Well Collision Avoidance - Separation Rule

Abstract

Abstract The well collision separation rule presented in this paper is a culmination of the work and consensus of industry experts from both operators and service companies in the SPE Wellbore Positioning Technical Section (WPTS). This is the second of two papers and complements the first paper SPE 184730 which described the collision avoidance management practices. These practices are fundamental in establishing the environment in which a minimum allowable separation distance (MASD) can be effectively applied. A standardised collision avoidance rule is recommended, complete with parameter values appropriate to the management of health, safety and environment (HSE) risk, and benchmarks for testing it. Together, these should help eliminate the disparate and occasionally contradictory methods in use today. The consequences of an unplanned intersection with an existing well can range from financial loss to a catastrophic blow-out and loss of life. The process of well collision avoidance involves rules that determine the allowable separation and the management of the associated directional planning and surveying activities. The proposed separation rule is based on the pedal curve method and is expressed as a separation factor, a dimensionless number expressed as a ratio of an adjusted centre to centre distance between wells divided by a function of the relative positional uncertainty between the two. The recommended values for the rule's parameters are based on a comparison of various industry models and experience. The relationships between key concepts such as separation factor, MASD and allowable deviation from the plan (ADP) are discussed, together with their interpretation and application. The dependency on the error distributions of the survey instrument performance models used to establish the tolerance lines is also discussed. The consequences of implementing a standardised separation rule across the industry are far reaching. It influences slot separations, trajectories, drilling practices, surveying programme and well shut-in. It is shown how the MASD can be related to a probability of crossing and being on the far side, or wrong side of an offset well. It is shown why this qualification is required for safe drilling practices to be preserved. Examples are presented to help the reader validate the calculations and software necessary to perform them. The limitations of the methods are explained and areas are highlighted for further work. The methods outlined here, taken together with SPE 184730, if implemented will improve efficiency in planning and executing wells and promote industry focus on the associated collision risks when drilling. The WPTS is also supporting the current development of the API Recommended Practices for Wellbore Positioning (RP78). Mathematical derivations or references are shown for all the calculations presented in the paper.