## 3 Results and Discussion
### Hybrid Huff-n-Puff in Micromodels  
To evaluate the potential application of the proposed hybrid huff-n-puff in EOR, the micro reservoir model resembling the complex pore structure of the geologic reservoir was used for the flooding experiments. Initially, the micro reservoir model was vacuumed and filled with crude oil. The total amount of oil in the reservoir is known as the original oil-in-place (OOIP). As mentioned earlier, these flooding experiments were conducted in three stages: first flooding, oil swelling, and second flooding. For the flooding experiments, DI water and DI water with 2% (\(w/v\)) SDS surfactant were used as the flooding fluids to analyze the effect of SDS. In the first stage, flooding was performed with DI water or DI water-SDS at the flow rate of 20 \(\upmu\)L/min for 30 min. This duration was selected based on the observation that flooding beyond 30 min yielded negligible additional oil recovery. The oil recovery was quantified through image processing of recorded video footage using MATLAB R2022b software. The process involved capturing an initial reference image of the oil-saturated porous medium before flooding to define OOIP. During the flooding process, changes in greyscale pixel values between the reference image and subsequent frames were analyzed to identify oil displacement, where darker regions represented oil and lighter regions indicated cleared areas. The pixel intensity differences were correlated to oil volume, providing a quantitative estimate of oil recovery. To ensure accuracy, consistent lighting conditions, camera calibration, and preliminary tests with known oil volumes were performed. This method has been used in many published articles [23]. Figure 7 shows time-dependent oil recovery during the first-stage flooding with pure DI water and SDS-DI water. In the case of pure DI water flooding, around 10% of OOIP was displaced over 30 min. Upon 2% (\(w/v\)) SDS addition in DI water, 13.5% oil recovery was achieved during the same flooding experiment period. This 3.5% improvement in oil recovery can be attributed to the adsorption mechanism of SDS surfactants, as explained earlier. Upon analyzing the video to validate results, we found that the breakthrough during flooding occurred at 808 s and 850 s in DI water and SDS-DI water flooding, respectively. Although the 42 s delay is not significant, it can be attributed to the comparatively broader sweep volume of the pore network by SDS-DI water. In other words, the SDS-DI water solution could penetrate deeper into the pores and take longer to reach the production port than DI water. (The drop in oil recovery for SDS-DI water, as shown in Figure 7, is due  
Figure 7: Oil recovery during DI water flooding and SDS–DI water flooding. to a small disturbance observed around 850 s. After breakthrough, residual oil from the tiny pores was mobilized and carried into the main channels along with the flooding fluid. This caused a temporary drop in the recorded intensity values, which is reflected as a spike error in the recovery curve.) Due to the adsorption mechanism and IFT reduction caused by surfactants, the SDS-DI water solution was able to imbibe the oil from tiny pores more effectively, thereby sweeping a larger pore volume. The oil imbibition phenomena can be visually seen during the experiment, as shown in Figure 8 (see Supplementary Video S1). Two images were taken approximately at 30 min (almost at the end of the flooding) of the DI water and SDS-DI water flooding experiment. The highlighted section shows the exact pore network in both cases, where it is apparent in the red-circled area that SDS-DI water flooding could reach deeper into the pore network than DI water. To simulate the hybrid huff-n-puff process in the tested micromodel, after the first flooding stage, an intermediate oil swelling by CO\({}_{2}\) (i.e., soaking) for 6 h was performed, followed by second flooding. This 6 h duration was chosen based on previous fluorescence-based microscopic experiments for CO\({}_{2}\) dissolution in oil.