Evaluation of Directional Survey Errors at Prudhoe Bay (includes associated papers 17467 and 18557 and 18581 and 18582 )

More than 1,000 directional surveys from wells in the Prudhoe Bay field are used to examine differences in well location at 8,000 ft (2438 m) subsea, as measured by gyroscopic and magnetic surveying instruments. The lateral, radial, and vertical errors are plotted as a function of horizontal offset. Mean lateral differences appear to be about 100 ft (30 m). Drillstring magnetization causes magnetic surveys to read north of gyros. Vertical and radial differences are negatively correlated, are chiefly a result of inclinometer errors, and are on the order of tens of feet. The Wolff-de Wardt (WdW) error model, with small modifications, does a good job of quantifying the error magnitudes, of describing the dependence of error upon horizontal offset, and of producing the ellipses of uncertainty.