Quantitative Studies of Relative Dip Angle And bed-Thickness Effects on Lwd Density Images acquired in High-Angle and Horizontal Wells

Abstract

Logging While Drilling (LWD) density images acquired in high-angle and horizontal (HA/HZ) wells can reveal much about the sedimentary structure of rock formations penetrated by the well. However, the effect of sedimentary structure on the measured density has only now begun to be explored. This paper describes numerical simulations undertaken to quantify the influence of relative dip angle and bed thickness on LWD density images acquired in HA/HZ wells penetrating thinly-bedded formations comprised of alternating sands and shales. Typically, the azimuthal binning scheme used to construct LWD density images divides the tool into 16 azimuthal sectors, each sector subtending an angle of 22.5 from the center of the tool. Count rate data are binned to angular sectors facing density detectors. Our objective is to assess the effects of adjacent beds on sector density measurements due to finite bed thickness and variable relative dip. The Monte Carlo N-Particle (MCNP) transport code is used to simulate LWD density measurements from several combinations of relative dip angle and bed thickness. Commercial count-rate processing techniques are applied to the short-spaced and longspaced detector measurements in each sector. The assumed source-sensor configuration corresponds to the commercial adnVISION675 LWD nuclear tool designed to operate with an 8.25-in stabilizer in an 8.5in borehole. Our study provides a way to estimate the corresponding depth shifts in true stratigraphic thickness (TST) observed in HA/HZ wells, which are caused by the difference in the radial lengths of investigation of the ® Mark of Schlumberger short-spaced and long-spaced sensors included in the tool.