SPE Drilling & Completion
Volume 25, Number 2, June 2010, pp. 168-176

SPE-108108-PA

Stress-Corrosion Cracking as an Alternative Time-Dependent Shale-Stability Model

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DOI  More information 10.2118/108108-PA http://dx.doi.org/10.2118/108108-PA

Citation

  • Park, N., Olson, J.E., and Holder, J. 2010. Stress-Corrosion Cracking as an Alternative Time-Dependent Shale-Stability Model. SPE Drill & Compl  25 (2): 168-176. SPE-108108-PA. doi: 10.2118/108108-PA.

Discipline Categories

  • 1.3.1 Wellbore Integrity/Geomechanics

Keywords

  • Time-dependent failure, stress corrosion, subcritical crack growth, wellbore stability, discrete element method

Summary

Wellbore stability in shale has been a crucial issue for drilling in all kinds of environments. The analysis of time-dependent wellbore stability in shales has largely concentrated on the influence of fluid chemistry and filtrate invasion into the formation to predict compressive failure using poroelasticity and continuum models. This paper presents another possible mechanism for time-dependent behavior--stress-corrosion cracking (subcritical crack growth). Using the discrete-element method (DEM) to simulate grain-scale processes, we apply the concept of time-dependent cracking to hole enlargement for vertical wellbores. We use a published example from the North Sea to verify the stress-corrosion model and demonstrate the application to wellbore stability in shale. Laboratory results on rocks indicate a wide range of susceptibility to stress-corrosion cracking related to rock petrology and contact-fluid chemistry. Using laboratory calibrated rock properties, we run 2D, plane-strain simulations of vertical-wellbore stability in shale, where hole enlargement is tracked through time. As a result of stress-corrosion cracking, the numerical models show a time-dependent failure history, with an initial stable period of varying duration (influenced by mud weight, rock properties, and in-situ stress), followed by a brief period of combined shear and tensile failure, and ending with stabilization at an enlarged, elliptically shaped geometry. Time to failure increases with increasing mud weight. Enlarged-hole shape changes from elliptical to roughly circular with decreasing stress anisotropy. These behaviors simulated by the stress-corrosion model coincide with previously reported field experience. This new modeling approach for time-dependent wellbore failure can be readily constrained with straight forward fracture-mechanics tests on rock samples and has the potential to also be applied to time-dependent, intermittent sand or fines generation during production.

© 2010. Society of Petroleum Engineers

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History

  • Original manuscript received: 14 February 2007
  • Meeting paper published: 16 April 2007
  • Revised manuscript received: 23 June 2009
  • Manuscript approved: 10 August 2009
  • Published online: 11 March 2010
  • Version of record: 14 June 2010