Characterization of geological formations by physical parameters obtained through full waveform acoustic logging

Abstract

In this paper, we will show through a field example that full wave form acoustic logging allows a quantitative evaluation of geological formations. For that purpose, conventional logs and their associated standard deviation (Std) must be computed (formation velocities, amplitudes, frequencies, etc.) since the Std is used to estimate the uncertainties associated with the log and to edit other logs. The missing values are then reconstructed by geostatistical interpolation (ordinary kriging and co-kriging). The shear velocity and density of the formation are also estimated in order to obtain mechanical parameters such as Poisson’s ratio or shear modulus. Since the converted refracted shear waves can be recorded in fast formations, a joint method based on the local measurement of the shear velocity by picking the arrival times of the refracted S wave and interpolation by co-kriging with P-wave velocity log has been used to compute a continuous shear velocity log. The Analysis of the dispersive properties of the Stoneley modes has then been used to estimate density variations and build iteratively a density log from an a priori density model. Furthermore, we will show that a dimensionless shape index can be used as a qualitative acoustic attribute to detect the presence of interfering waves, anomalic zones and to obtain a measurement of the attenuation. We will also show that P-wave attenuation , P-wave frequency and acoustic porosity logs can be fruitfully used to compute an acoustic permeability log.