Experimental study on Sc-CO2 fracturing of granite under real-time high temperature and true triaxial stress

Abstract

Sc-CO₂ fracturing would be a potential stimulation method for Hot Dry Rock. A series of Sc-CO₂ fracturing experiments were performed on granite under different temperature and stress conditions. Quantitative and qualitative analysis of injection pressure curves and cracks were conducted to explain the Sc-CO₂ fracturing mechanism under high temperature and high stress conditions. Under the same stress conditions, as the temperature increases, the breakdown pressure decreases. Concurrently, the volume and length of macro-cracks on the sample surface decrease, whereas the volume of micro-cracks within the sample increases. Under the same temperature conditions, as the stress increases, the breakdown pressure increases. However, this increasing trend is less noticeable at high temperatures. Compared with hydraulic fracturing, due to the lower density and viscosity of CO₂, Sc-CO₂ fracturing takes longer from injection to breakdown and has lower breakdown pressure. The effect of high temperature on fracturing mainly manifests in the generation of microscopic thermal cracks and a reduction in viscosity and density of Sc-CO₂. Low viscosity and low density CO₂ are more likely to penetrate into the thermal cracks of the sample, generating a diffuse micro-crack network, leading to an increase in pore pressure and a reduction in effective stress near the wellbore. Consequently, there is propagation of these micro-cracks, resulting in an increase in the volume of micro-cracks while the volume and length of macro-cracks decrease, ultimately leading to a decrease in breakdown pressure. High stress primarily influences the fracture process by reducing the opening width of microscopic thermal cracks. This reduction inhibits the diffusion of Sc-CO₂ through these cracks, ultimately leads to an increase in breakdown pressure. The findings of this experimental study provide a theoretical basis for efficient fracturing and crack creation in hot dry rock reservoirs.