W. C. Paske, S. G. Mack, M. V. Rao, L. Spross, and J. R. Twist
Sperry-Sun Drilling Services
Abstract
Formation evaluation measurements made while drilling now include natural gamma ray, resistivity, neutron porosity and density sensors. As in the case with comparable wireline measurements, measurement while drilling formation evaluation tools are dependent on the borehole condition and require a caliper measurement to correct for washouts. This caliper measurement is now available and is made using a non-mechanical sensor which computes the effective diameter of a non-uniform borehole. This calculation is achieved by using pairs of detector banks which have been both azimuthally and axially displaced to provide unique non-degenerate borehole measurements. The hole size computation is made for every data sample and is displayed as a continuous curve in the same manner as a mechanical wireline caliper.
A field study comprised of several wells has shown that hole size corrections applied to MWD sensors improves data quality in hole enlargements up to 3 inches over bit size. Field validation of the MWD caliper’s consistency and repeatability is obtained through the observation that wiped borehole diameters are equal to or larger than the corresponding values recorded while drilling. Good agreement exists between the MWD measurement and conventional wireline calipers on a series of wells with mud weights on the order of 9-10 ppg, moderate inclinations, and with drilling ROP’s up to 500 ft/hr. Fine features of the dipmeter calipers are often mimicked by the MWD measurements with a consistency that precludes chance. Using the dipmeter caliper as a standard, MWD cement volume estimates were consistently lower (as expected of an earlier measurement) by between 3 and 10 percent. In those boreholes which exhibited a tendency towards elliptical holes, the MW]) caliper is observed to more accurately estimate hole sizes than single arm mechanical calipers due to their tendency to ride along the major axis of a non-circular borehole.
MWD logs may now be borehole corrected using a hole size measurement obtained at the time of the log. After applying such corrections, the resulting MWD neutron and density values are closer to the borehole corrected wireline logs. MWD hole size caliper measurements also provide cement volume estimates immediately after drilling.