ABSTRACT

ABSTRACT

An analysis of data of borehole breakouts as an indication of orientation of in-situ stresses is presented. Wellbores drilled in Alaska and Colorado provided data for this investigation.

Two field cases illustrating borehole breakouts at opposing failure directions are discussed. The first case refers to an offshore well drilled in the Gulf of Alaska. The failure zone is predicted to take place centered on the diameter in the direction of the least horizontal principal stress. The second case refers to the failure in a coal seam in a wellbore drilled in the Piceance Basin (Colorado). The failure mode was located normal to the direction of the least horizontal principal stress. Both failures can be explained by the von Mises failure criteria.

1 INTRODUCTION

Since the beginning of this decade breakouts have been used as indicators of the orientation of the principal stresses. Breakout (also referred as borehole ellipticity and borehole spalling) are zones of failure lying on opposite diameters of the wellbore. Failure leading to spalling results when the stresses at the borehole wall exceed or are equivalent to the local rock strength. Measurements of the spalled cross-sections, in vertical wells, with an ultrasonic televiewer[5] disclosed broad depressions aligned in the direction of the minimum principal stress. Other observations [4,6,7,10] also indicate that breakout azimuth is in the direction of the minimum principal stress. Breakouts aligned in the direction of the maximum principal stress were first reported by Jones, et. al.[7]. Breakouts in the direction of the maximum horizontal principal stress have been observed in friable rocks (e.g., coal seams) only.

This paper describes an analysis of wellbore breakouts for boreholes drilled in Alaska and Colorado. The first well is an example of breakouts aligned with the direction of the minimum principal stress, while the spalled zones for the other well are aligned with the direction of the maximum principal stress.

2. FIELD SPALLING OBSERVATIONS

Maintaining stable wellbore is of primary importance during drilling of oil and gas wells. Wellbore stability requires a proper balance between the in situ stresses, wellbore fluid pressure and mud chemical composition. Most oil field stability studies have used the von Mises failure criterion. This is based on the second invariant of deviatoric stress and differs from the shear stress by 0 to 15 percent. In the von Mises criteria the second invariant of the deviatoric stress (vJ2) at the wellbore is obtained from:

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and the mean effective stress, (Pc - Po), where

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and Po is the pore pressure. In equations 1 and 2, sr, s¿ and sz are the radial, tangential, and vertical stresses at the borehole wall, respectively. These parameters are obtained by superposition of the Kirsch [9] solution for biaxial far-field stresses and the solution for the stress distribution due to the application of a pressure (p) in the cylindrical cavity:

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