Study on acoustic anisotropy correction method of shale based on staggered grid finite difference
The presence of bedding characteristics in shale can lead to pronounced acoustic anisotropy and significant discrepancies logging data obtained from vertical and horizontal wells.Consequently,the petrophysical interpretation model employed for vertical well reservoir calculation cannot be directly extrapolated to horizontal wells.To tackle this challenge,we have introduced the staggered grid finite difference algorithm,based on experimental data on the wave velocity anisotropy of shale.The simulation of acoustic wave transmission and reception is initially conducted in a vertical wellbore model(VTI medium).Subsequently,the propagation of the acoustic field in a wellbore(TTI medium)is simulated by applying bond transformation to the elastic coefficient matrix,assuming non-zero inclination angle of the well.The ordinate represents the relative value of P-wave slowness difference between deviation angle and zero deviation angle,while the abscissa represents the corresponding well deviation angle for establishing an acoustic anisotropy correction formula.The simulation and application results demonstrate that the waveform curve within the borehole aligns with the waveform curve acquired through the Real Axis Integration method(RAI).Furthermore,the slowness similarity correlation algorithm(STC)accurately determines the formation P-wave slowness,exhibiting a strong correlation with experimental measurements.In the presence of 28 formation elastic parameters,it achieves an average relative error of merely 2.3％.The relationship between the relative value of P-wave slowness difference and well inclination angle suggests that minimal changes occur in the relative value of P-wave slowness difference when the well inclination angle is less than 30°.However,beyond an inclination angle of 30°,a rapid increase is observed.In the case of a horizontal well(90°),the relative value of P-wave slowness difference reaches its maximum.The P-wave anisotropy correction formula was employed to rectify the slowness values of the P-wave curve for horizontal wells.The relative error analysis was subsequently conducted on the effective porosity of the horizontal well calculated before and after the P-wave correction,and compared to the effective porosity derived from core analysis of the pilot well.The results indicated a significant enhancement in the accuracy of effective porosity calculation following P-wave correction,thereby demonstrating the efficacy of this method and its applicability for P-wave slowness correction in shale horizontal wells.
Staggered gridFinite differenceAcoustic anisotropyShaleHorizontal well
NETL
NSTL
万方数据
