Carbon dioxide pre-fracturing has shown high application potential in improving oil recovery in conglomerate reservoirs. However, the influence of CO₂ on the physical properties of reservoir rock and its diffusion behavior within the reservoir matrix have not been systematically studied. This paper integrates CO₂-saturated water soaking experiments, true triaxial fracturing experiments and field-scale tests to demonstrate that CO₂ soaking induces quartz reduction and clay mineral increase, leading to a decrease in porosity and mechanical strength. Clay-cemented conglomerates experience a greater loss in compressive strength and a higher reduction in permeability compared to calcareous-cemented counterparts under identical CO₂ soaking. In the horizontal principal stress direction, CO₂ fracturing achieves a greater fracture penetration depth than slickwater fracturing or CO₂ pre-injection followed by slickwater fracturing. CO₂ pre-fracturing reduces breakdown pressure by 15%-5% and increases fracture complexity. Field tests confirm a reduction in injection pressure and improved effective stimulation. However, dnarrower fracture width and higher tortuosity may limit proppant transportation.

Huff-and-puff is a key technology for the efficient recovery of oil and gas from tight reservoirs. Active water and CO₂ are two huff-and-puff media with great development potential; however, their effects on enhanced oil recovery and the contribution of imbibition displacement to enhanced oil recovery need further investigation. In this paper, short cores were spliced into long cores for huff-and-puff experiments, and then nuclear magnetic resonance testing was performed to test the transverse relaxation time spectrum of different core sections at different huff-and-puff cycles. Subsequently, the enhanced oil recovery effects, limited effective distances, and influencing factors of active water and CO₂ huff-and-puff were evaluated. Meanwhile, a comparative experiment without well soaking in some specific huff-and-puff cycles was designed to quantitatively split the contribution rate of elastic displacement and imbibition displacement. The results show that active water huff-and-puff mainly mobilizes crude oil in large pores, while CO₂ huff-and-puff can also mobilize crude oil in small pores. The cumulative oil recovery of active water and CO₂ after 4 cycles of huff-and-puff was 24.78% and 40.89%, respectively, and the limited effective distances were 6-8 cm and 8-10 cm, respectively. Elastic displacement is considered the main enhanced oil recovery mechanism of active water and CO₂ huff-and-puff, while imbibition displacement accounts for 20.86% and 31.52%, respectively. Due to its good diffusion and mass transfer ability, CO₂ can more fully participate in the mechanism of imbibition displacement and further improve oil recovery. The findings of this paper can provide valuable theoretical and field data support for the application of huff-and-puff technology in tight reservoirs.