Between the survey stations
A typical MWD program takes static surveys at connections, often about 30 m apart. This is approximately 98 ft. Actual spacing depends on the survey program and drilling practice.
Minimum curvature represents the path between stations as a circular arc. Steering, formation changes, and BHA behaviour can produce curvature that differs from this assumption. Widely spaced stations can miss those changes.
Tortuosity describes irregularity in the wellbore path. Planned curvature is not necessarily unwanted tortuosity. The measured detail depends on the survey spacing, sensor response, and data quality.
Unresolved curvature can change contact forces and calculated torque and drag. A fitted friction factor can absorb some effects of the assumed path. Local curvature can also affect casing and completion running, but a dogleg value alone does not establish a running limit. Equipment stiffness, dimensions, clearance, and the length of the curved section also matter.
How sampling can affect TVD
True vertical depth (TVD) depends on inclination integrated along measured depth. Curvature between stations can therefore change the calculated TVD, even when the station orientations are accurate.
The net difference depends on the inclination pattern and the survey spacing. Curvature above and below the station-based estimate can partly balance. A repeated sampling pattern can instead produce an accumulating difference.
The figure compares three estimates of one lateral. Its static surveys fall near the low points of successive build-and-drop cycles. This pattern does not occur in every well.
The lateral runs from MD 3,000 m to 4,200 m through an 8 m target. Static surveys are 30 m apart. The denser inclination series uses nominal 1 m spacing, while azimuth remains fixed.
The static surveys fall near the cycle lows. The denser estimate leaves the target near MD 3,767 m.
Detail A, MD 3,720 to 3,810 m, 4 static surveys. The denser inclination series uses nominal 1 m spacing with interpolated gaps. It builds on each slide and drops back near the station value while rotating, differently in every stand. TVD rise across these 3 stands: Static MWD 0.00 m, Sag corrected 0.16 m, HFS 0.43 m.
Detail A, MD 3,720 to 3,840 m, 5 static surveys. The denser inclination series uses nominal 1 m spacing with interpolated gaps. It builds on each slide and drops back near the station value while rotating, differently in every stand. TVD rise across these 4 stands: Static MWD 0.00 m, Sag corrected 0.21 m, HFS 0.61 m.
| At TD · MD 4,200 m | Mean inc. | TVD |
|---|---|---|
| Static MWDevery 30 m · no sag corr. | 90.00°from landing | 2,500.0 m4.0 m below top |
| Sag correctedevery 30 m · +0.10° | 90.10°from landing | 2,497.9 m1.9 m below top |
| HFSnominal 1 m · sag corr. | 90.30°from landing | 2,493.8 m2.2 m above top |
| x · listing to HFS | 6.3 mout of target from MD 3,767 |
NoteOne 1,200 m lateral in an 8 m target. Static spacing is 30 m; dense inclination spacing is nominally 1 m. Azimuth is fixed. The assumed sag correction is +0.10°. The dense series has 25 missing samples in 9 gaps, bridged by interpolation. Vertical distances are drawn ×30; table values are in metres. Mean inclination is from the landing point. The total TD difference includes 2.1 m from sag and 4.2 m from the denser estimate.
The figure assumes a constant +0.10° sag correction. This moves the estimated TD approximately 2.1 m shallower. The denser inclination estimate adds approximately 4.2 m, giving a total difference of 6.3 m from the uncorrected static listing.
The denser estimate leaves the top of the target near MD 3,767 m. These values apply to these conditions. They do not establish field correction accuracy or an expected HFS benefit. The sag correction's magnitude and sign depend on the actual BHA and conditions.
Measurements and processing
High Frequency Surveying uses suitable directional measurements to estimate more detail between conventional survey stations. It improves the description only where the data has sufficient coverage and quality.
Continuous inclination can constrain the TVD estimate. It does not independently measure changes in azimuth. The current inclination-based analysis uses azimuth from the accepted static survey. Dense azimuth can support a fuller trajectory estimate where suitable azimuth measurements are available.
The data review includes:
- Measurement identity: sensor identity, units, axis or heading conventions, and drilling activity.
- Depth and time alignment: measured depth, sensor offsets, depth lag, and timestamps.
- Coverage: actual along-hole spacing, gaps, and rejected measurements.
- Measurement quality: noise, offsets, changing errors, and agreement with accepted static surveys.
A high sampling rate in time does not guarantee uniform spacing along the hole. Drilling speed and the instrument response affect the detail available.
The processing compares the dense measurements with accepted static surveys. It records the treatment of offsets, noise, filtering, and gaps. Static and continuous measurements can need different treatment. Referencing, calibration, and sag corrections apply only where the available data and measurement method support them.
What the analysis reports
The comparison can provide:
- Windowed curvature estimates, with the window length, filtering, and units stated. These describe resolved changes in orientation, not curvature at an unlimited resolution.
- Path differences, including TVD differences between the accepted estimates. The comparison needs consistent tie points, depth units, and direction references.
- An overlay against the definitive survey listing, showing the supported interval and data gaps.
Very short calculation intervals can turn measurement noise into apparent severe doglegs. A stated analysis window helps the reader interpret the result. A tortuosity measure must also have a defined method before results from different wells can be compared.
Position adjustments derived from the dense data remain estimates. Keep them distinct from measured local orientation, especially if an adjusted listing represents interval position rather than the original station angle.
Uses of the analysis
- Torque and drag: provide a more detailed path for a mechanical model, with suitable friction and operating inputs.
- Casing and completion running: support checks of curved sections using the equipment geometry, stiffness, clearance, and running conditions.
- Casing wear: provide trajectory inputs for contact-force and wear calculations. These also need operational conditions and wear factors.
- Geological review: support log interpretation and comparison with the geological model, subject to depth alignment and geological uncertainty.
References
- ISCWSA: Introduction to Wellbore Positioning, Chapter 24 discusses tortuosity and planned curvature.
- Willerth and colleagues: Improving Wellbore Position with High-Resolution Data (2017) explains sampling effects and correction limits.
- ISCWSA 41st meeting: Novel Method for Wellbore Tortuosity Analysis (2015) records the discussion of dogleg noise and analysis windows.
- ISCWSA XCL technical supplement explains course-length uncertainty and interpolated reporting points.
- SLB: High-Definition Survey Service describes continuous inclination filtering and comparison with static surveys.
- Nobbs and colleagues: Stiff String Casing Design, Tortuosity and Centralisation describes the mechanical inputs needed for casing analysis.