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发展一种利用钻孔张性裂缝（DTF）在高温高围压条件下估算大深度岩体地应力的新技术。DTF的特征在于两个因素，即两个宏观纵向裂缝在井壁上的周向位置和宏观纵向裂缝组成的短裂纹的倾角。因此，在钻孔的每个深度范围处，两组信息可用于估计应力状态。因此，如果有三个以上的深度范围的DTF的，它是可能的，原则上建立六个以上的观测方程，为六个未知的应力分量。基于这一事实，一个逆问题已经被构造，以及用于解决逆问题的新技术，其中采用统计方法来克服由于不可避免地包括在DTF数据中观察到的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。如果可能的话，钻孔崩落数据也可以纳入新技术。的有效性已被证实，通过应用该技术的虚拟数据集，构造数值。最后，新技术被应用到现场DTF数据从三个钻孔，以证明新技术的现场数据的适用性。这三个钻孔分别是汤森HWR试验场的TG-2、日治HDR试验场的HDR-2和英国HDR试验场的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.