Abstract

In response to increasing concerns over the environmental effects of the disposal of drill cuttings to the sea, a number of North Sea operators have begun to dispose of cuttings through injection into underground strata. Phillips Petroleum, as part of the Eko II redevelopment program for the Ekofisk field, conducted an evaluation of cuttings disposal options prior to selecting cuttings reinjection for all cuttings disposal from the new 2/4X wellhead platform.

In support of the selection of the final alternative for reinjection of cuttings, reinjection of slurrified cuttings via a dedicated well into the mid-overburden overlying the northeastern flank of the reservoir, an extensive engineering study was performed. Within the study, a number of issues were evaluated and reviewed: the relevant experiences of other North Sea operators; the economic evalution of various mud systems and disposal alternatives; a complete review of the geologic structure of the overburden for determining the optimal location of reinjection; numerical simulations of fracture behavior and performance; well design considerations; final selection of the well location; contingency planning; and a subsidence risk assessment. The subsidence risk assessment, performed in cooperation with an outside consultant, evaluated the unique concerns of reinjection into the overburden overlying a compacting reservoir.

Cuttings reinjection was initiated late in 1996 and by early 1998 more than 700,000 barrels of slurry had been reinjected into a single completion interval. Difficulties with the well have been relative minor, though the well has plugged-up several times, and the reinjection operation is considered successful. Building upon these results, plans are now being implemented for cuttings reinjection via dedicated wells at the Eldfisk field, another compacting reservoir lying to the southeast of Ekofisk.

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