18.6 Definition of Ellipse Axes

There is no recognized industry standard for defining the dimensions of the ellipse of uncertainty. The axes can be either the true 3D axes length or a projection usually in the horizontal plane or the perpendicular plane (looking down the wellbore). TVD Spread is always the vertical TVD occupied by the ellipse of uncertainty.

Care should be taken when interpreting an ellipse of uncertainty report. Different companies report different definitions of the ellipse. The real uncertainty envelope is a 3 dimensional body and most software applications project this into a plane. In the graphic above, it can be seen that the plane may be vertical, horizontal or perpendicular to the wellpath. As a result the quoted semi major and semi minor axes (and possibly orientation) may mean any of the following;

• The 3D dimensions of the ellipse closest to the lateral and high side vectors.
• The horizontal projected ellipse major and minor axes and orientation from north.
• The perpendicular projected major and minor axes and orientation from high side

TVD Spread nearly always means the TVD occupied by the ellipse but can be quoted as a plus or minus number or an enclosing value.

Historically there has been no standardization of these terms but clearly they will produce different results for the same well path with the same error model.