Laboratory visualization of supercritical CO₂ fracturing in tight sandstone using digital image correlation method

Hydraulic fracturing is a crucial technology for reservoir stimulation in unconventional reservoirs. Conventional water-based fracturing has several potential issues of environmental pollution and waste of water resources. Anhydrous fracturing fluids, such as supercritical CO₂ (SC–CO₂), are now utilized for fracturing in recently years. However, the fundamental mechanism of fracture initiation and propagation during SC-CO₂ fracturing has not been fully understood. A tri-axial fracturing visualization equipment is developed in this work, which can conduct fracturing experiments under reservoir conditions (150 °C and 50 MPa). The injection pressure, fracture propagation and strain field of the entire hydraulic fracturing procedure can be quantitatively monitored using a high-speed camera and digital image correlation (DIC) technology. Results demonstrate that low-viscosity fluids (SC–CO₂) tend to induce micro-fractures, mainly generating mode I-II composite fractures. However, relatively high-viscosity fluids (Liquid-CO₂) tend to form a simple pattern, mainly mode I fractures. SC-CO₂ shows strong diffusivity and can easily diffuse into micro-fractures. As a result of the increased pore pressures in the rock around the wellbore, SC-CO₂ fracturing has a breakdown pressure that is 22% lower than L-CO₂ fracturing. The fracture initiation time of SC-CO₂ fracturing is about 34% later than that of L-CO₂ fracturing. An equivalent strain difference coefficient is proposed and indicates that the strain concentration effect of SC-CO₂ fracturing is more obvious than L-CO₂ fracturing. Meanwhile, the DIC measured full-field displacement field indicates that the fracture width produced by SC-CO₂ is 1.31 mm, which is narrower than that produced by L-CO₂ of 1.58 mm. This experiment shows that the DIC method can provide an intuitive, accurate, and effective method for quantifying fracture characteristics. The results can provide an empirical reference for further analysis of the fracturing mechanism.