Debris Protection System For Liner Tops

ABSTRACT This paper will introduce a new design of debris protection system for liner tops. The system has been used extensively in the North Sea and has solved the problem of well debris (e.g. formation cuttings/fines, mud solids, cement solids, foreign bodies etc.) entering the liner top P.B.R. and sticking the setting tools. INTRODUCTION Well debris entering liner top polished bore receptacles (P.B.R.'s) during cementing operations has been a notorious problem for many years, and in some cases has led to liner banger setting tools becoming stuck in the liner top P.B.R. This can lead to the setting tool being cemented in place and result in an expensive fishing job or side-track. This paper will describe how Enterprise Oil experienced severe problems with well debris entering the liner top P.B.R. and sticking the liner hanger setting tool, In addition, it will describe how a prototype debris protection system was developed and tested prior to offshore application, and the subsequent successful use of the tool on Nelson development wells. DESCRIPTION OF PROBLEM During drilling of the early Nelson development wells in the UK North Sea, severe problems were experienced with well debris entering the liner top P.B.R. As a result, difficulties occurred retrieving the liner hanger setting tool. Although in all cases the setting tools were successfully recovered, it was evident that there had been a significant risk of cementing the liner running string into the liner top P.B.R. In all cases, the well debris entering the liner top P.B.R. consisted of fine cavings of shale originating from the base of the Balder formation (see fig. 1). The setting depth of the 9.5/8" casing strings on Nelson development wells is governed by two factors, these being :The Eocene formation must be cased off prior to drilling the Forties formationCasing must be set prior to drilling into a loss zone. 85 feet TVD below the top of the Balder formation. The Eocene formation consists of highly reactive and over-pressured shales which become unstable if drilled with mud weights less than 14.0 ppg, and the Forties formation is normally pressured sandstone in which differential sticking may occur if drilled with mud weights above 10.0 ppg. Therefore, to allow the Forties sandstone to be successfully drilled without inducing borehole collapse in the Eocene, the 9.5/8" casing string is set in the Balder formation. In addition, the Balder also contains a known loss zone when drilling with 14.0 ppg mud, above which the 9.5/8" casing must be set. These criteria dictate the setting depth of the 9.5/8" casing shoe. Having run and cemented the 9.5/8" casing string, the 14.0 ppg mud used to drill the 12. ¼" hole section is displaced to 10.0 ppg mud prior to drilling the 8. ½" hole section.