Experience with Temporarily Sealing Leaking Tubing Annuli with Extended-Life Polymer Gel Plugs in the Greater Ekofisk Area

Abstract

SPE Members Abstract Extended-life polymer gel plugs have met rigorous requirements as a sealant for restoring a barrier in the tubing annulus when tubing-annulus leakage has occurred. These requirements include urgency to restore annulus integrity dynamic placement in unbalanced well conditions, and ease of removal without milling the tubing. Very attractive economic benefits have resulted. Introduction Casing deformation resulting from reservoir compaction has caused 45 well workovers, starting in 1981, in three of the Greater Ekofisk Area fields. The Ekofisk, Eldfisk, and West Ekofisk fields are located in the southern portion of the Norwegian sector of the North Sea (Fig. 1). Compaction of up to 30 ft has deformed casing both in the chalk reservoirs and in the overburden formations. Deformation in the overburden leads to tubing damage and leakage causing the well to be plugged and scheduled for a sidetrack workover when a rig is available. This paper describes experiences with extended life polymer gel (ELPG) plugs to seal leaking tubing annuli, thus allowing the wells to continue to produce until workover is practical. Gel characteristics determined by laboratory testing are presented, the placement techniques in wells are described, and field experience is given with regard to plug life and removal for well workovers. The present well failure rate of five to eight wells per year along with the shut-in time of 6 months or more before a rig is available has significant economic consequences. The demanding requirements for an annulus sealing method include urgency to restore the annulus barrier, dynamic placement in unbalanced well conditions, effective sealing of the annular area, and obtaining a plug which can be easily removed without milling. Gel plugs have proven to be a reliable and cost effective sealing system for this application. CASING DEFORMATION Casing deformation caused by reservoir compaction (Fig. 2) is the dominant mechanism for well failure at Greater Ekofisk. As the originally overpressured, high-porosity chalk formations are being depleted, reservoir compaction results. P. 201^

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