Inversion-Based Interpretation of Logging-While-Drilling Resistivity and Nuclear Measurements: Field Examples of Application in High-Angle and Horizontal Wells

摘要

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 nonpay reservoir zones. Therefore, the borehole measurements simultaneously respond to multiple and heterogeneous layer properties, thus giving rise to challenging well-log features such as resistivity polarization horns, anisotropy in apparent resistivity, and "bull's-eye" features on nuclear-log 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, we document the application of inversion-based interpretation and highlight the advantages of joint petrophysical inversion of nuclear and resistivity measurements in HA/HZ wells. The examples show that conventional interpretation intuition often developed in vertical wells may be inappropriate in HA/HZ wellsbecause of bed geometry, shoulder beds, well trajectory, and differing volume-of-investigation (VOI) effects. Results indicate that estimated hydrocarbon pore volume (HPV) using conventional petrophysical analysis can yield errors as high as 10%.We show that inversion-based interpretation is a viable method to emphasize sensitivity to petrophysical properties, including electrical anisotropy, interpreted from LWD measurements acquired in HA/HZ wells. This is especially significant across thinly bedded formations, where formation layering is below the spatial resolution of the measurements and shoulder-bed effects mask the detection of hydrocarbon sweet spots from array propagation-resistivity logs. In the examples, variations in water saturation influenced by reservoir quality become conspicuous only after applying the inversion-based interpretation method. Subsequently, assessment of reservoir HPV improves the diagnosis of pay and nonpay reservoir zones.