FRACTURING MECHANISM, PHASE TRANSFORMATION, AND ENERGY ENHANCEMENT EFFECT OF CO₂ FRACTURING FLUIDS IN TIGHT RESERVOIRS

CO₂ fracturing and energy enhancement represent pivotal technological directions for the development of tight oil and gas reservoirs. This paper systematically compares the thermophysical properties of CO₂ and water under varying temperature and pressure conditions. Tight reservoir samples from the Chang 7 Member of the Yanchang Formation in the Longdong area of the Ordos Basin were selected for physical simulation experiments of CO₂ fracturing. By comparing the differences among CO₂,slickwater,and gel fluids as fracturing media,and incorporating acoustic emission monitoring and CT-based fracture network reconstruction,the study analyzes the initiation and propagation of fractures under CO₂ fracturing. The results show that CO₂,characterized by its compressibility,low viscosity,and high diffusivity,dynamically transitions between liquid and supercritical states during the fracturing process,influenced by injection rate,formation temperature,and fracture morphology,among other factors. It is confirmed that the high compressibility and diffusivity of CO₂ facilitate efficient pressure transmission,promote the opening of weak structural planes,significantly reduce fracture pressure,and enhance fracture height extension. Under the temperature and pressure conditions of the tight reservoirs in the Longdong area,liquid CO₂ undergoes complex transformations from storage and wellbore injection to fracturing operations and flowback stages. Owing to its unique thermophysical properties,CO₂ undergoes multiple volume changes and phase transitions,releasing substantial energy to drive fluid flow within the reservoir and enhance reservoir energy. This research elucidates the mechanisms of CO₂ fracturing and highlights its significant potential in reservoir stimulation and production improvement,providing theoretical and technical support for the efficient development of tight reservoirs.