Heterogeneity and fractures significantly influence oil and gas migration. However, current research remains insufficient in clearly visualizing CO₂ displacement mechanisms under such reservoir conditions. This study combines microscopic visualization experiments and numerical simulations to investigate three-phase flow characteristics during CO₂ huff-n-puff, CO₂ displacement, and water huff-n-puff in heterogeneous fractured reservoirs. The research elucidates the formation mechanism of residual oil and the characteristic of CO₂ storage during huff-n-puff, and further illustrates the influences of soaking time and injection pressure. Results indicate that CO₂ huff-n-puff significantly mitigates the adverse effects of heterogeneity. The primary cause of residual oil formation is insufficient displacement energy to overcome various flow resistances. CO₂ primarily exists in pore throats as bubbles and is stored in dissolved form in the oil and water phases. As the soaking time and injection pressure increase, the proportion of continuous residual oil decreases noticeably, leading to a higher oil recovery factor. When the injection pressure exceeds the minimum miscible pressure (MMP), the diffusion coefficient of CO₂ increases significantly. Moreover, the presence of vertical fractures effectively expands the diffusion range of CO₂. These findings provide a theoretical basis for the applications of carbon dioxide enhanced oil recovery (CO₂ EOR) and carbon capture, utilization, and storage (CCUS) technologies.

Combined with unconventional oil and gas field development technology and based on microfluidic technology, a set of unconventional oil and gas field chemical working fluid flow simulation experiment system was designed, and the unconventional oil and gas field chemical working fluid micro-flow simulation experiment device and method were established, which realized the visualization and quantitative characterization of chemical working fluid flow characteristics under the condition of reservoir structure, and can be used as an important auxiliary means to study the flow characteristics of chemical working fluid, catalyst migration and phase transformation of oil and gas under the condition of unconventional reservoir structure. The experimental design is conducive to stimulating students' enthusiasm for learning and research, broadening their interdisciplinary research vision, and cultivating their comprehensive analysis and application practice ability. It also helps students form the ability to combine theory with engineering practice to solve practical engineering problems on the site.

The efficient pyrolysis and conversion of organic matter in organic-rich shale, as well as the effective recovery of pyrolysis shale oil and gas, play a vital role in alleviating energy pressure. The state of carbon dioxide (CO₂) in the pyrolysis environment of shale reservoirs is the supercritical state. Its unique supercritical fluid properties not only effectively heat organic matter, displace pyrolysis products and change shale pore structure, but also achieve carbon storage to a certain extent. Shale samples were made into powder and three sizes of cores, and nitrogen (N₂) and supercritical carbon dioxide (ScCO₂) pyrolysis experiments were performed at different final pyrolysis temperatures. The properties and mineral characteristics of the pyrolysis products were studied based on gas chromatography analysis, X-ray diffraction tests, and mass spectrometry analysis. Besides, the pore structure characteristics at different regions of cores before and after pyrolysis were analyzed using N₂ adsorption tests to clarify the impact of fracturing degree on the pyrolysis effect. The results indicate that the optimal pyrolysis temperature of Longkou shale is about 430 ℃. Compared with N₂, the oil yield of ScCO₂ pyrolysis is higher. The pyrolysis oil obtained by ScCO₂ extraction has more intermediate fractions and higher relative molecular weight. The ScCO₂ can effectively improve the pore diameter of shale and its effect is better than that of N₂. The micropores are produced in shale after pyrolysis, and the macropores only are generated in ScCO₂ pyrolysis environments with temperatures greater than 430 ℃. The pore structure has different development characteristics at different pyrolysis temperatures, which are mainly affected by the pressure holding of volatile matter and products blocking. Compared to the surface of the core, the pore development effect inside the core is better. With the decrease in core size, the pore diameter, specific surface area, and pore volume of cores all increase after pyrolysis.

Tight oil resources are abundant in the world. It is very important to strengthen the research on the development theory and technology of tight oil reservoirs for ensuring national energy security. Natural gas huff-n-puff can effectively improve the oil recovery of tight oil reservoirs. However, the pore-scale oil production characteristics and the mechanisms of natural gas huff-n-puff in matrix-fracture cores are poorly understood. The influence degree of important factors on oil recovery is not clear and the interactions between factors are rarely considered. In this paper, the oil production characteristics and mechanisms of natural gas huff-n-puff in tight cores with different fracture lengths were quantitatively analyzed by combining nuclear magnetic resonance (NMR) with numerical simulation technology. The influencing factors and their interactions were evaluated by the response surface method (RSM). The results show that tight cores mainly consist of medium pores (0.1–1 μm) and small pores (0.01–0.1 μm). The fracture mainly increases the proportion of macro-pores (1–10 μm) and medium pores. In the natural gas huff-n-puff process, crude oil from macro-pores (1–10 μm) and medium pores is mainly developed, and the contribution percentage of crude oil in medium pores to oil recovery is the largest, up to 98.28%. The position of gas–oil contact (GOC) moves deeper as the number of huff-n-puff cycles increases. The contents of CH₄ and CO₂ in the oil phase remain at a high level within the GOC, while between the GOC and the component sweep front, the contents of CH₄ and CO₂ in the oil phase decrease with the increase in dimensionless distance. The gas component sweep volume is increasing with the increase in fracture length. Moreover, the injected natural gas mainly extracts C₃–C₁₀ components from crude oil. The reduction law of crude oil viscosity is consistent with the migration laws of CH₄ components along the path. Compared with soaking time and gas diffusion coefficient, the injection pressure is the most significant factor underlying the recovery of natural gas huff-n-puff in tight cores. Besides the influence of single-factor, the interaction effects of gas injection pressure and diffusion also should be considered to determine the huff-n-puff parameters in the field implementation of natural gas huff-n-puff in tight reservoirs after fracturing.