Evaluation of Separation Factors Used in Wellbore Collision Avoidance

Separation factor (SF) is a widely used parameter for specifying the safe distance between two wells, and for monitoring safe distance while drilling. A variety of SF formulas is commonly applied in the industry. This paper demonstrates that different SF formulas may give significantly different results when applied to the same scenarios. This may create confusion about the interpretation and validity of the various SF definitions. More worryingly, the application of an incorrect SF formula may lead to wrong decisions with respect to well placement. A valid SF formula must adhere to fundamental principles of mathematical statistics, as elucidated in this paper. The paper further reviews commonly used SF formulas against these principles. The evaluation shows that several SF formulas may give either overly optimistic or unnecessarily pessimistic results, and, therefore, should not be used. These conclusions are supported by numeric examples. SF formulas in common use apply to a point-to-point model. However, an important application of the SF parameter is the monitoring of changes in SF along a wellbore. This implies the calculation of SF for successive point pairs, resulting in an SF listing or graph. Notable conclusions of the study are that none of the currently used formulas produces both intuitive and correct SF graphs, and that the validity of an SF graph cannot in general be assessed from its visual appearance alone. Furthermore, the current common practice of selecting the point pairs by solely geometric-distance criteria should be changed, because it frequently leads to optimistic SF values. All these findings should be of major concern to the industry.