Reducing Uncertainty With Seismic Measurements While Drilling

This paper discusses both seismic checkshot data inversion and seismic waveform look-ahead imaging while drilling. We investigate the estimation under real-time data uncertainties of interval velocity profiles calculated from checkshot measurements acquired while drilling. It is found that real-time checkshots may suffer from downhole time-picking errors in addition to an unpredictable clock drift uncertainty. We developed a method to account for these uncertainties and to provide drillers with reliable formation interval velocity models in real time, regardless of whether the direct arrival times are detected by an automated downhole process or picked by an interpreter uphole once the seismograms have been transmitted to the surface through mud-pulse telemetry. By quantifying the posterior uncertainties in the interpreted velocity measurements, we then demonstrate: 1) the value of making the seismic waveforms recorded in downhole memory available in real time at the surface and 2) the benefit of quantifying and accounting for the standard deviations, which describe the downhole time-picking errors, in the case when the real-time seismograms are not available. This is exemplified with the application of the proposed methodology to measurements collected in deep water in the Gulf of Mexico, with inverted real-time velocity models that are consistent and comparable to the ones obtained from travel times picked after the drilling is complete. Finally, a case study is shown combining real-time checkshot inversion and downhole-to-surface transmitted seismic waveforms to look ahead while drilling for an optimal well steering in high-probability hydrocarbon sands.