THE EFFECTS OF POROSITY-PERMEABILITY-CLAY CONTENT ON THE VELOCITY OF COMPRESSIONAL WAVES

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 - 6.11-PHI - 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 Vp(seismic) = 5.27 - 5.4-PHI - 2.54C + 0.001K, r = 0.93 for porosities 6-36 percent.