Abstract: The wall of borehole tends to deform under in-situ stress state, resulting in a change in the cross-sectional shape of the borehole. Accordingly the morphology of borehole after deformation also reflects the stress state of borehole. Based on the classical rock mechanics theory, the geometric shape of borehole under the plane two-dimensional stress is studied, and the relationship between the stress and geometric parameters of the circular hole after deformation is established. The stress solution based on the elliptical parameters of the borehole is realized. The measuring method and measuring technology of borehole elliptical parameters is proposed, forming a new in-situ stress measuring method based on borehole shape analysis. The conclusions are as follows: (1) under the action of plane two-dimensional stress, the geometric shape of the circular hole turns to be ellipse, and the relationship between the ellipse parameter and the stress is derived; (2) the parameters of ellipse can be resolved by using the diameter values in three different directions. Besides, the contact micro-optical aperture measurement technology is developed to measure the morphology of borehole. The feasibility of this new measurement technology is analyzed from the principle point of view. Furthermore, the possible problems of this technology in measuring the in-situ stress were discussed; (3) based on the indoor simulation tests, the feasibility of the technical principle is verified. The accuracy of measurement results is also discussed through error analysis and example calculation.

Key words: in-situ stress measurement, borehole shape, borehole diameter deformation, micro-optical, elliptical shape measurement