When a piece of rock at depth is cut out to be a core sample by drilling, it becomes free from rock stresses and expand in radial direction. The expansion occurs in an asymmetric manner with the relief of anisotropic in-situ stresses, and it results in a sinusoidal variation of core diameter with a period of 180 deg. in the circumferential direction. The circumferential variation of core diameter is given theoretically as a function of in-situ stress. These new findings can lead various ideas to determine the in-situ stress from circumferential variation of core diameter measured after the core retrieving. In the most simple case when a single core is only available, the difference between the maximum and minimum components of in-situ stress in a plane perpendicular to the drilled hole can be estimated from the maximum and minimum core diameters. If several cores with different orientation are available, all of three principal components of three dimensional in-situ stress can be determined. The technique of side-wall coring is one of possible ways to take core samples with different orientations even after a borehole has been drilled. The theoretical relationship between the core expansion and rock stress has been verified through the examination of the core prepared in laboratory experiments and retrieved field cores.
Geomechanical approaches applied to oil and gas exploitation at deep depths are becoming common place. Shale gas development is a typical example for which such approach is inevitable. Rock stress is a principal factor dominating the geomechanical behaviors of rock. We have developed recently a method using core samples, which is referred to as Diametrical Core Deformation Analysis (DCDA) (Funato and Ito, 2017). Core samples expand elastically in an asymmetric manner by being relieved from anisotropic in-situ stresses, and the magnitude and orientation of in-situ stress can be estimated from the circumferential variation of the measured core diameters.