Abstract: Coal cleat is not only the main reservoir space of natural gas, but also the key channel of seepage, which directly determines the storage capacity and permeability of coalbed methane reservoir. Therefore, the evaluation of cleats is critical. Cleat has a great influence on the elastic properties of coal, making seismic data and acoustic logging highly sensitive to cleat characteristics. Consequently, the study of equivalent elastic constant and wave velocity anisotropy of cleated coal is the basis for using these data to evaluate the development of cleat. Digital rock physics is a common method to study the elastic properties of unconventional rocks. However, in the numerical simulation based on the wave field, the existence of cleats will induce wave reflection and refraction, resulting in an extremely complex waveform that hinders the reliable extraction of S-wave arrivals. To solve this problem, this study proposes a method that combines finite element analysis and stress perturbation-energy derivation to calculate the velocity of longitudinal and transverse waves by determining the equivalent elastic constants. Numerical simulations were conducted on models containing different cleat configurations to investigate the variation of wave velocity with cleat development. The results show that when the face cleats penetrate the rock model, the cleats will destroy the continuity of the coal matrix, resulting in the systematic attenuation of the equivalent elastic constants ( especially in the direction perpendicular to the face cleats ), and the P-wave velocity in the vertical face cleat direction and the slow S-wave velocity in the parallel face cleat direction decrease sharply, approaching the theoretical lower limit. In the case that the surface cleats do not penetrate the rock model, the effect of cleats on the wave velocity of coal is gradually weakened. With the increase of cleat density, the equivalent elastic constant of coal decreases, accompanied by a reduction in P-wave and S-wave velocities in all directions. Comparing the results of the two cases, relative to a fully penetrating cleat, the wave velocity of the non-penetration cleat model decreases less with the increase of the cleat density, and the decreases in P-wave velocity perpendicular to the cleat plane and slow S-wave velocity parallel to the cleat plane are significantly less pronounced. In this study, a new method based on digital core simulation calculation of P-wave and S-wave velocity is proposed, elucidating the anisotropic wave velocity response law of coal under different cleat conditions, providing theoretical guidance for coal reservoir evaluation and development.
 

coal cleats,finite element method,numerical simulation,P-wave,S-wave