Tor/Ekofisk wells drilled in the Danish sector of the North Sea have historically suffered from borehole instability problems when intersecting the Upper/Lower Lark and Horda shale formations using either water-based mud (WBM) or oil-based mud (OBM). An extensive wellbore stability investigation was carried out, focused primarily on improving shale-fluid compatibility. It was augmented by a lookback analysis of historical drilling operations in order to identify practical solutions to the borehole instability problems.

A state-of-the-art experimental rock mechanics and shale-fluids compatibility investigation was carried out featuring X-ray diffraction and cation exchange capacity characterizations, shale accretion, cuttings dispersion, mud pressure transmission and a new type of borehole collapse test for 11 different mud systems (WBM, OBM and high-performance WBM). The results of this investigation were then combined with the results of a comprehensive well lookback study. The integrated study clearly identified the root cause(s) of the Tor/Ekofisk well problems and highlighted comprehensive practical solutions, which were subsequently implemented in the field.

The borehole instability problems at Tor/Ekofisk in the Lark/Horda shales have a characteristic time-dependency, with wellbore cavings occurring after 3-5 days of open-hole time. The problems were not related to mud weight selection, but were instead caused by mud pressure invasion into the shales, which destabilizes them over time. An extensive experimental testing program revealed that this effect occurs in both WBM and OBM to equal extent, which explains why non-optimum field performance has historically been obtained with both types of mud systems. New high-performance WBM (HP-WBM) formulations were identified that significantly improve upon the mud pressure invasion and borehole collapse behavior of conventional OBM and WBM systems, yielding extended open hole time that would allow the hole sections in the Lark/Horda shales to be drilled, cased and cemented without triggering large-scale instability. Lookback review also indicated that secondary causes of wellbore instability, such as barite sag, extensive backreaming and associate drillstring vibrations should be minimized for optimum drilling performance. A new HPWBM system, together with improved operational guidelines, was successfully implemented in the field.

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