Acoustic velocity and attenuation of rocks: Isotropy, intrinsic anisotropy, and stress-induced anisotropy

Acoustic properties of reservoir rocks depend in complex ways on many physical parameters, only some of which are independent of each other. An improved understanding of the interrelationships among rock acoustic properties and petrophysical parameters has been essential to merge geophysical observations with petrophysical parameters for refined characterization of geological subsurface structures and elaborated petroleum reservoir exploration.; This dissertation presents results from new studies of geophysical and petrophysical properties of sedimentary rocks via sophisticated experiments and physical modeling approaches that investigate isotropic, intrinsic anisotropic and stress-induced anisotropic properties of rocks.; An extensive series of experiments on unconsolidated sand-clay mixtures and glass-beads were designed, and the results revealed a few rigorous relationships among porosity and clay-content, permeability and clay-content, cementation and porosity, and their effects on the acoustic properties at various confining pressures under both dry and saturated conditions. Consequently, a new physical concept, namely "critical porosity", was devised and discussed.; Acoustic velocity and attenuation anisotropy of shaly rocks were studied through a series of experiments not only because many shaly rocks hold intrinsic anisotropy but also because elastic properties of shaly rocks have been difficult to predict and have been a challenging subject in rockphysics. In three sets of Freeman Jewett Shale samples, the intrinsic transverse isotropic pattern was observed under both dry and saturated condition. More importantly, the time-dependent variations of acoustic velocity and attenuation during pore-pressure equilibrium led to a new method of hydraulic permeability measurement to those low permeable rock samples. The dynamic measurements on these shale samples under various controlled high pore-pressures have provided insight to the applicability of effective pressure theory to low permeable rocks.; Finally, stress-induced velocity and attenuation anisotropy were examined via an experimental apparatus that applied true three-dimensional polyaxial loading to eight different rock samples. The results suggest that stress-induced anisotropy is a function of finite strain. Thus, the three-dimensional stress pattern may be more readily inferred from dynamic acoustic anisotropy in soft rocks than does in hard rocks. A pseudo-hodogram method was developed to visualize the relationship between the three-dimensional attenuation anisotropy and the three-dimensional stress pattern.