Position uncertainty and well clearance
Both the drilling well and each offset have an estimated position and associated uncertainty. A separation factor compares well spacing with position uncertainty under a defined calculation method. The method specifies how borehole dimensions, confidence levels, and additional margins enter the calculation. Relative uncertainty calculations must also account for applicable correlations between the wells' errors.
A separation factor is meaningful only with its calculation method and action threshold. It is not a direct collision probability. A separation factor of 2 does not mean that a well is twice as safe. The ISCWSA guide to collision-avoidance calculations explains how different methods can produce different results.
A breach of an action limit triggers the response specified by the operator's collision-avoidance policy. Responses may include improved surveys, trajectory changes, or additional operating controls.
What improved survey accuracy can support
For fixed well positions and a fixed calculation method, reducing relevant relative uncertainty can improve the separation factor. Corrected surveys can also change the calculated positions, so clearance must be recomputed. Improved accuracy may support:
- Closer spacing, where the operator's clearance requirements are met.
- Fewer operating restrictions, where the updated assessment supports their removal.
- Technical review, with documented corrections and an appropriate error model.
The important corrections depend on the encounter geometry and the uncertainty of both wells. Azimuth, inclination, depth, and reference errors can all matter.
Geomagnetic referencing, IFR1, and IFR2 can reduce magnetic reference uncertainty. Sag correction can reduce inclination errors caused by BHA misalignment. Validated MSA estimates and corrects systematic sensor errors. Residual uncertainty remains and must be represented in the selected error model. These residual errors contribute to the position uncertainty used in the clearance calculation.
Data and survey coverage
Clearance assessments require verified offset surveys, compatible coordinate references, and appropriate error models. The ISCWSA guide to collision-avoidance management describes these requirements.
During drilling, the assessment must also consider uncertainty beyond the latest survey station, including the interval to the bit and the next survey. A calculation at the latest survey station does not cover that interval by itself.
Watching the offsets while drilling
Engineers can use a normal-plane travelling cylinder plot to watch offsets while drilling. The drilling wellbore, or its plan, forms the reference at the centre. Each offset appears by distance and direction in a plane perpendicular to the reference wellbore at the selected station.
The plot shows relative geometry. A plotted distance need not be the closest distance between the wellbores. Clearance decisions use the operator's specified calculation methods, limits, and procedures. Our article on travelling cylinder plots explains how to read one, and why two plots of the same wells can disagree.