What the ellipse of uncertainty is
Survey measurements let us calculate an estimated wellpath. An error model estimates position uncertainty at each survey station. The three-dimensional region is an ellipsoid of uncertainty (EOU). Its projection onto a plane is an ellipse of uncertainty.
The region represents a stated confidence level under the model's assumptions. It does not guarantee that the true position lies inside the region, and it does not account for gross survey errors. Examples include defective tools and incorrect data entry.
Position uncertainty informs collision avoidance, geological target assessment, and relief-well planning.
How it's computed
The ISCWSA error model provides a framework for calculating wellbore position uncertainty. ISCWSA stands for Industry Steering Committee on Wellbore Survey Accuracy. Its Error Model Maintenance Committee maintains the framework. Its generic toolcodes include models originally developed by the Operators Wellbore Survey Group (OWSG).
An instrument performance model (IPM) describes the residual errors expected for a survey tool under stated conditions. It must match the survey method and applied corrections. Its error terms include sensor biases, scale factors, misalignments, reference errors, and depth errors.
Each term has a propagation mode that defines how its errors are correlated between stations, survey intervals, and wells. These modes include random, systematic, global, and well-by-well.
Software propagates these terms along the wellpath and combines their contributions into a covariance matrix at each station. The matrix describes uncertainty in each direction and how the directional errors vary together. It defines the ellipsoid's three principal axes and their orientation. The selected confidence level determines their lengths.
The matrix can be expressed in NEV (north/east/vertical) coordinates or HLA (highside/lateral/along-hole) coordinates. NEV describes uncertainty relative to the Earth; HLA describes it relative to the local wellbore.
Each survey interval uses its applicable IPM. Uncertainty from earlier survey intervals carries into later sections at a tie-on.
What changes the size of the ellipse
The EOU depends on the well geometry, survey program, error model, applied corrections, and confidence level. Validated corrections can reduce specific uncertainty contributions. Residual uncertainty remains after correction.
| Service | Uncertainty addressed |
|---|---|
| MFM referencing | Magnetic reference uncertainty in declination, dip, and field strength, where relevant |
| IFR1 | Uncertainty associated with local crustal magnetic fields |
| IFR2 | Uncertainty associated with time-varying external magnetic fields |
| MSA | Residual sensor bias, scale-factor, and sensor-axis alignment errors |
| Sag | Residual sensor-to-borehole misalignment, mainly affecting inclination |
In a long lateral, azimuth uncertainty can strongly affect lateral position. Inclination uncertainty, including residual sag, can strongly affect true vertical depth (TVD). The largest contributions depend on the well geometry and survey program.
Using error models for survey QC
Error-model assumptions can also support survey quality control (QC) checks. For magnetic MWD surveys, these checks compare measured total magnetic field, gravity, and dip with reference values. Predicted limits depend on the model, location, and tool orientation. A result outside those limits requires investigation. Passing these checks does not prove that the survey is free from error.
These checks cannot detect every source of position error. For example, sensor-to-borehole misalignment can affect inclination without failing total-field, gravity, or dip checks. The ISCWSA field acceptance presentation describes these limits.
TALLENTECH applies these checks when it receives the required survey data.