Abstract
The elastic moduli of four sandstone samples are measured at seismic (2−2000 Hz) and ultrasonic (1 MHz) frequencies and water- and glycerin-saturated conditions. We observe that the high-permeability samples under partially water-saturated conditions and the low-permeability samples under partially glycerin-saturated conditions show little dispersion at low frequencies (2−2000 Hz). However, the high-permeability samples under partially glycerin-saturated conditions and the low-permeability samples under partially water-saturated conditions produce strong dispersion in the same frequency range (2−2000 Hz). This suggests that fluid mobility largely controls the pore-fluid movement and pore pressure in a porous medium. High fluid mobility facilitates pore-pressure equilibration either between pores or between heterogeneous regions, resulting in a low-frequency domain where the Gassmann equations are valid. In contrast, low fluid mobility produces pressure gradients even at seismic frequencies, and thus dispersion. The latter shows a systematic shift to lower frequencies with decreasing mobility. Sandstone samples showed variations in Vp as a function of fluid saturation. We explore the applicability of the Gassmann model on sandstone rocks. Two theoretical bounds for the P-velocity are known, the Gassmann–Wood and Gassmann–Hill limits. The observations confirm the effect of wave-induced flow on the transition from the Gassmann–Wood to the Gassmann–Hill limit. With decreasing fluid mobility, the P-velocity at 2–2000 Hz moves from the Gassmann–Wood boundary to the Gassmann–Hill boundary. In addition,, we investigate the mechanisms responsible for this transition.
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This work was supported by 973 Program “Fundamental Study on the Geophysical Prospecting of the Deep-layered Oil and Gas Reservoirs” (No. 2013CB228600).
Ma Xiao-Yi received his Ph.D. from China University of Petroleum (Beijing) in 2018. Main research interests are experimental rock physics.
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Ma, XY., Wang, SX., Zhao, JG. et al. Velocity dispersion and fluid substitution in sandstone under partially saturated conditions. Appl. Geophys. 15, 188–196 (2018). https://doi.org/10.1007/s11770-018-0683-8
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DOI: https://doi.org/10.1007/s11770-018-0683-8