Estimating Stress at Great Depth from Borehole Breakout

本研究的目的是阐明钻孔崩落的成核机制，并研究钻孔崩落的生长过程，强调钻孔底部的影响。首先，通过有限元分析，研究了底板周围三维弹塑性应力场的特点，特别是底板周围的奇异应力场。在此基础上，建立了模拟三维应力场的伪二维应力场模型，并利用伪应力场对生长过程进行了研究。结果表明：（1）在孔底附近，应力集中发生在与远离孔底处不同的周向位置;应力集中区具有很强的局部性; 2）对于裂缝成核过程中对局部应力场不敏感的岩石，在钻孔底部不会形成井眼突; 3）钻孔过程中应力场随钻孔底部到观测点距离的变化可以看作是钻孔过程中某一深度处应力随时间的变化。因此，崩落可以看作是在无限大单调递增载荷作用下的弹塑性介质中的圆柱形孔洞壁上形成的。崩落宽度与纯弹性应力状态下的崩落宽度基本相同。表明Haimson等人提出的关于宽度和远场应力之间关系的假设是正确的。

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.