, and the CO2flooding displaced mainly residual oil saturation in high oil saturation regions and homogenized and improved the oil saturation distribution. This study demonstrated that the combination of polymer flooding and CO2flooding was viable.In the study of supercritical CO2flooding, the physical experiment simulation was used to study the displacement process and reduction of crude oil effective permeability due to asphaltene precipitation. Meanwhile, using X-ray CT technique studied the influence of asphaltene precipitation on porosity and permeability. In this paper, a total of three CO2coreflood tests under immiscible conditions were performed through the so-called dry, secondary, and tertiary oil recovery processes, respectively. Research indicated that both asphaltenes precipitation and metal carbonates precipitation reduced the crude oil effective permeability. The oil effective permeability reduction becomes the largest to32.75%in the secondary water flooding and subsequent CO2tertiary flooding and smallest to17.84%in the CO2dry flooding. Comparing segmented CT images, it is found that asphaltenes deposited onto the sand grains and blocked the small pore, and the porosity of porous media decreased. Some asphaltenes blocked the throat and isolated the pores, which destroyed the connectivity. In addition, the porosity-based permeability model was developed to study the effective permeability reduction due to porosity reduction. Based on Kozeny-Carman equation and experiment data, the porosity-based permeability was calculated. The porosity-based permeability reduction approximates the Darcy-formula permeability reduction and both kinds of permeability reduction have the same variation tendency,which demonstrates for artificial core the method of permeability reduction estimation based on porosity reduction of porous media is viable.. |
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