Temperature Dependent Shale Strength and its Effect on Borehole Stability
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Borehole instability in shale has been a costly problem for the petroleum industry for years.The instability problems are related to the high complexity of shale in terms of mechanical and chemical properties. For instance the low permeability and the high sensitivity for nonnative fluids, leads to a time dependent stability in shale sections. Further a change in temperature may affect both the mechanical and chemical properties of shales, which againwill affect the borehole stability. However no thorough investigation of the temperature effect on the unconfined compressive strength, important for the stability of shale formations, is found in the literature. In order to enhance this knowledge laboratory experiments on Pierre outcrop shale is carried out. The strength of this particular shale is measured through a punching technique using small samples, which enables a large test matrix and reduced testing time. Further a temperature chamber integrated in the loadframe provides the elevated temperatures. Theresults from this testing indicates a significant decrease in the measured shear strength of about 30% when the temperature is increased from room temperature up to 90°C. This temperature effect may be explained by a release of bound water from the interlayer space, reducing the stiffness of the shale and hence also the strength. Other effect could be reduced solid-fluid friction and/or chemical reactions reducing the grain stiffness. Further this weakening effect on shale strength was used in a borehole stability model to evaluate the effect on the mud weight window. A case where the unconfined compressive strength value found at room temperature was used, was compared to a case where the UCS was corrected for the in-situ temperature. The results indicates that a limited effect is seen for a vertical well, while for a horizontal well the mud weight window almost closes.This means that mechanical testing on shale strength needs to be conducted at the correct in-situ temperature or corrected for, before the results are used in borehole stability analysis.