Chandra S. Rai

Laboratory observations of shear-wave propagation in anisotropic media

Shear‐wave exploration has been revitalized in recent years with the recognition and proper treatment of shear‐wave velocity anisotropy. The subsurface anisotropy we are concerned with is referred to as azimuthal anisotropy and, for simplicity, has been assumed to exhibit hexagonal symmetry or transverse isotropy with a horizonal unique axis. However, before one can address the interpretation of shear‐wave field data, one must provide (as input to the discussion) proper shear‐wave data. To do this, one must accept the validity of R.M. Alford’s 1986 rotational transformations. We have devised a series of laboratory experiments which clearly show the equivalence implied in Alford’s algorithm between physical and mathematical rotation. Furthermore, these studies give new insight into the complexities of field data acquired under conditions of vertically misaligned azimuthal anisotropy.

A new concept in quantitative core characterization

Geophysical emphasis in exploration and exploitation is focusing more and more on using 3-D seismic data acquisition and processing, analysis of amplitude variation versus offset (AVO), tomography, and direct shear wave studies. However, absent in all these approaches is the direct capability to quantify a specific geologic property, such as lithology or pore fluid. It is only through knowledge of how seismically mappable parameters, such as amplitudes and velocities, relate to rock properties that the tie between seismics and geology can be made.

Sensitivity Studies In Forward AVO Modeling

AVO is commonly used as an exploration tool to detect hydrocarbons. AVO is a function of several rock and fluid parameters such as compressional velocities, shear wave velocities, and densities. The AVO response is therefore sensitive to errors in these rock and pore-fluid properties. The rock parameters are often associated with uncertainties, which may be due to measurement errors or heterogeneities in the rocks. Since some of these rock properties are often estimated from other rock properties, uncertainties may be caused by use of imperfect models in the estimation. BiotGassmann’s equations of fluid-substitution are used at an intermediate stage in the AVO forward modeling, to predict the compressional and shear wave velocities of hydrocarbonsaturated rocks from reference dry or brine-saturated rock properties. These equations, too, are sensitive to various rock and fluid properties.

Means for obtaining shear wave velocities

The present invention provides a method of logging a subterranean formation through a wellbore containing a fluid, wherein no shear wave signal is obtainable utilizing conventional sonic well logging tools suspended in the wellbore. The shear wave signal is not obtainable usually because the acoustic velocity of the wellbore fluid is greater than the shear wave velocity of the formation to be logged. In the present method, the acoustic velocity of the wellbore fluid is adjusted until a shear velocity signal is obtainable by lowering the acoustic velocity of the wellbore fluid until it is less than the shear wave velocity of the formation. Thereafter, the formation is logged through the wellbore to obtain a shear wave signal.