ANALYSIS OF DENSITY IMAGE DIP ANGLE CALCULATIONS

摘要

Logging While Drilling (LWD) density images havebeen shown to be useful in determining the relative dipangle of beds intersecting the borehole. In a typicalcalculation, the dip angle alpha is given by the equationα = arctan((A-B)/(D+2ΔR)), where A and B are thepeak and trough, respectively, of a sinusoidal fit to theimage of the bed boundary, D is the borehole (or tool)size, and ΔR is the depth of investigation of the tool.Traditionally, ΔR is considered constant for a given toolgeometry; however, for an LWD density tool, DR is afunction of the formation density, mud weight and toolstandoff, while the accuracy of the values assigned to Aand B are related to such factors as logging speed andthe depth resolution of the density measurement. In thisstudy, we present an in-depth analysis of thecomputation of relative dip angle from density imagesbased on numerical simulation as well as laboratorytests and field data. It includes the computation of ΔRand the effects of the borehole environment on ΔR.The effect of errors in the values of A and B in the dipangle equation are also investigated. We discuss theaccuracy of the calculated dip angle as a function of thedip angle and demonstrate that accurate (relative) dipangles can be obtained with an LWD density tool.