Accurate MWD Density Measurements With Very Large Standoffs

The most critical variable in MWD density measurements is standoff. Standoff is usually eliminated as a variable in wireline logging by pressing the tool against the formation with a backup arm. However, it is unavoidable in MWD measurements, although its effects may be substantially reduced through use of a stabilizer blade. Accurate corrections for small or moderate standoffs may be obtained by comparing long and short spaced detector responses if larger standoffs occur, either because of washout or though the deliberate use of drilling friendly "undergauge" tools, corrections obtained from the comparison method become increasingly less accurate. Thus, alternative methodologies are required to obtain accurate density values in enlarged boreholes. To correct MWD density measurements for large standoffs a new statistical method has been developed which uses a time series of many short duration counting samples. Through a statistical analysis of the time series, the method first determines if additional processing for excessive standoff is required. If it is, the short duration samples for both the long and short spaced detectors are sorted according to their count rates, and an improved avenge count rate for each detector is obtained through rejection of those samples taken at larger than acceptable standoffs. The improved count rates are fed back into the correction algorithm to obtain an improved corrected density value. A demonstration test of the method conducted in a water filled, 12 inch diameter limestone test formation (maximum standoff over 3 inches) resulted in improvement from a measurement error of more than 0.5 g/cm3 without the method, to an error of less than 0.03 g/cm3 with the method invoked. The correction method requires no well-specific input, and is implemented completely in downhole software. The tool must be eccentered and rotating so that minimum standoff is experienced during part of the tool rotation. Since changes in count rate indicate variation in standoff, the effectiveness of the method requires a minimum amount of mud to formation density contrast. However, in situations where the lack of contrast reduces the effectiveness of the method, the algorithm is capable of determining from sample to sample the parameters which optimize the sorting algorithm. Implementation of the method in the downhole software will be described, and a field example employing the method will be presented.