Stress orientation on the Norwegian continental shelf derived from borehole failures observed in high-resolution borehole imaging logs

Abstract

Knowledge of stress orientation is crucial for the understanding of many processes in the earth's crust such as tectonic development, earthquake occurrence, and fluid transport along faults. In the case of the Norwegian continental shelf, which is a prolific area for hydrocarbon production, this knowledge also plays an important role for engineering decisions with respect to wellbore stability and reservoir management. Therefore, data on stress orientation have been collected extensively over the past few years in this region. However, nearly all data published for the Norwegian continental shelf for depths less than 6km are based on the evaluation of four-arm caliper data, a method with intrinsic limitations. For greater depths stress orientation is derived from earthquake focal plane mechanisms. This article presents stress orientation data derived from an extensive set of image logs throughout the Norwegian continental shelf. The analysis is based on the identification of compressive and tensile failures of the borehole wall in electrical and acoustic borehole imaging logs. This method for the determination of the stress orientation is found to be highly reliable and capable of delivering detailed and accurate results, and thus is far superior to the analysis of breakouts from four-arm caliper logs. The analysis of image logs shows that the stress orientation is consistent both regionally and with depth. Strong variations in stress orientation, which are often interpreted from earlier analyses of four-arm caliper data, are not observed. This may indicate that some of the earlier four-arm caliper analyses may have been biased by the misinterpretation of drilling-induced artefacts as stress-induced breakouts. Stress-induced failures of the borehole wall are observed in only about 40% of the wells analysed in this paper. This does not support the conclusion that the horizontal stresses are isotropic. It just indicates that the stress difference in combination with the borehole conditions (downhole pressure and temperature) was insufficient to induce these failures.