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Dependence of Elastic Wave Velocities on Fluid Content, Porosity, and Pore Structure in Rocks of Varying Lithology Under Pressure
Abstract
It is crucial to understand how porosity, fluid saturation, pore structure, and lithology affect the elastic wave properties of rocks for seismic exploration. This study aims to investigate the sensitivity of elastic wave velocities toward changes in pressure, pore fluid, porosity, mineralogy, and pore structure.
This research shows that the velocity change highly depends on the porosity of rocks and pressure. The variation in elastic wave velocities usually is more when pressure and porosity are lower. Such velocity variation is explained by the presence of microcracks in low-porosity rocks. The cracks are not responsible for the velocity change at high pressure as the cracks close and pore space becomes rounded with incremental stress, and thus the variation in velocity decrease. Compressional wave velocity greatly increases under low pressure when a rock is saturated with fluid since the microcracks play a significant role in low-porosity rocks. P-wave velocity in high-porosity rock also shows high sensitivity to fluid under pressure. However, shear wave velocities in low- and high-porosity rocks are not affected by the fluid in pores with varying stress. Pore fluid causes substantial increases in bulk modulus and Poisson’s ratio at low differential pressure; thus, bulk modulus and Poisson’s ratio can be treated as fluid indicators where reservoir rocks are over-pressured and effective/differential pressure may be low.
This research also indicates that the velocity change may not be only caused by microfracture but also by different pore structures. In this research, Sun's rock physics model (2000) was used to demonstrate how different pore structures cause velocity variations under pressure. As an elastic parameter, the frame flexibility factor (γ) enables a representation of the velocity change with pressure. γ decreases, and the aspect ratio increases as pressure increases, resulting in P-wave velocity increasing. A decrease in frame flexibility is also related to the increase in grain size index for similar low-porosity rocks at high pressure.
The theoretical results of this study could be a useful guide for enhancing reservoir characterization and reducing the risks of oil and gas exploration. Especially when a rock physics model is used, it is possible to understand better the considerable effect of pore structure on elastic wave velocities in rocks under varying pressure. Additionally, more precise fluid substitution modeling could be developed for monitoring oil/gas production, enhanced oil/gas recovery, and CO2 flooding.
Subject
pore structurerock physics
microcracks
elastic wave velocities
porosity
fluid content
fluid substitution
Citation
Demirboga, Gozde (2023). Dependence of Elastic Wave Velocities on Fluid Content, Porosity, and Pore Structure in Rocks of Varying Lithology Under Pressure. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /198866.