Method and apparatus for investigating drill string stand-off and drilling fluid sound speed while drilling

摘要

MWD apparatus for measuring the stand-off distance of a drillstring (20) from a borehole wall (24) uses a single acoustic transducer (34) mounted on a drill collar (26) near a tubular member (30), such as an upset or stabilizer blade, which is of greater diameter than the collar (26). Control apparatus (36) determines roundtrip times (RTT) of pulses from the transducer (34) to the borehole wall (24) and back. The minimum stand-off is the known radial difference (R2-R1) between the MWD drill collar (26) and the tubular member (30) when the drillstring (20) is side-walled against the borehole wall (24). RTT values are measured and statistically analyzed to identify and verify the minimum stand-off distance (RTTmin). The RTT values may be analyzed directly or using histogram techniques. The sound speed of the drilling fluid (26) is calculated as twice the radial difference (R2-R1) divided by the minimum stand-off (RTTmin). The sound speed is used to calculate the stand-off distance corresponding to other RTT values.