Mud Velocity Corrections For High Accuracy Standoff/Caliper Measurements

Ultrasonic measurement of the tool standoff and borehole diameter has many established uses in the well logging community: correcting neutron porosity and density logs, borehole imaging of rugosity and fracture detection, borehole shape measurements and cement volume calculations. Another possible application involves multistage drilling (Altoff, 1998, Bfrchak, 1993, Orbin, 1991).For all of these applications, accuracy in the ultrasonic measurement is imperative (usually to within ± 0.10" for caliper, and 0.05" for standoff). Larger errors can greatly effect the mud corrections in the standoff-based density log. The ultrasonic detection method must ensure that in lab conditions the ultrasonic standoff/caliper gauge is accurate to within these specifications. This can be achieved through proper a combination of ultrasonic transducer design and positioning and phase-sensitive averaged detection methods. However, such accuracy is meaningless if the large (−.20%) uncertainty in the sound velocity is not addressed. Our mud velocity correction technique was done using lab measurements of ultrasound velocity in drilling muds for various mud weights, temperature and pressure. A correction algorithm was then derived that reduces mud velocity error for known field conditions to, in principle, only a few percent. We present lab data demonstrating how the required accuracy is achieved when the fluid (water) sound speed is well known. We then present field examples of the logs where no velocity correction is made. In the case of heavy muds, a consistent offset as much as 0.3" in the caliper reading is observed above the expected borehole diameters. Wireline logs and known casing inner diameters verify these discrepancies. Once the correction algorithms are applied, excellent correlation (>0.10" error) between MWD and wireline is produced. In some cases, the MWD caliper is actually smaller than wireline caliper, showing regions where washout or damage has occurred. Still, problems remain with this post drilling correction, It is of no use to the standoff based binning as that data cannot be post-corrected. In a closed loop rotary drilling system, where the borehole diameter is expected to be in gauge nearly throughout, artificially large caliper values are, to say the least, a major concern to the driller. A system has been created so that, given the expected drilling parameters, the caliper and standoff values are adjusted for real-time accuracy. Log examples will demonstrate that mud velocity approximations can accurately adjust the MWI) caliper and standoff prior to drilling, thus producing highly accurate standoff-based binning of nuclear density data.