Characterization of meso-structure of glutenite reservoirs by ultrasonic characteristics and the velocity heterogeneity

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Different from fine-grained sedimentary reservoirs such as sandstone and shale, glutenite reservoir is rich in gravel. Its physical and mechanical properties are affected by gravel content and gravel size and are related to the cementation and support pattern between the matrix and gravel. Characterizing the meso-structure of glutenite reservoirs and its influence on the physical and mechanical properties is of great significance for the evaluation, exploration and development of glutenite reservoirs. Thus, an experimental study on ultrasonic characteristics was conducted on glutenite of different meso-structures. Measurements were performed along different travel paths in each sample to study the effect of gravel. Combined with the meso-structure of samples, the scattering effect on ultrasonic waves by gravel was analyzed. The results show that (1) Gravel content, gravel size and the support pattern between the matrix and gravel have an important influence on the ultrasonic waveform, frequency spectrum and velocity. The higher gravel content or larger gravel size is, the more severe waveform disturbance is. And the wave velocity decreases with the increase of gravel content or gravel size. (2) In glutenite, the scattering of waves by gravel is responsible for the ultrasonic characteristics. The higher the gravel content or larger the gravel size is, the stronger the scattering effect is. (3) Because the gravel size and wavelength in glutenite are approximately the same, the effective medium theory does not work on ultrasonic propagation in glutenite. (4) Thus, we established the velocity heterogeneity model to evaluate the meso-structure of glutenite. The rise of the velocity heterogeneity reflects the increase of gravel content, gravel size and meso-heterogeneity, and the transition of support pattern from matrix-support to gravel-support. This approach is more concise and effective and may potentially realize downhole meso-structure characterization by logging.