High-angle (HA) and horizontal (HZ) wells are drilled to maximize reservoir exposure, hence improve hydrocarbon production. Because attaining the desired well-placement targets while drilling is challenging, HA/HZ wells commonly traverse multiple bed boundaries, including pay, and non-pay reservoir zones. Therefore, borehole measurements simultaneously respond to multiple layer properties. The physical relationships governing how bed geometrical properties affect logging-while-drilling (LWD) measurements in HA/HZ wells are complex. These relationships give rise to conditions and challenging well-log features such as resistivity polarization horns, anisotropy in apparent resistivity, and "bull's-eye" features on nuclear borehole images. It is for these reasons that conventional well-log interpretation methods developed for vertical wells often produce unreliable petrophysical results in HA/HZ wells.

Using field examples of HA and HZ wells, this paper documents the application of inversion-based interpretation. We highlight the advantages of joint petrophysical inversion of LWD nuclear and resistivity measurements for improved formation evaluation. The field examples consist of highly-deviated, HA, and HZ wells drilled across thinly-bedded and anisotropic calcite-cemented siltstone formations. Petrophysical properties of the thin beds, including coefficient of electrical anisotropy and hydrocarbon pore volume (HPV), are estimated using the inversion-based method.

Results from a field example of a HZ well interval indicate that the estimated HPV from conventional petrophysical analysis can yield errors as high as 10%. Despite deviated and HA intervals, shoulder-bed effects of thin beds within identified formation tops mask the detection of hydrocarbon sweet-spots from array propagation resistivity logs. Water saturation variations, influenced by reservoir quality, become conspicuous only after applying the inversion-based interpretation method to LWD nuclear and resistivity measurements. Subsequently, assessment of reservoir HPV in the HA/HZ field examples improves the diagnosis of pay and non-pay reservoir zones.

The field examples show that conventional interpretation intuition often developed in vertical wells may be inappropriate in HA/HZ wells because of bed geometry, shoulder beds, well trajectory, and differing volume-of-investigation (VOI) effects. Therefore, inversion is a viable method to emphasize sensitivity to petrophysical properties interpreted from LWD measurements acquired in HA/HZ wells, especially across thinly-bedded formations where formation layering is below the spatial resolution of the measurements.