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Rock Mechanical Anisotropy, and its Impact on Borehole Stability
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Petroleum Engineering and Applied Geophysics.
2014 (English)MasteroppgaveStudent thesis
Abstract [en]

In drilling operations, small margins between wellbore collapse and hydraulic fracturing can become aggravated in anisotropic formations. A stuck pipe resulting from borehole failure can give serious economic consequences, as it may require sidetracking or even abandoning the well. The prediction of the minimum mudweight to prevent hole collapse is a complex process affected by rock properties, stress configuration and wellbore orientation. An understanding of the underlying physics, in particular on the failure models for anisotropic strength is required to combat wellbore collapse. In addition, understanding the behavior of elastic stresses for wellbore geometry is of high importance. Existing models show that the predicted mudweight to prevent failure increases if anisotropic formation strength is assumed. The motivation and goal for this study is to achieve a greater understanding of how rock anisotropy may influence the wellbore stability. Three strength and two stiffness models predicting behavior in transversely isotropic material have been investigated. Data from 171 compression tests on a selection of mudrocks was used to calibrate the parameters in each model. Goodness of fit analyses were conducted to determine the preferred model. Further, this model was used in predictions of shear failure in borehole stability analyses. The impact of isotropic and anisotropic stress configurations on borehole stability were studied. In addition, mudweight limits for intrinsic and structural strength anisotropy, and the difference between using strength anisotropic and strength isotropic criteria was investigated. The mudrock selection consisted of four shales and one mudstone displaying directional anisotropy. The models in Fjær et al. (2014) were found to be preferred for the rock selection. They predicted the trends in stiffness and strength for both intrinsic and weakplane failure. The borehole stability analyses predicted higher minimum mudweight when accounting for anisotropic stresses. This effect became more pronounced when accounting for intrinsic and structural strength anisotropy. Conclusions drawn from the study include Fjær & Nes’ model being the preferred model for anisotropic strength and stiffness prediction for this rock selection. The model was able to predict the trends in both intrinsic and structural anisotropy, suggesting that it may be used for a variety of transversely isotropic rocks. Strength anisotropy in stability analyses displayed up to 32 % higher minimum mudweight limit compared to prediction based on strength isotropy. This emphasizes the importance of accounting for rock

Place, publisher, year, edition, pages
Institutt for petroleumsteknologi og anvendt geofysikk , 2014. , 108 p.
URN: urn:nbn:no:ntnu:diva-26029Local ID: ntnudaim:11915OAI: diva2:743694
Available from: 2014-09-04 Created: 2014-09-04 Last updated: 2014-09-04Bibliographically approved

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