In-situ stress monitoring based on variation in electrical property of rock associated with micro-crack closure

众所周知，岩石的电阻率被定义为孔隙度、孔隙连通性和孔隙流体电阻率的函数。正如格洛弗等人的实验所表明的那样，岩石的电阻率也随应力而变化。（1994年）。电阻率对应力的这种依赖性是可能的，因为孔隙度和孔隙互连性应随应力条件而变化。另一方面，当在地层中钻出钻孔时，在钻孔附近引入应力扰动。从电阻率与应力的关系可以推断，应力扰动可能引起井眼周围电阻率的附加扰动。电阻率扰动和应力扰动一样，都是远场应力的函数。因此，如果我们能以某种方式测量电阻率分布，就有可能从钻孔周围的电阻率分布估计远程应力。为了实现这一想法，我们首先进行了实验室实验，在单轴加载条件下，使用圆柱形样品测量岩石的电阻率。然后，我们研究如何在轴向和横向的试样的电阻率随轴向载荷的变化。在此基础上，提出了一种由井旁电阻率分布估计地应力状态的新方法。

It is known that electrical resistivity of rocks are defined as a function of porosity, pore inter-connectivity and resistivity of pore fluid. Resistivity of rocks changes with also stress as has been shown experimentally by Glover et al.(1994). Such dependency of resistivity on stress is likely, since porosity and pore inter-connectivity should change with stress condition. On the other hand, when a borehole is drilled into a formation, a stress disturbance is introduced in the vicinity of the borehole. As inferred from the relationship between resistivity and stress, the stress disturbance possibly induces an additional disturbance in resistivity around the borehole. The resistivity disturbance should be a function of remote stresses as same as the stress disturbance. Therefore, it may be possible to estimate remote stresses from resistivity distribution around the borehole, if we can measure the resistivity distribution somehow. To accomplish this idea, first we conduct laboratory experiments to measure resistivity of rock under uniaxial loading condition by using a cylindrical specimen. Then we examine how the specimen's resistivity in axial and transverse directions of the specimen changes with axial loading. Based on these results, we propose a new method to estimate in-situ stress state from resistivity distribution in the vicinity of borehole.