Estimating Stress at Great Depth from Borehole Breakout

点击翻译按钮获取中文摘要

The objective of the present study is to clarify the mechanism of nucleation of borehole breakout and to examine the growth process of the borehole breakout, emphasizing the effect of the bottom of the borehole. Firstly, the characteristics of the three-dimensional elastoplastic stress field around the bottom were studied by FEM analysis, paying special attention to the singular stress field around the bottom. Then, a model of a pseudo two-dimensional stress field simulating the three-dimensional stress field was constructed, and the growth process was examined by using the pseudo stress field. The results are summarized as follows :1) Near the bottom of the borehole, stress concentration takes place at a circumferential position that is different from that at a borehole section far from the bottom. The region of stress concentration is very localized.2) Thus, for a rock that is insensitive to a local stress field in fracture nucleation, borehole break out is not formed at the bottom of borehole.3) Variation of the stress field with respect to the distance between the bottom and the observation position, can be seen as the time variation of stress at a fixed depth during drilling. The geometry of borehole breakout calculated by following exactly the loading path just mentioned is almost the same as that estimated by two-dimensional approach.4) Therefore, the breakout can be considered to be formed on the wall of a cylindrical cavity in an elastoplastic medium subjected to a monotonically increasing load at infinity.5) The width of the breakout is almost the same as that estimated by purely elastic stress state, indicating the validity of the hypothesis proposed by Haimson et al. on the relationship between the width and the far field stress.