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Variations in the P-wave velocity under different pressures. The x-axis is the variation of porosity, and the y-axis is the P-wave velocity. The triangle symbols represent P-wave velocities at a pore pressure of 8 MPa and a CO2 saturation of 20%. In addition, the black colour represents the P-wave velocity under the original porosity (0.18–0.29), while the red, blue, green, grey and purple colours represent the P-wave velocities under porosities that are 1–5%, respectively, higher than the original porosity. The star symbols represent the P-wave velocities for a pore pressure of 15 MPa and a constant porosity, while the black, red, blue, and green colours represent the P-wave velocities under CO2 saturations of 5%, 10%, 15%, and 20%, respectively.