18.5 Collision Risk

It would be wrong to think that if two uncertainty envelopes were touching there was always a high risk of collision. We often use between 2 and 3 standard deviations when determining the size of our ellipses which means that probability of being outside the ellipse is very small.

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.