Today, directional drilling, with no doubts, has brought massive benefits –ultimately has helped to increase reserves, increase production with even fewer number of wells, reduce cost, reduce footprint and environmental impact.

With its enormous benefits also is a risk –the risk of well collision. With the advances made in directional drilling and wellbore surveying the probability of occurrence has been reduced significantly and both service companies and clients have come up with policies to ensure that the risk is kept at ALARP.

Today, three parameters are widely used for collision risk assessment: separation factor, minimum separation distance and centre-to-centre distance. There are different methods used for calculating these parameters and the minimum acceptable value are defined by clients and directional drilling service companies.

The deepwater (GoG) development under review in this paper was developed using the subsea production manifold system. The wells were drilled and tied through a jumper to the manifold; each manifold has six slots. This implied that for every manifold, at least six wells will be closely spaced and thus posing the risk of collision especially during tophole and surface hole sections.

However, at the initial development plan, the well trajectories have been optimized to ensure there were little or no collision concerns as defined by the company rules on what is minimum acceptable separation factor and minimum separation distance. So deploying directional drilling technologies and high accuracy wellbore surveying tools, the initial wells were drilled without any collision concerns.

However, 6years after the first oil, the asset management team has asked for in-fill wells in a drive to increase production and improve recovery. This brought with it special collision concerns; special because these wells are in production and are all tied to an FPSO. Any collision incident will be very catastrophic.

This called for a thorough planning and a more systematic approach to ensure that the in-fill wells are drilled without colliding with existing wells. This paper is aimed at highlighting the systematic approach used to manage this inherent collision risks from the planning to execution of these in-fill wells. A total of 8 in-fill wells have been drilled successfully without any collision.

The success of the systematic approach is largely from the synergy built amongst the client, the drilling contractor, the directional drilling and surveying company in developing and implementing it. At this point, it this has been proven that collision avoidance is not just about solving equations and making calculations (in fact there are different methods for calculating the three defining parameters) but a systematic procedure that must be put in place from the planning, execution to post-execution.

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