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Variations in the P-wave velocity under different porosity, pressure and saturation conditions. The x-axis is the variation of porosity, and the y-axis is the P-wave velocity. The triangle symbols represent a pore pressure of 8 MPa, and the star symbols represent a pore pressure of 15 MPa. The black colour represents the P-wave velocity trend at a CO2 saturation of 5% and a porosity of 0.18–0.29, while the red, blue, green, grey and purple colours represent the P-wave velocity trends at CO2 saturations of 10%, 15%, 20%, 25%, and 30% and porosities that are 1–5%, respectively, higher than the data represented by the black colour.