ABSTRACT
An analysis of laboratory work of borehole breakouts as an indication of magnitude and orientation of in-situ stresses is presented. Bore- holes drilled in laboratory rock specimens provided data for this investigation. Borehole spalling experiments were conducted to determine if failed borehole profiles provide a means for estimating the far-field stresses. Tests were conducted in 2.54 to 3.81 cm diameter boreholes drilled in the center of a 30.5 cm x 30.5 cm x 45.7 cm blocks of Austin Chalk and hydrostone. In two tests loads were applied after boreholes were drilled and in a third test the hole was drilled into a stressed block. Tests examined tensile and shear failure aligned with the minimum principal stress direction Zoback, et. al.[1] model predicted shear spall depth to better than 15 percent. The difference between the predicted and experimental circumferential length of the spall region ranged from 41 to 66 percent. Calculation of far-field stresses based on the spall profile yielded unreasonably large values of the far field in-situ stresses.
1 INTRODUCTION
Knowledge of the magnitude of in-situ stresses with depth is important to the validation of earthquake prediction, to the placement of structures, to the design of hydraulic fracturing treatments. The only field technique which has been demonstrated to be viable for in-situ stress determination at depths greater than a few tens of meters is based on the interpretation of shut-in pressures of small scale hydraulic fracture at low pump rates. Unfortunately, these tests are expensive to perform due to rig and service costs. The alternative approach is to infer the in-situ stresses (magnitude and/or direction) from measurements routinely performed in wells, e.g., from geophysical logs. Along these lines there are methods for in-situ stress estimate based on the formation elastic properties and temperature. Other techniques include stress gradients, core relaxation, and the subject of this discussion, wellbore failures. Extension failures and borehole breakouts (the latter also referred to as borehole ellipticity and borehole spalling) results when the stresses at the borehole wall exceed or are equivalent to the local rock strength. Measurements of the spalled cross-sections with an ultrasonic televiewer[1] disclosed broad depressions aligned in the direction of the minimum principal stress. Other observations [1-4] also indicate that breakout azimuth is in the direction of the minimum principal stress. Field observations including that of wellbore failure modes, together with the laboratory developed failure envelope can provide bounds, as well as directions, for the principal in-situ stresses [7]. This paper describes an analysis of laboratory data of borehole breakouts as an indication of magnitude and orientation of in-situ stresses.
2. RELATION OF IN-SITU STRESSES TO SPALLATION ZONE PROFILE
Zoback, et. al. [1] proposed evaluating in-situ stresses from the measurements of the spall region profile (depth and circumferential length) for known well conditions. The method is based Kirsch's solution[5] in combination with the failure criterion of MaClintock and Walsh[6]. Kirsch's complete elastic solution around the wellbore is given by:
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