Estimation of In Situ Rock Stress at a Great Depth by Means of Drilling Induced Tensile Fractures

本项目的主要目标是开发一种利用钻井诱发的拉伸裂缝(DTF)在高温和高围压条件下估计大深度岩体地应力的新技术。DTF的特点是两个因素，即两个宏观纵向裂缝在井壁上的周向位置和由宏观纵向裂缝组成的短裂缝的倾角。因此，在一个钻孔的每个深度范围内，有两组信息可用于估计应力状态。因此，如果DTF有三个以上的深度范围，则原则上可以为六个未知应力分量建立六个以上的观测方程。基于这一事实，构造了一个反问题以及求解该反问题的新技术，其中采用统计方法来克服由于观测到的DTF数据中不可避免地包含误差而引起的模糊性。此外，还开发了适用于使用个人计算机求解逆问题的程序代码。如果有的话，钻孔破裂的数据也可以被纳入新技术中。通过将该技术应用于数值构造的虚拟数据集，验证了该方法的有效性。最后，将新技术应用于三个钻孔获得的野外DTF数据，验证了新技术对野外数据的适用性。这三个井分别是尤诺森重水堆实验场的TG-2、Hijiori HDR实验场的HDR-2和英国HDR实验场的RH-15。所有这些试验都令人满意。

The main objective of the present project is the development a new technique estimating in situ stress in rock masses at great depths under high temperature and high confining pressure conditions, by taking advantages of drilling-induced tensile fractures (DTF). DTF is characterized by two factors, i.e. circumferential position of two macroscopic longitudinal fractures on the borehole wall and inclination of short cracks consisting of the macroscopic longitudinal fractures. Thus, at each depth range of a borehole, two sets of information are available for estimating stress state. Therefore, if there are more than three depth ranges with DTF's, it is possible in principle to set up more than six observation equations for six unknown stress components. Based on this fact, an inverse problem has been constructed, as well as the new technique for solving the inverse problem, where a statistical approach is employed to overcome the ambiguity due to the errors that are inevitably included in DTF data observed. Furthermore, a program code that is applicable for solving the inverse problem by using personal computers, has also been developed. Data of borehole breakout can also be incorporated in the new technique, if available. The validity has been confirmed by applying the technique to fictitious data sets that were constructed numerically. Finally, the new technique was applied to field DTF data obtained from three boreholes to demonstrate the applicability of the new technique to field data. The three boreholes are TG-2 of Yunomori HWR test site, HDR-2 of Hijiori HDR test site and RH-15 of British HDR test site. All these trials are satisfactory.