Building a continuous velocity model from acoustic data and drilling measurements

Abstract

On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. During the drilling, some parameters such as rate of penetration (ROP) and Torque have been continuously recorded. Full Waveform Acoustic logging (FWAL) and seismic experiments, such as refracted tomography, were conducted. A linear relationship between Torque-to-ROP ratio and acoustic velocity has been computed, in a root mean square sense, to obtain an estimated P-wave velocity log from drilling parameters. A specific procedure based on zoning process applied on acoustic data is used to force the torque to respect the trends of variation of the P-wave velocity. After calibration with acoustic velocity in the 30 – 192 m depth interval, and validation with tomographic velocity in the 0 – 12 m depth interval, drilling parameters allow a prediction of P-wave velocity from the surface up to the terminal depth of the borehole, with a 10% relative uncertainty. The acoustic velocity log from FWAL is by that way extended over the total heigh of the borehole.