An Integrated Solution of Extended-Reach Drilling Problems in the Niakuk Field, Alaska: Part I - Wellbore Stability Assessment

Abstract

Abstract The Extended Reach Drilling (ERD) program in the Niakuk Field, North Slope, Alaska was suspended in January 1998 due to hole problems in the 8.5" sections of two ERD wells. An integrated wellbore stability study was undertaken to identify and address the problems. The work undertaken involved detailed characterisation of the troublesome shales and calculation of mud weights required for acceptable hole quality and well stability. This paper shows how instability in the two suspended wells was attributed to both compressive and tensile failure of the shale formations. The primary cause of well failures was drilling with too low a static mud weight, typically 0.5 ppg lower than recommended for acceptable hole quality. In addition, the wells used a viscous mud, possessing a high Equivalent Circulating Density (ECD) relative to its static mud weight. This resulted in a time-dependent, fatigue-induced extension mode of borehole instability. This mode of failure is not common but is highlighted as a mechanism requiring attention in ERD well planning. The paper documents the back analysis of well failures, synthesis of geological information, the analysis approach adopted for assessing chemical and mechanical instability, and a forecast of required mud weights for future ERD wells. The paper then presents a summary of subsequent well drilling case histories which show improvements in hole quality due to decreased instability. This resulted in a reduction of non-productive time and large cost savings to the business. In particular, one of the suspended 8.5" hole sections has now been side-tracked with negligible hole enlargement over a 5730ft section through previously troublesome shales. In conclusion the paper emphasises the need for a proper wellbore stability assessment prior to drilling challenging ERD wells.