Slow formations are defined as those formations where the shear velocity is slower than the borehole fluid velocity. In order to measure shear velocities in such formations, sonic tools incorporating dipole transmitters were developed. Since their introduction more than a decade ago, dipole sonic logging tools have provided a wealth of information about the acoustic properties of slow formations, including the detection of anisotropy and inhomogeneity. The latter mechanism manifests itself as radial variations in compressional and shear slownesses, which it is now possible to quantify.
The Cymric field is located in the San Joaquin valley near Bakersfield, California. One of the producing zones, the Opal A diatomite, is an extremely slow formation with compressional slownesses approaching 200 us/ft and shear slownesses approaching 1000 us/ft. In this paper, we examine the results from a wideband crossed dipole sonic tool that was recorded in a well drilled in the Cymric field. The tool is specifically designed for 3D acoustic characterization of formations. This is achieved through a formation typing scheme that classifies formations into one of four categories: A isotropic homogeneous, B isotropic inhomogeneous, C anisotropic homogeneous, and D anisotropic inhomogeneous. Slowness frequency dispersion analysis is used in the classification scheme as each of these categories has a unique signature when dispersion analysis is employed.
When inhomogeneity is present, a new technique, known as dipole radial profiling, can be applied to measure the radial variation of shear slowness away from the borehole. This can be used to determine the extent of possible formation damage, alteration, or filtrate invasion and provide a measure of the far field shear slowness.
These methods are utilized to acoustically characterize the extremely slow formations present in the Cymric field.