KEYWORDSlow carbon, CO 2 flooding, flooding mechanism, carbon capture utilization and storage, enhanced oil recovery Research statusCO 2 flooding has been used globally to enhance oil recovery for more than 30 years and is becoming more popular. Therefore, an increasing number of researchers are engaged in research on CO 2 flooding (Chen et al., 2015). There are many reported CCUS-EOR projects from domestic and foreign investigators, as shown in Table 1. Research on CO 2 flooding began in the 1950s (Whorton et al., 1952), and some European countries had earlier used CO 2 flooding to enhance oil recovery. The former Soviet Union first began studying CO 2 injection enhanced oil recovery (EOR) technology in 1952. In 1968, CO 2 flooding tests were carried out in the Tuimajin Oilfield, which achieved a 15% increase in the final crude oil recovery (Qin et al., 2015). CO 2 flooding developed the

Fabrication method and device of ultra-small gradient-index (GRIN) fiber probe were investigated in order to explore the development of ultra-small probes for optical coherence tomography (OCT) imaging. The beamexpanding effect of no-core fiber (NCF) and the focusing properties of the GRIN fiber lens were analyzed based on the model of GRIN fiber probe consisting of single-mode fiber (SMF), NCF and GRIN fiber lens. A stereo microscope based system was developed to fabricate the GRIN fiber probe. A fiber fusion splicer and an ultrasonic cleaver were used to weld and cut the fiber respectively. A confocal microscopy was used to measure the dimensions of probe components. The results show that the sizes of probe components developed are at the level of millimeter. Therefore, the proposed experimental system meets the fabrication requirements of an ultra-small self-focusing GRIN fiber probe. This shows that this fabrication device and method can be employed in the fabrication of ultrasmall self-focusing GRIN fiber probe and applied in the study of miniaturized optical probes and OCT systems.

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The weight loss of cement slurry is the main cause of early annular air channeling and accurate experimental evaluation of the law of loss change is the key to achieve compression stability and prevent this undesired phenomenon. Typically, tests on the pressure loss of cement slurry are carried out for temperature smaller than 120°C, and this condition cannot simulate effectively the situation occurring in high temperature wells. For this reason, in this study a series of experimental tests have been conducted considering a larger range of temperatures, different retarders and fluid loss additives. The results show that with an increase in the temperature, the weight loss curve of cement slurry changes from a "two-stage" to a "three-stage" behavior, and the risk of channeling increases accordingly. On increasing the amount of retarder and fluid loss additive, the transition time of cement slurry displays a non-monotonic behavior (it decreases first and then increases). It is found that the optimized retarder and fluid loss additive dosage are 0.2% and 2.5%, respectively.

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