INVERSION OF SECTOR-BASED LWD DENSITY MEASUREMENTS ACQUIRED IN LAMINATED SEQUENCES PENETRATED BY HIGH-ANGLE AND HORIZONTAL WELLS

摘要

We show that inversion processing improves thepetrophysical interpretation of logging-while-drilling(LWD) density measurements acquired in high-angleand horizontal (HA/HZ) wells. Our interpretationmethod consists of first detecting bed boundaries fromshort-spacing (SS) detector and bottom-quadrantcompensated densities by calculating their variancewithin a sliding window. Subsequently, a correlationalgorithm calculates dip and azimuth from the densityimage. Depth shifts that vary azimuthally and dependon relative dip angle, together with the effectivepenetration length (EPL) of each sensor, refinepreviously selected bed boundaries. Subsequently,inversion combines all the sector-based densitymeasurements acquired at all measurement points alongthe well trajectory to estimate layer densities. Weimplement the inversion with a recently developedlinear approximation that accurately and efficientlysimulates borehole density measurements.To verify the reliability and applicability of theinversion method, we first use forward simulations togenerate synthetic density images from a modelconstructed from field data. Results indicate thatinversion improves the interpretation of azimuthaldensity data as it consistently reduces shoulder-bedeffects. We appraise inversion results obtained fromfield measurements by quantifying the correspondingintegrated porosity-feet yielded by inversion methods incomparison to standard techniques that use simplecutoffs on field-processed compensated density.Integrated porosity-feet of inverted synthetic densitymeasurements increase by 4.4% with respect to non-inverted field measurements. By comparison, integratedporosity-feet from inversion results that include onlybottom sectors improve by 27.8% with respect to thatcalculated with field-compensated, bottom-quadrantdensity measurements. Experience shows that anadditional benefit of inversion methods is their abilityto detect and quantify the inaccuracies attributed toincreasing tool standoff in the upper sectors of themeasurement.