238 / 2022-02-15 18:09:48

Frequency-dependent attenuation and dispersion caused by fractures at microscopic and mesoscopic scales: Three-dimensional numerical study

coalbed methane,fractures,dual-scales,dispersion,attenuation,3d numerical model

Resource Development and Utilization > 7. Geology of Mineral Resources Associated with Coal Measures

In the context of carbon peaking and carbon neutrality, coalbed methane (CBM), as clean energy, is of interest due to its huge reserves around the world. Fractures are both the reservoir and the transport channel of CBM. Therefore, seismic exploration on fractures is of great significance. In the case of coal seams fluid flow in mesoscale fractures is arguably the main cause of intrinsic seismic attenuation across the entire exploration seismic and acoustic frequency range. However, its quantitative analysis is complicated by the fact that the coal seam matrix is frequency dependent. The reason for this is the presence of microscopic heterogeneity, such as microcracks or broken particle contacts, resulting in the so-called modified dry frame stiffness being complex-valued and frequency-dependent, which in turn may affect the viscoelastic response to mesoscale fractures where fluid flow. In this work, we present a 3D numerical model consisting of fractures embedded in a solid matrix containing a series of microcracks. The frequency-dependent effective stiffness is obtained from the numerical modeling, and the dispersion and attenuation are derived therefrom. Furthermore, the numerical results are compared with the published analytical solutions related and analyse the impact of their assumptions. Two important areas for improvement in the analytical solution are finally obtained: the calculation of the frame stiffness modulus and the frequency dependence of mid-frequency attenuation and dispersion.