Severe hole stability problems were encountered in a recent exploration well in the Norwegian North Sea. The problems occurred when drilling through Tertiary shale sections interbedded with permeable sand layers. Drilling was initially performed with water based drilling fluid. However, being unable to reach the section target after more than two weeks of operation, the section was plugged back and a sidetrack was drilled using an oil based drilling fluid without encountering major operational problems.

On the basis of the post-drill analysis of drilling data, well logs, drill cuttings and borehole cavings sampled from the well, this paper describes how the complex combination of drilling fluid salt concentration and geological constraints may be utilized to ensure successful future drilling operations in this part of the North Sea.

Cuttings and preserved cavings collected during the drilling operation were selected from several depth intervals identified as potentially troublesome from drilling experience and log data. Determination of cuttings mineralogy enabled better prediction of how the time dependency of the stable drilling fluid density window is influenced by interaction between the shale and the drilling fluid. Mechanical strength is a key input parameter when predicting borehole stability. Dedicated rock mechanical punch measurements on cavings were used to confirm the prediction of strength from log data alone. Examination of caving surfaces revealed the possible presence of in-situ fractured rock. Such fractures would require special measures while drilling to maintain stability.

Subsequently a borehole stability sensitivity analysis was performed focusing on time dependent stability in the shale formations. The analysis used cuttings and cavings properties and logs as input. In particular, the modelling showed how the optimum KCl concentration in the drilling fluid changes with depth. The modelling further identified a relatively large sensitivity towards borehole inclination – even at fairly small inclinations. This paper thus illustrates the significance of properly accounting for rock mechanical aspects when planning new wells.

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