SEISMIC SIGNATURES OF RESERVOIR TRANSPORT-PROPERTIES AND PORE FLUID DISTRIBUTION

We investigate the effects of permeability, frequency, and fluid distr ibution on the viscoelastic behavior of rock. The viscoelastic respons e of rock to seismic waves depends on the relative motion of pore flui d with respect to the solid phase. Fluid motion depends, in part, on t he internal wave-induced pore pressure distribution that relates to th e pore microstructure of rock and the scales of saturation. We conside r wave-induced squirt fluid flow at two scales: (1) local microscopic flow at the smallest scale of saturation heterogeneity (e.g., within a single pore) and (2) macroscopic flow at a larger scale of fluid-satu rated and dry patches. We explore the circumstances under which each o f these mechanisms prevails. We examine such flows under the condition s of uniform confining (bulk) compression and obtain the effective dyn amic bulk modulus of rock. The solutions are formulated in terms of ge neralized frequencies that depend on frequency, saturation, fluid and gas properties, and on the macroscopic properties of rock such as perm eability, porosity, and dry bulk modulus. The study includes the whole range of saturation and frequency; therefore, we provide the missing link between the low-frequency limit (Gassmann's formula) and the high -frequency limit given by Mavko and Jizba. Further, we compare our mod el with Biot's theory and introduce a geometrical factor whose numeric value gives an indication as to whether local fluid squirt or global (squirt and/or Biot's) mechanisms dominate the viscoelastic properties of porous materials. The important results of our theoretical modelin g are: (1) a hysteresis of acoustic velocity versus saturation resulti ng from variations in fluid distributions, and (2) two peaks of acoust ic wave attenuation-one at low frequency (caused by global squirt-flow ) and another at higher frequency (caused by local flow). Both theoret ical results are compared with experimental data.