NUMERICAL STUDY OF PORE STRUCTURE EFFECTS ON ACOUSTIC LOGGING DATA IN THE BOREHOLE ENVIRONMENT

Existing methods of well-logging interpretation often contain errors in the exploration and evaluation of carbonate reservoirs due to the complex pore structures. The differences in frequency ranges and measurement methods deviated between the acoustic well logs and indoor ultrasonic tests cause inconsistent results. Based on the elastic wave equation and the principle of the control variable method, a 2D axisynunetric borehole model with complex pore structures was developed, and the numerical simulation method for acoustic log was constructed. The modeling results show that the power function can well describe the effects of pore structure on the acoustic waves, while the velocity of the Stoneley wave is not sensitive to the pore structure. Crack-like pores with pore aspect ratio (AR) less than 0.1 significantly affect the velocities of P- and S-waves, whereas "spherical" pores have fewer effects. The models with larger pore sizes have high velocities of P- and S-waves. The velocities calculated by the equivalent medium theory are always higher than the numerical simulation results. The velocity deviation caused by the difference in frequency is much smaller than the pore structure. A fractal approach to quantify the effects of pore structures is applied in the acoustic logging data. The fractal dimension increases with the pore AR or size when the porosity is constant, which can be described by a simple power function. This gives us new ideas and methods for pore structure evaluation in the lower frequency range than the conventional petrophysical model.