Velocity Dispersion and Wave Attenuation of Sandstone and Shale under Different Pressures and Saturations in Seismic Frequencies

The elasticity of subsurface structures is a key assumption of seismic exploration. However, there is no perfectly elastic medium in the Earth. The inelasticity gives rise to velocity dispersion and wave attenuation, which was observed in both the field and laboratory. Many theories and models were proposed to explore the mechanism of velocity dispersion and wave attenuation. However, there is still insufficient reliable data in the seismic frequency band to support these theories and models.

In order to study the mechanism of velocity dispersion and wave attenuation in the seismic frequency band, a newly designed low-frequency measurement (LFM) system was built. Efforts were made to reduce random and systematic errors and to increase the reliability and stability. Also, a workflow of the LFM system from sample preparation to data processing was introduced to ensure the consistency of the low-frequency measurement. The new system can measure the moduli, velocities, and their attenuation with a low tolerance from 4 Hz to 600 Hz.

Two sandstone samples were measured at different saturations and pressures. Measured dispersion and attenuation of dry sandstones are quite weak. Thus, rock frame of sandstone can be considered elastic. Under water saturation conditions, the sample with high clay contents has significantly strong dispersion at high degrees of water saturation, whereas, the sample with an extremely low portion of clay contents are not dispersive and dissipative at any degree of water saturation. By analysis, clay contents can result in patchy water saturation and reduce the rock’s permeability. This suggests heterogeneities of rock frame and pore fluid’s distribution is the main reason for dispersion and attenuation in the seismic frequency band.

Four dry shale samples were measured at different pressures. Measured data shows inorganic shale is elastic under dry conditions. However, dry shale with a significant amount of organic matter, especially pre-existing hydrocarbon, has a strong dispersion and attenuation. This suggests organic matter can change the structure of shale’s rock frame, which transforms dry shale from elastic to inelastic.