When operating mature or late life oil and gas fields the objective is focused on using the existing infrastructure to economically recover as much of the in-place oil and gas as possible. For offshore fields at the later stages there can often be insufficient reserves to justify any new infrastructure in the asset. The primary approach to enhancing production now centers on interventions in existing wells to sustain output from current reservoir locations. As these wells become less economically viable, the strategy shifts toward utilizing depleted wells to access remaining reserves within the reservoir by drilling sidetracks into identified hydrocarbon pockets.

Given that the field will be heading towards the end of its life, there needs to be a very strong focus on the investment returns to keep the whole operation economic. In the North Sea there are many large assets that have been operating for many decades. The fields have been developed with fixed platforms using platform wells, floating platforms where the subsea wells can be accessed by the platform's drilling rig and more remote wells that can only be accessed by a mobile rig. When looking at drilling new reservoir targets from the existing field platforms, consideration must be given to viability of each sidetrack, the size of the recoverable volume in the reservoir target and type of well design to reach the reservoir target. The risk elements considered in this process are associated with each sidetrack plan including volume risk of the reservoir target, plug and abandonment risks for slot recovery and wellpath design risk.

To mitigate the risks for the development program, a portfolio approach can be considered to balance the return and the risks associated with the program to successfully extend the production life of the asset. In many old assets the largest remaining opportunities may be on the periphery of the reservoir, which may require the longest reach wells and may be riskier, whereas smaller pockets of bypassed oil may be closer to the existing wells in the middle of the field and can be reached with shorter sidetracks from deeper depths in the mother bore.