Quantitative Studies of Relative Dip Angle And bed-Thickness Effects on Lwd Density Images acquired in High-Angle and Horizontal Wells
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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 mea-sured density has only now begun to be explored in detail.This thesis report describes numerical simulations undertaken to quantify the in-fluence 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.We use the Monte Carlo N-Particle (MCNP) transport code to simulate LWD den-sity measurements from several combinations of relative dip angle and bed thickness.Commercial count-rate processing techniques are applied to the short- and long-spaced detector measurements in each sector. The assumed source-sensor configu-ration corresponds to the commercial adnVISION6751LWD nuclear tool designedto operate with an 8.25-in stabilizer in an 8.5-in borehole.Several combinations of relative dip angle and bed thickness indicate that numer-ically simulated density images properly quantify bed thickness, bed density, andrelative dip when the thickness of the bed is larger than two inches and when relative dip angle is larger than 75◦. This conclusion stems from the study of density images constructed with compensated and alpha-processed sector density measurements.Furthermore, 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 differences in the radial lengths of investigation of the short- and long-spaced sensors included in the tool.We describe a field example of LWD tool response across thinly-bedded formationsin a HA well intended to validate the numerically simulated LWD density measure-ments. Comparison of measured and simulated density images confirms the relia-bility of numerical simulations and their value in assessing petrophysical properties of thinly-bedded rock clastic sequences penetrated by HA/HZ wells.