Abstract

Compressional velocities V P were measured in laboratory samples at ultrasonic frequencies (0.5-1.5 MHz) and under varying confining and pore fluid pressures (up to 40 MPa). Forty-two water saturated sandstones having a range of porosities Phi (2 to 36 percent), permeabilities K (0.001 to 306 mD) and clay contents C (negligible to 30 percent) were studied. I found that at 40 MPa the compressional velocity is inversely proportional to clay content. P-wave velocity decreases with increasing porosity, but the scatter is large even at very high pressures. The velocity-porosity scatter is reduced when the clay content is included. The dependence of V P on permeability for a wide range of porosities (6 to 36 percent) is indeterminable due to a large scatter. When the rocks are grouped into identical porosities the scatter is reduced and V P increases with decreasing clay content and increasing permeability. However, the effect of permeability alone on V P was found to be negligible in rocks with identical porosity, lithology, and negligible or similar clay content. Hence, the velocity-permeability relationship is controlled by the velocity-clay content and permeability-clay content interrelations.For all samples, the compressional-wave velocity V P in km/s at ultrasonic frequencies and 40 MPa is related to porosity Phi (fractional), clay content C (fractional) and permeability K (millidarcy) by V P = 5.66 - 6.11Phi - 3.53C + 0.0007K, r = 0.96 where r is the correlation coefficient. The relationship shows empirically that the permeability effect is very small compared to that of porosity and clay content.By calculating the elastic moduli, I extrapolated from ultrasonic to seismic frequencies and obtained V (sub P(seismic)) = 5.27 - 5.4Phi - 2.54C + 0.001K, r = 0.93 for porosities 6-36 percent.

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