The Relationship Between the Shape of a Disced Core and Three-Dimensional in-Situ Stresses Estimated by a Tensile Principal Stress Analysis.

Abstract

Based upon the assumption that core discing results from tensile stresses within and below a core during boring, the direction of the principal tensile stress was analyzed in detail for the stress conditions where core discing is likely to occur to investigate the relationship between the shape of a disced core and in-situ stresses for the case of a long disced core. Main results obtained in this study are summarized as follows: 1) In the central parts of the end surfaces, a relatively flat plane is formed. The azimuth of the normal direction of the plane coincides with that of the minimum principal stress, ƒÐ3. The inclination, Om of the normal direction from the core axis is approximately one thirds of that,.753 of ƒÐ3. By using the two equations, (1) and (3), a method for estimating more accurate ƒÓ3 (•}23%) was proposed. 2) By combining the additional equation, (1) on the magnitudes of ƒÐ3, the mean stress, um and the stress in the direction of the core axis, aZ with the previously proposed equation, (2), which is the condition of core discing, two of the above stresses can be determined if Om is measured and if one of them is determined independently. For the vertical borehole, by assuming uz to be an overburden pressure, um and u3 can be determined from the two equations. 3) When the difference between the maximum principal stress, ƒÐ1 and the intermediate principal stress, σ2 is large enough, a saddle shaped disc is formed and the shape becomes more distinct with the difference.For