Rock Stress Measurement by Hydraulic Fracturing with Hemispherical-ended Borehole

在预测大型岩洞结构稳定性方面的最新进展表明，需要在现场进行系统的应力测量，例如系统地测量洞穴周围的应力分布。在水力压裂中，为了确定岩体内某一点的状态应力，需要通过对钻孔施加水压来测量钻孔表面的破裂现象，这是地应力的函数。半球形井眼水力压裂与常规水力压裂非常相似，不同之处在于施加压力的位置，即半球形井底表面的压力。该方法有可能在单个钻孔中测量岩体的三维应力状态。首先，提出了无限大岩体中球形空腔的水力压裂方法。然后利用理论上确定的空腔表面应力系数对断裂现象进行了分析。结果表明，用该方法可以确定岩石应力的两个大小和三个方向。其次，提出了半球形井底水力压裂的概念，并用有限元方法确定了应力系数，以分析井底破裂现象。最后，对一般三维地应力作用下半球形井壁的破裂现象进行了分析，发现这些破裂现象主要受差异应力、主应力方向、岩体抗拉强度等因素的影响。结果表明，岩石应力可通过半球形井底水力压裂来确定。

Recent development in relation to prediction of structural stability of large rock cavern has indicated the need for systematic stress measurements in the field, e.g. systematic measurement of the stress distributions around caverns. In the hydraulic fracturing to determine the state stress at a certain point within a rock mass, the fracture phenomena of a borehole surface, which is a function of the field stress, needs to be measured by applying hydraulic pressure into the borehole. Hydraulic fracturing with hemispherical-ended borehole is very similar to conventional hydraulic fracturing except for the location of applying hydraulic pressure, namely the pressure at the borehole bottom surface which is formed hemispherically. This method has a possibility to measure the three dimensional stress state in rock mass in a single borehole.Firstly, hydraulic fracturing of spherical cavity in infinite rock mass is suggested. Then the fracture phenomena are analyzed by the stress coefficients determined theoretically on the surface of the cavity. It is clarified that two magnitudes and three directions of the principal stresses of rock stress are determined by this method. Secondly hydraulic fracturing with hemispherical-ended borehole is suggested and the stress coefficients are determined by FEM for analyzing fracture phenomena at borehole bottom surface. Finally, analyzing the fracture phenomena on the hemispherical-ended borehole surface under general three dimensional rock stresses, it is made clear that those are dominated by the deferential stress, the directions of principal stresses, the tensile strength of rock mass and so on. It is concluded that the rock stress can be determined by the hydraulic fracturing with hemispherical-ended borehole.