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Pressure and Fluid Effect on Frequency Dependent Velocities in Fully Saturated Tight Sandstone
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 79th EAGE Conference and Exhibition 2017, Jun 2017, Volume 2017, p.1 - 5
Abstract
A multi-frequency band elastic parameters measurement system was used to measure the dispersive nature of elastic waves in a tight sandstone under nitrogen gas and glycerin saturation. The measurements were performed at effective pressure Peff = [2–35] MPa and at temperature 23°C, 40°C and 60 °C. For nitrogen gas saturated (dry) rock, velocities remained almost unchanged from low seismic to high ultrasonic frequencies at each effective pressure. But under glycerin saturation, the saturated tight sandstone exhibited obvious velocity variations in the seismic frequency region, which were largely damped by the increase in effective pressure. Furthermore, an increase in fluid viscosity caused a shift of the dispersion curve to lower frequencies, suggesting a fluid flow mechanism where the characteristic frequency was inversely proportional to fluid viscosity. A simple squirt flow model was used to estimate the frequency effect and fitted the behavior observed in our dataset relatively well. Although the magnitude of velocities in measurement and estimation did not match exactly, the characteristic frequency changed with viscosity and effective pressure as predicted by the squirt flow theory. So for the “crack-pore” tight sandstone, it is the squirt flow mechanism that causes the dispersion phenomenon in the seismic frequency band.