Well position relative to a geological target
A geological target represents the intended reservoir location for well planning. Its geometry can describe a layer, surface, polygon, or volume. A box is one possible simplification. The ISCWSA overview of geological and driller's targets describes common target shapes and their limitations.
The survey provides an estimated well position with associated uncertainty. An ellipse of uncertainty shows a confidence contour in a selected plane. The fraction of its area inside a target does not generally equal the probability of hitting that target. The model estimates that probability from the position distribution and target boundaries, under stated assumptions.
What the analysis measures
Define the success criterion before calculating a probability. The criterion might be target entry, position at a specified depth, or length within the reservoir. A probability at one survey station does not establish the probability that a whole lateral stays in zone. Errors at different stations can be correlated, as the ISCWSA error model describes.
State when the analysis assumes fixed target boundaries. Include uncertainty in the geological interpretation where it affects the result. Survey uncertainty can limit confidence in placement, while formation measurements provide additional evidence.
During planning, compare survey options against the required placement confidence and project constraints. Include costs where they are available. Corrected surveys can change the estimated well position as well as its uncertainty, so the assessment must use the updated positions.
Selecting accuracy enhancements
Assess uncertainty in the directions relevant to the target boundaries. Ellipse axes need not align with vertical and lateral directions. The useful corrections depend on the well trajectory and target orientation.
- Thin targets can be sensitive to inclination error. Measured-depth error and geological uncertainty can also matter. Sag correction can reduce inclination error caused by BHA deflection. The error model must include residual uncertainty.
- Narrow targets can be sensitive to azimuth uncertainty. Validated magnetic referencing, IFR1, IFR2, and MSA can reduce relevant errors. Residual uncertainty remains.
- Curvature between survey stations can change calculated TVD and displacement. Validated high-frequency surveys can reveal curvature missed between standard stations. The effect depends on the trajectory and data quality.
Long laterals need suitable survey accuracy and ongoing geological interpretation. Formation measurements help update the model and guide steering decisions. Research on geosteering under uncertainty describes how these updates can support placement decisions.