Pore-Scale Modeling of Pore Structure Effects on P-Wave Scattering Attenuation in Dry Rocks

Abstract

Underground rocks usually have complex pore system with a variety of pore types and a wide range of pore size. The effects of pore structure on elastic wave attenuation cannot be neglected. We investigated the pore structure effects on P-wave scattering attenuation in dry rocks by pore-scale modeling based on the wave theory and the similarity principle. Our modeling results indicate that pore size, pore shape (such as aspect ratio), and pore density are important factors influencing P-wave scattering attenuation in porous rocks, and can explain the variation of scattering attenuation at the same porosity. From the perspective of scattering attenuation, porous rocks can safely suit to the long wavelength assumption when the ratio of wavelength to pore size is larger than 15. Under the long wavelength condition, the scattering attenuation coefficient increases as a power function as the pore density increases, and it increases exponentially with the increase in aspect ratio. For a certain porosity, rocks with smaller aspect ratio and/or larger pore size have stronger scattering attenuation. When the pore aspect ratio is larger than 0.5, the variation of scattering attenuation at the same porosity is dominantly caused by pore size and almost independent of the pore aspect ratio. These results lay a foundation for pore structure inversion from elastic wave responses in porous rocks.

A Propagation Loss Coefficient Model of Low-Frequency Elastic Wave in Coal Strata Set

“…e medium parameters are shown in Table 1. It is 2000 m/s, the shear wave velocity is 1000 m/s, the density is 1.3 g/cm 3 , and the coal seam thickness is 4 m. e upper and lower layers are surrounding rocks, which are isotropic media. e P-wave velocity is 4000 m/s, the S-wave speed is 2000 m/s, the density is 2.6 g/cm 3 , and the thickness is 10 m.…”

“…We select 60 Hz as the simulation frequency, R � 0.014 cm as the particle size, and φ � 0.25 as the porosity. Different coal seam densities are chosen to evaluate the model, with values in the range of 1.1-1.9 g/cm 3 . e simulation results of the relationship between the loss and coal density are shown in Figure 3.…”

“…e simulation result is similar to that obtained with COMSOL, but our simulation curve is smoother. e relationship between loss and particle size for a medium density of ρ � 1.4 g/cm 3 , a frequency of 60 Hz, a porosity of φ � 0.25, and particle size ranging from 0.008 to 0.02 cm is shown in Figure 4. Figure 4 illustrates that the loss increases gradually with increasing particle size.…”

“…In some cases, the formula model can be directly used to estimate attenuation factors of the observed seismic reflection data. In other cases, the model is considered overly complicated by some researchers, who instead selected other methods [2][3][4], such as the control variable method, even though such practices are inadequate at describing the losses. e control variable method has been used to analyze the influence of the frequency, propagation velocity, propagation distance, and quality factor on amplitude loss [5].…”

Experiments on acoustic measurement of fractured rocks and application of acoustic logging data to evaluation of fractures

“…However, the single material simulation experiment had neglected the complexity of the mineral and pore distribution of the rock, so that the method was still faced with many problems in actual application. In addition, due to the fact that the acoustic wave attenuation was much more complicated than the change of the acoustic wave velocity, it was difficult to explain the principles of the acoustic wave attenuation using the physical model (Morris et al 1964;Jose et al 2013), thus most of the methods of acoustic wave amplitude were derived by means of numerical simulation, but the boundary conditions were too simple to match many problems in actual applications (Shi et al 2004;Wang et al 2015;Shragge et al 2015).…”