Abstract: Supercritical carbon dioxide fracturing fluid is sensitive to temperature and pressure, and accurate prediction of wellbore temperature, pressure and phase state during fracturing directly affects the final fracturing effect. As a result, a transient wellbore temperature and pressure model of supercritical carbon dioxide fracturing considering axial heat conduction, Joule-Thomson effect, expansion/compression work, and frictional heat was established. Based on the model, the effects of injection temperature, displacement, drag reduction effect, and tubing size on the wellbore temperature, pressure and phase state were analyzed. The results show that the decrease of wellbore temperature leads to an increase in carbon dioxide density and a decrease in flow velocity, which causes the wellhead pressure to decrease simultaneously with the wellbore temperature. The higher the injection temperature, the smaller the displacement, the higher the resistance reduction rate, the larger the tubing size, the higher the bottom hole temperature, and the lower the wellhead pressure. Among them, the wellhead temperature increases by 10℃, and the bottom hole temperature increases by about 7℃; the resistance reduction rate increases by 20%, and the wellhead pressure decreases by about 7MPa. Increasing the injection temperature, the cross-sectional area of the flow channel, and reducing the displacement while using the thickener/resistance reducer can promote the carbon dioxide to reach the supercritical state at the bottom of the well. This article has strong guiding significance for the optimization design and field application of supercritical carbon dioxide fracturing.

Key words: supercritical carbon dioxide, fracturing, wellbore, transient temperature and pressure, phase state control