Experimental work and numerical modelling have been applied to study mechanisms related to time dependent borehole stability during drilling in shales. Fully coupled numerical modelling showed that consolidation effects may act on a time scale of days, due to the low permeabilities of shale. Creep testing of a North Sea Tertiary shale showed that above 80% of the failure stress, the observed strain rates indicated that delayed borehole failure could occur within a period of 15 days. When exposing outcrop shale with close to zero swelling clay content to non-native fluids, substantial changes in mechanical properties were observed. Elevated temperature (80°C) also seemed to cause a weakening both in static and dynamic properties.

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