SEISMIC ATTENUATION AND PORE-FLUID VISCOSITY IN CLAY-RICH RESERVOIR SANDSTONES

The frequency dependence of seismic attenuation in a suite of clay-ric h reservoir sandstones was investigated in the laboratory. Compression al- and shear-wave velocities (V-p and V-s) and quality factors (Q(p) and Q(s)) were measured as functions of pore-fluid viscosity at an eff ective pressure of 50 MPa and at an experimental frequency of about 0. 8 MHz using the pulse-echo technique. The experimental viscosity range d from 0.3 to 1000 centipoise, which gives equivalent frequencies for a water-saturated sandstone of 2.6 MHz to 780 Hz, assuming a global-ho w loss mechanism. Two types of behavior were observed: high permeabili ty (greater than 100 millidarcies) sandstones tend to show variable Q( p) and Q(s) which are similar in magnitude to those predicted by the B lot theory over the viscosity range 0.3 to about 20 centipoise (equiva lent frequency range 2.6 MHz to about 39 kHz); low permeability (less than 50 millidarcies) sandstones tend to show almost constant Q(p) and Q(s) over the experimental viscosity range that are not predicted by the Blot theory. The Blot theory does not predict the observed Q(p) an d Q(s) values in the high permeability sandstones for viscosities grea ter than about 20 centipoise, where the observed ap values are general ly much lower than the Blot predicted values. High permeability sandst ones show small velocity dispersions with changing pore-fluid viscosit y that are consistent with the Blot theory. Low permeability sandstone s show relatively large increases in velocity with increasing viscosit y not explained by the Blot theory, which are consistent with a local flow loss mechanism. The results indicate the presence of two dominant loss mechanisms: global flow (at least down to about 39 kHz in water- saturated rocks) in high permeability sandstones with only small amoun ts of intrapore clay, and local flow at ultrasonic frequencies in low permeability, clay-rich sandstones.