Inclination is the angle between the wellbore axis and true vertical, measured in degrees. Along with azimuth, inclination defines wellbore trajectory and is critical for directional drilling, target intersection, and accurate depth calculations.
Measurement Tools and Accuracy
Modern measurement-while-drilling (MWD) systems achieve ±0.1° to ±0.2° inclination accuracy after calibration, per the ISCWSA Error Model Rev5.13. Near-bit inclination (NBI) sensors can correlate within 0.01°-0.15° of conventional MWD readings.
Survey station intervals should not exceed 100 ft (30 m) to maintain position accuracy within acceptable limits for most applications.
TVD Accuracy Requirements
True vertical depth (TVD) calculations depend on accurate inclination surveys. Critical applications require TVD accuracy better than 3/1000 (0.3%) on step-out—a standard that ISCWSA documentation notes is "very difficult to achieve with MWD in open hole."
The ISCWSA MWD Error Model Rev5 includes 64 error sources: 4 depth-related, 4 misalignment, 20 sensor errors, and 32 geomagnetic reference terms.
Why It Matters
Inclination accuracy affects multiple drilling decisions: TVD calculations for casing point selection, pore pressure and fracture gradient predictions, target intersection planning, and anti-collision monitoring in multi-well pads. Errors compound with depth.
Automation Advantage
Autonomous drilling systems integrate real-time inclination data with drilling optimization technology to maintain planned trajectory while maximizing ROP. Consistent parameter control reduces the hunting and correction cycles that characterize manual directional drilling.