ASSESSMENT OF SHOULDER-BED, INVASION, AND LAMINATION EFFECTS ON BOREHOLE SONIC LOGS: A NUMERICAL SENSITIVITY STUDY

摘要

We quantify the relative effects of shoulder beds, layer thickness, invasion, and sand-shale laminations on monopole and dipole borehole sonic logs. This is accomplished through numerical simulation of sonic waveforms that includes all of the propagation modes within the 1-10 KHz range. Numerical simulations assume infinitesimal monopole and dipole sources as well as an array of infinitesimal receivers deployed in close similarity with one commercially available wireline sonic tool. Propagation modes and their velocities are assessed via slowness-time-coherence processing of the simulated waveforms. For the case of shoulder-bed and layer-thickness effects, we consider soft and hard formations bounded by shale layers. The thickness of the formation is changed to consider cases where transmitter and receivers are completely included in the bed and partially distributed across different beds. Simulations indicate that shoulder- and bed-thickness effects can account for up to 30% variations in the P-wave velocities measured with a monopole source and 31% in the measured S-wave velocities measured with a dipole source compared to those measured across infinitely thick beds. We reproduce the sonic response of sand-shale laminated formations by including thin shale layers within pay sands. The thickness of shale layers is varied to consider the effect of proportion of sand and shale on the measurements. Simulations indicate that shale laminations can account for up to 7.3% variations in the P-velocities measured with a monopole source and up to 5.8% variations in the S-velocities measured with a dipole source with respect to those of effective lamination velocities. Effects of invasion are simulated for the case of waterbase mud invading oil-bearing formations. Our simulations indicate that P-wave sonic logs can undergo up to 23% variations due to invasion under realistic assumptions of petrophysical and elastic properties of soft and hard formations.