Introduction
Many Operating companies and Directional Drilling contractors have policies that define how they manage the risk of well collision. Such policies invariably include one or more rules that define how close the reference well can be drilled to an offset well.
Collision avoidance policies normally specify the corrective actions triggered by comparison of the nominal well proximity with a minimum allowable separation distance. The prime purpose of this paper is to explain the ways in which minimum allowable separation is commonly defined and calculated.
The adoption of a particular minimum allowable separation rule, no matter how conservative, does not ensure acceptably low probability of collision. Many other factors contribute, including the level of compliance by office and rig personnel with collision avoidance procedures, and the completeness and correctness of the directional database.
Most of the terminology used in this document is explained in the Collision Avoidance Lexicon, which was also produced by the Collision Avoidance Work Group. Both documents are available on the ISCWSA web site.
Recommendations made in this document are based on consensus within the Work Group; no greater authority is claimed.
1.0 Classification
Collision avoidance rules can be generalised as falling into one of four categories based on the criterion used. The abbreviations S, E, R and P have been adopted here for convenience, but they are not an Industry standard nomenclature. A collision avoidance policy may make use of more than one type of rule.
- 1.1 S type - Well separation distance
- Since it does not account for position uncertainty, the S type rule is usually considered too simplistic to be used as the primary rule for a collision avoidance policy, but it may be the only meaningful rule at shallow depth and close proximity. Other rules may give a false sense of security when position uncertainty is very low and wells are very close, as might be the case at shallow depth. An S type rule is therefore sometimes used in conjunction with another rule as part of a collision avoidance policy.
- 1.2 E type - Position uncertainty ellipse or ellipsoid separation distance
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Collision Avoidance Calculations - Current Common Practice