Axial resolution enhancement of borehole acoustic measurements via inversion-based interpretation supported with ultrasonic data

Abstract

Conventional borehole acoustic measurements deliver P and S wave slowness logs that inherently average in-situ rock properties along the receiver array of the acoustic instrument. These acquisition and processing conditions often limit the accuracy and resolution of the estimated rock elastic properties across heterolithic sedimentary sequences. We introduce an inversion-based interpretation method for borehole acoustic measurements that improves their vertical resolution by complementing them with ultrasonic borehole images. Results consist of high-resolution, layer-by-layer P and S wave slownesses. The combination of borehole acoustic measurements with borehole ultrasonic images enhances the definition of small rock features such as thin beds or vugs. We verify the new inversion-based interpretation method with synthetic borehole measurements and field acoustic logs acquired across sandstone-shale laminated formations and spatially heterogeneous carbonates. High-resolution layer-by-layer compressional and shear slownesses obtained with the new inversion method give rise to wider variations of calculated elastic properties than with standard acoustic logs for improved petrophysical and geomechanical evaluation. It is also found that implementing a common set of layers for the estimation of layer-by-layer rock elastic properties mitigates biases due to discrepancies in the intrinsic resolution of the various input measurements.