A new method of stress measurement based on elastic deformation of sidewall-core with stress relief during coring

When rock is cored at depth by drilling, anisotropic expansion occurs with the relief of anisotropic rock stresses, resulting in a sinusoidal variation of core diameter with a period of 180 deg. in the core roll angle. The circumferential variation of core diameter is given theoretically as a function of rock stresses. These new findings can lead ideas to estimate the rock stress from circumferential variation of core diameter measured after the core retrieving. When several core samples with different orientation are available, all of three principal components of 3D rock stress can be estimated. If we apply an advanced technology of the rotary sidewall coring tool, we can have cores with the orientation of core axis deviated from the originally-drilled borehole axis by a right angle, 90 deg.. By using both of a sidewall core and a core of the originally-drilled borehole, we can determine absolute magnitude of 3D in-situ stress. The theoretical relationship between the core expansion and rock stress has been verified through the examination of core samples prepared in laboratory experiments and retrieved field cores.