Application of multi-lateral well engineering has been successfully applied as an emerging technology in the Northern North Sea to drain more marginal reservoirs which would otherwise be uneconomic. Calibration of the technique to local conditions is vital and the success in the North Sea offers tremendous follow-on opportunities for global applications.

Following a successful onshore trial, Shell Expro's Northern Business Unit and their lead well engineering contractor, KCA Drilling Ltd, drilled and completed TA-14, a cased hole multi-lateral. This consisted of an appraisal and subsequent abandonment of the Triassic formation, followed by the drilling of two geosteered horizontal laterals into the shallower Brent reservoir. Techniques developed on TA-14 were then employed on an existing well. TA-19, to suspend the mainbore, side-track and recomplete as a dual lateral. Re-entry exists into the horizontal lateral with continued production available from the mother bore. Recently, TA-17 has been completed as a dual lateral horizontal Triassic oil producer with selective production from, and re-entry capabilities into both cased hole laterals. One lateral developed Triassic reserves proven by TA-14 while the other appraised the neighbouring block.

A steep learning curve has been experienced throughout the construction of multi-lateral junctions. This includes the milling and fishing process, management of debris and equipment design. Further well engineering strategies for cementing, clean up, perforating and completions have been developed.

Benefits of the multi-lateral technique are pervasive. Savings on TA-14 type wells can be achieved by applying the technique in a combination of variable and low quality reservoirs in one single well. Slot constraints can be alleviated using the TA-19 technique by side-tracking existing producers at an earlier stage. This strongly accelerates production whilst safeguarding the often underestimated tail production from the mother borehole. By using the TA-17 technique, two objectives can be met from one borehole, which would have otherwise required two new wells.

Lessons learnt have accelerated the evolution of multilateral systems on the Tern and applications have been identified for multi-laterals in early and mid 1998. Other future applications of the multi-lateral methods are expected to involve coiled tubing drilling and water injection wells with a very large economic window of opportunity.

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