Successful Installations and Predictable Performance of Solid Expandable Drilling Liner in Greater Ekofisk Field, Offshore North Sea

Abstract

Drilling the lower overburden section in specific parts of the Greater Ekofisk Area (GEA) fields can be very troublesome. Wells in these parts may intersect shales with high gas content in the upper section (requiring high mud weight) and unstable zones with massive lost circulation risk (requiring low mud weight) near the base of the interval. These challenges have raised the need for a contingency drilling liner to "split" the section in two parts. Rather than changing the basic well design, the operator fronted the development of an 8-5/8″ expandable drilling liner with high collapse resistance for this purpose. This string provides 8.514″ post-expansion drift ID that accommodate an 8 ½″ bit size for the reservoir section, which is critical for GEA well design strategy. In the past five years, the operator has successfully installed 31 800 ft of 8-5/8″ expandable liner in 27 different wellbores with near perfect track record. The average liner length installed is 1 140 ft per wellbore, with an average installation time of 2.8 rig days. The solid expandable tubular (SET) drilling liner has been utilized both as part of the planned well design and as contingency liner. It has, on occasions, been worked down with parameters outside the stated specifications and still been successfully expanded afterwards. The 8-5/8″ expandable liner is now a proven system and has been one of the key enablers to achieve well objectives by maintaining hole size in a predictable manner. It provides a better drilling window for reservoir drilling and reservoir liner cementing compared to a conventional 7-3/4″ liner alternative. It also enables further contingency solutions in case other difficulties arise in the reservoir section. This technical paper describes how the operator in the overcame a significant geological challenge by working with an expandable pipe supplier to develop a unique size and strength of expandable liner that fits with the base case GEA well design. The paper also reviews the installation experiences, associated risks, performance, and key learnings with expandable liners.