关键词: 沂沭断裂带, 地应力场, 超声波钻孔电视, 钻孔诱发裂缝, 主应力方向

Abstract: The parameters of regional in-situ stress field are very important for the analysis of regional crustal stability, extraction of deep energy resource materials, and deep infrastructure construction. Moreover, the principal stress orientation is one of the important characteristic indicators of regional in-situ stress field. At present, there are many methods to determine the stress field, mainly including the method based on borehole(in-situ measurement)or focal mechanism solution(inversion). Comparing the way of in-situ measurement and inversion measurement, the in-situ measurement not only can measure the orientation of the in-situ stress, but also can obtain the magnitude of the in-situ stress. In the existing in-situ stress measurement and estimation methods based on borehole, the stress orientation is mainly determined by borehole induced failure structures when drilling or coring. Due to the concentration of tectonic stress in the borehole wall after drilling, the rock mass in the borehole wall area may be damaged, and borehole induced failure structures including borehole breakout(BO)and borehole-induced tensile fracture(DITF)will occur. Usually, borehole breakout occurs near the minimum principal stress orientation, and tensile cracks occur near the maximum principal stress orientation. In these two types of borehole induced failure structures, the DITF is more widely used to determine stress in-situ orientation, because DITF is the direct response of the orientation of the maximum horizontal stress in the stress field. The Yishu fault zone is the main active fault in the southern middle segment of Tan-Lu fault zone and the M_S8.5 Tancheng earthquake in 1668 occurred in this area, so determining the characteristics of the stress field in this area has important reference value for the judgment of Yishu fault stability. In order to accurately determine the orientation of the modern stress field of the Yishu fault zone, we drilled three boreholes with a depth of about 400m in the Yishu fault zone from 2012 to 2014, the connection direction of the three borehole positions is perpendicular to the strike of fault zone and covered the main faults and its branch faults. In the three deep boreholes, we use high-precision ultrasonic drilling TV system to implement a three-year repeat scan logging and hydraulic fracturing in-situ stress measurement with a view to analyzing the characteristics of the regional in-situ stress field. Through logging images, a total of 199 vertical fractures were accurately identified based on the characteristics of the damaged structure, including 99 drilling-induced fractures, 43 hydraulically induced fractures, and 57 growth fractures in the later stages(natural growth), simultaneously, we use the cyclic statistical method to determine the dominant horizontal principal stress orientation of the Yishu fault zone. As the result, the principal stress orientation is about 92.28° which is basically no change compared with previous research results, and the natural growth of vertical fractures was discovered for the first time near the dominant orientation. In order to verify this phenomenon, we compare the rock tensile strength of the borehole core with the stress profile obtained from the measured in-situ stress value in this research. Through Brazil Splitting Strip Test, Brazil Splitting Pyramid Test, Hollow Rock Pillar Test(oil injection and water injection)and the direct tensile test, we obtained the rock tensile strength of about 6~11MPa. Theoretical analysis results show that the maximum circumferential stress in the borehole is larger than the tensile strength of the rock, which satisfies the basic conditions for causing natural tensile failure, and it is theoretically possible to form the natural tension fracture in the current stress environment. This new phenomenon can further verify the accuracy of ultrasonic drilling TV in determining in-situ stress orientation, and acoustic borehole televiewer with high resolution can be widely used to in-situ stress measurement in the future.

Key words: Yishu fault zone, in-situ stress field, acoustic borehole televiewer with high resolution, drilling-induced fracture, orientation of in-situ stress