The Marnock field is a retrograde gas condensate accumulation in the central area of the North Sea. Compared to conventional North Sea reservoirs it is near HPHT with a pressure of 9123 psia and a temperature of 310°F.

The development plan comprised six horizontal wells, with average reservoir sections of 750m length. Drilling was conducted in two phases and has delivered four wells capable of the original plateau rate expected from six wells.

Well completion design for this field presented significant challenges. The main ones were; installing downhole sand control in a horizontal well, flow back of 1.9 sg drilling mud through sand screens and the use of an isolation valve to allow the upper completion to be installed under balance.

Completion operations were initially delayed by equipment failures, and difficulties achieving the hole quality required for installation of sand screens. The main elements of the completion design were revised to a modular approach, reducing interdependency and adding flexibility to the operation. Improvements in the drilling and preparation of the reservoir section were also implemented.

The four wells completed are capable of delivering sand free production, with individual well rates of up to 120mmscf/d.

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