Research Background The development of unconventional oil and gas resources has made shale oil an important energy source. CO₂ fracturing technology is considered to be able to increase fracture complexity and improve oil and gas production. However, in some continental shale oil fields, the actual results are not satisfactory. This prompts us to deeply explore the influence of CO₂ on shale reservoirs, so as to better guide shale oil development.
Research Methods (1) Experimental Design This study uses nano - indentation experiments to explore the influence of CO₂ treatment on the micro - mechanical properties of shale fracture surfaces, and then analyzes the changes in fracture conductivity. The UMT - 1 nano - mechanical test system and scanning electron microscope (SEM) are used to conduct a detailed analysis of shale samples. After treating the samples with CO₂ - aqueous solution, the hardness and indentation depth are measured by nano - indentation tests. In addition, physical simulation experimental equipment is used to study the changes in shale fracture conductivity and pore permeability before and after CO₂ treatment. (2) Experimental Process In the nano - indentation experiment, the experimental conditions are precisely controlled, and the shale samples before and after treatment are tested to obtain hardness and indentation depth data. The physical simulation experiment simulates the actual situation of shale reservoirs and observes the influence of CO₂ treatment on the pore permeability in different areas (near - fracture area and far - from - fracture area).
Research Conclusions (1) Changes in Micro - mechanical Properties CO₂ treatment significantly affects the micro - mechanical properties of shale fracture surfaces. After treatment, the shale surface hardness is reduced by about 22.54%, and the indentation depth is increased by 15.83%. This indicates that CO₂ softens the shale surface, resulting in deeper embedding of proppants in the fractures, thereby greatly reducing the fracture conductivity. (2) Differences in Fracture Conductivity and Pore Permeability The influence of CO₂ treatment on fracture conductivity and pore permeability is significantly different in different areas. The pore permeability in the near - fracture area is improved, which may be because the action of CO₂ has changed the local pore structure to a certain extent. However, the permeability in the far - from - fracture area is significantly damaged, and the permeability loss after CO₂ treatment exceeds 70%. (3) Influence on Long - term Production Capacity Due to the decrease in fracture conductivity and matrix permeability, the long - term production capacity is damaged. In the development of high - clay shale reservoirs, the formation damage that may be caused by CO₂ fracturing must be carefully considered. Although CO₂ fracturing initially increases shale oil production, in the long run, due to the damage to matrix permeability and fracture conductivity, it may affect the long - term production of oil wells.
Conclusions and Prospects This study clarifies the potential damage mechanism of CO₂ to fracture conductivity and matrix permeability in shale reservoirs, which has important guiding significance for optimizing shale oil development strategies and reducing formation damage. In the future, we will further explore the interaction mechanism between CO₂ and shale reservoirs, explore more effective fracturing technologies and measures to improve the benefits of shale oil development. At the same time, it is also necessary to conduct more extensive verification and application in combination with actual oil field data to provide strong support for the sustainable development of the shale oil industry.