Characterizing broad-band seismic dispersion and attenuation in carbonates with fractures and cavities: a numerical approach

SUMMARY Carbonates are highly heterogeneous and commonly develop large-scale fracture-cavity systems due to the strong diagenesis effect of dissolution, faulting and karstification. Understanding the seismic wave velocity and attenuation signatures is crucial for hydrocarbon exploration and recovery, carbon capture, geothermal exploitation and groundwater management using seismic methods. Nevertheless, due to the sample size limitation, traditional core-scale experiments and sonic logs fail to effectively unravel the essential factors controlling elastic and anelastic responses of those carbonates with large-scale heterogeneity. We developed a 2-D geology-constrained method for constructing digital rock models of carbonates with fracture-cavity systems, integrating outcrop data and statistical analyses. Using finite element simulations based on Biot's quasi-static equations of poroelastic consolidation, we examine the effects of cavity shape, size and number, as well as fracture number, length, width and angle, on wave dispersion and attenuation characteristics. Deep carbonate rocks with fracture-cavity systems exhibit wideband attenuation spanning over three orders of magnitude (from < 0.01 to >10 Hz), driven by coupled pore pressure relaxation mechanisms in the heterogeneous system. Horizontal attenuation was found to be roughly five times greater than vertical attenuation (as fractures are predominantly in a semi-vertical direction), highlighting significant attenuation anisotropy. While cavities have minimal effects on attenuation, fracture density and geometry—especially longer and narrower fractures—significantly influence wave dispersion and attenuation behaviour. These results underscore the importance of integrating both seismic velocity and attenuation data to enhance reservoir characterization reliability, and highlight the potential of using comprehensive broad-band seismic data and large-offset seismic data to improve geophysical interpretations of complex deep carbonate reservoirs.