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When considering collision risk we often use the separation factor calculated in a plane at right angles to the offset well. This simple formula is one method of calculating separation factor. A separation factor of one would indicate that the ellipses were just touching. By way of example, if the hole size occupied one third of the ellipse and the ellipse was set at 3 standard deviations, the probability of these wells colliding would be less than 1 in 300,000 but the risk rises rapidly as the ellipses overlap. The separation factor is an excellent way of drawing attention to high risk areas quickly then more detailed analysis may be needed to determine the safety of continuing to drill.
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.
Figure 83: The ellipse of uncertainty mapped in 3D software.
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.
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