Abstract

Stress-Dependent Complementary Variations in P- and S-Wave Velocities for Rocks Containing High-Compliance Porosity: an Effective-Medium Model and Comparison with Experiments

V.Zaitsev, A.Shulga (Institute of Applied Physics RAS, Nizhny Novgorod, Russia); P.Sas (Katholieke Universiteit Leuven, Mechanical Engineering Department, Heverlee, Belgium)

e-mail: vyuzai@hydro.appl.sci.nnov.ru

A 3D effective-medium model describing variation in rock elastic moduli due to presence of high-compliant defects is considered. The key point is the description of the defects in terms of their normal and shear compliance. The application of the model to known experimental data on pressure dependencies of S- and P-elastic wave velocities in dry and saturated sandstones indicated excellent agreement with the theoretical fits. The ratio of the normal-to-shear compliance of the defects was determined for different rock samples. The results confirmed that, in saturated rocks, defect’s shear compliance dominates. For dry rocks, normal compliance dominates. An unexpected result is that, for some dry sandstones, the normal compliance exceeds the shear one significantly stronger, than conventional models of cracks and contacts predict. An example of a rock exhibiting pronounced negative Poisson’s ratio is revealed. Poisson’s ratio transition from negative to positive values at increased confined pressure is well described by the model. For dry rocks, the revealed defect properties indicate necessity of essential improvement of existing defect models. For saturated sandstones, the performed modified examination unambiguously indicates strong domination of the local (squirt-flow), rather than global (Biot’s) dispersion, including examples, for which an unknown dispersion mechanism was previously supposed. The work was supported by RFBR (grants No 00-05-64252, 02-02-16237).

Section : 6