Abstract

Borehole failures like drilling-induced fractures and breakouts observed inimage logs offer the possibility to derive high quality stress orientations. Image logs from 16 wells in the Northern North Sea are analysed for theoccurrence of borehole failures. While in wells to the west of the Viking Graben very few stress related borehole failures are observed abundantdrilling-induced fractures are detected in all of the image logs in wells to the east of the graben. Also the orientation of SHappears to be slightly different for the western and the eastern part of the investigated area. An SHorientation of approximately NI000E is found in the western part, while the SHorientation is found to be approximately N800E for the wells in the eastern part. Investigating the conditions for fracture initiation in three of the wells allows to constrain the possible magnitude of SH. For all three wells the occurrence of drilling-induced fractures can only be explained if theSHmagnitude is at least slightly greater than the vertical stress, which indicates a strike-slip tectonic regime(Sh<Sv<SH).

Introduction

There is increasing awareness for the state of stress being an important parameter for engineering tasks in reservoir development, and for the geoscientific understanding of fault seal and hydrocarbon migration. While quite sophisticated tools for e.g. prediction of wellbore stability and sand production are available, the knowledge of the present-day in-situ stress field is still often based on scarce and poor quality data.

Formerly, stress orientations were commonly derived from breakout orientations determined from the analysis of four-arm caliper data. As this tool provides only two diameters of the borehole cross section, the accuracy of derived stress orientations is generally low, and special care has to be taken to differ key seats from breakouts. This has in many former analyses lead to a significant scatter in the orientation of SHwhich often was explained by local influence of topography, faults, or salt layers, while it may be due to the immanent weaknesses of the four-arm caliper analysis.

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