Integrated determination of principal stress and tensile strength of rock based on the laboratory and field hydraulic fracturing tests
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摘要: 钻杆式水压致裂原地应力测试系统的柔性会影响最大水平主应力的计算精度。利用空心岩柱液压致裂试验获得的岩石抗拉强度来取代重张压力计算最大水平主应力是降低钻杆式测试系统柔性的负面影响的重要途径。在福建某隧道深度为65 m的钻孔内开展了8段的高质量水压致裂原地应力测试，随后利用钻孔所揭露的完整岩芯开展了17个岩样的空心岩柱液压致裂试验。利用空心岩柱液压致裂所得的抗拉强度平均值为8.40 MPa，与经典水压致裂法确定的岩体抗拉强度8.22 MPa接近。对于20 m的范围内8个测段的原地应力量值，最小水平主应力平均值为8.41 MPa，基于重张压力Pr的最大水平主应力平均值为16.70 MPa；基于空心岩柱抗拉强度的最大水平主应力量值平均值为16.88 MPa，两种方法获得的最大水平主应力平均值基本一致。最大最小水平主应力与垂直主应力之间的关系表现为σH > σV > σh，这种应力状态有利于区域走滑断层活动。通过对比分析可知，对于钻杆式水压致裂原地应力测试系统，当测试深度小且测试系统柔性小时，基于重张压力和基于空心岩柱抗拉强度得到的最大水平主应力量值差别不大，这说明基于空心岩柱的岩石抗拉强度完全可以用于水压致裂最大水平主应力的计算，同时基于微小系统柔性的水压致裂测试系统获得的现场岩体强度也是可靠的。Abstract: The compliance of the drilling-rod hydraulic fracturing in-situ stress measurement can affect the determination accuracy of the maximum horizontal principal stress. Utilizing tensile strengths based on the hollow cylinder hydraulic fracturing test to replace reopening pressures to calculate maximum horizontal principal stresses is a very promising option to cut down the negative effects from the drilling-rod test system. Eight hydraulic fracturing in-situ stress measurement tests were conducted in a 65 m-deep borehole in an under-construction railway tunnel in Fujian Province. Seventeen hollow cylinders made from the cores recovered from the same borehole were fractured hydraulically in laboratory. The average tensile strength based on the hollow cylinder hydraulic fracturing test is 8.40 MPa, which is close to 8.22 MPa, that determined by the classic hydraulic fracturing in-situ stress measurement. For the 8 in-situ stress measurements within a very narrow depth range of 20 m, the average value of the minimum horizontal principal stress is 8.41 MPa, and the average value of the maximum horizontal principal stress based on the hollow-cylinder tensile strength is 16.88 MPa, which is close to 16.70 MPa, the average value that calculated by the reopening pressure. The relationship between the three major principal stresses is σH > σV > σh, which is favorable for the strike-slip faulting. Based on the comparative analysis, for the drilling-rod hydraulic fracturing test system, when the test depth is shallow and the system compliance is minor, there are no marked differences in the calculated maximum principal stresses between based on the reopening pressures and the hollow-cylinder-test tensile strengths, which proves that the tensile strengths based on the hollow-cylinder test can be utilized to calculate the maximum horizontal principal stress during the hydraulic fracturing in-situ stress measurement; at the same time, the tensile strength of rock mass in the test interval, determined by the field hydraulic fracturing test with minor test system compliance, is definitely reliable for use.
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图 1 空心岩柱试验力学模型与加载示意图
σz—轴向应力；σr—径向应力；σθ—切向应力；Pr—内孔液压力；Pa—轴向压力；r1—岩样内孔半径；R2—岩样外半径
Figure 1. Mechanical model of the hollow cylinder rock specimen test and the loading diagram
图 2 某隧道内钻孔水压致裂原地应力测量压力-时间曲线
Figure 2. Recorded P-t curves of hydraulic fracturing campaign in a railway tunnel
图 3 微型压裂后的岩样照片
Figure 3. Photos of the rock specimen after the hydraulic fracturing tensile test
图 4 不同尺寸空心岩柱测试结果统计图
Figure 4. Statistical histogram of hollow cylinder tests of different geometrical sizes
表 1 水压致裂原地应力测量成果表
Table 1. Summary of hydraulic fracturing stress measurements
深度
/mPb Pr Ps P0 T σH σh σV σH
方向/MPa 21.7~22.3 15.74 7.04 7.41 0.41 8.70 14.78 7.41 11.39 80° 23.7~24.3 15.30 8.47 8.57 0.43 6.83 16.81 8.57 11.44 97° 25.7~26.3 17.87 9.87 8.82 0.45 8.00 16.14 8.82 11.49 27.7~28.3 19.88 8.71 9.30 0.47 11.17 18.72 9.30 11.55 29.7~30.3 16.69 7.75 8.39 0.49 8.94 16.93 8.39 11.60 31.7~32.3 16.02 8.49 7.57 0.51 7.53 13.71 7.57 11.66 39.7~40.3 13.39 8.46 9.69 0.59 4.93 20.02 9.69 11.87 41.7~42.3 18.33 8.68 9.11 0.60 9.65 18.05 9.11 11.93 
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表 2 基于两种方法计算的水压致裂原地应力测量成果表
Table 2. Summary of hydraulic fracturing stress measurements based on two methods
隧道内测试
深度/m自然
埋深/mσh/
MPaσV/
MPa①σH1/
MPa②σH2/
MPa③平均差
系数21.73~22.28 422.01 7.41 11.39 14.78 14.28 1% 23.73~24.28 424.01 8.57 11.44 16.81 18.18 5% 25.73~26.28 426.01 8.82 11.49 16.14 16.34 1% 27.73~28.28 428.01 9.30 11.55 18.72 15.75 8% 29.73~30.28 430.01 8.39 11.60 16.93 16.19 2% 31.73~32.28 432.01 7.57 11.66 13.71 14.38 3% 39.73~40.28 440.01 9.69 11.87 20.02 23.29 8% 41.73~42.28 442.01 9.11 11.93 18.05 16.61 4% ①—基于公式(5)计算，Pr采用经典水压致裂法利用P-t曲线确定；②—基于公式(3)计算，岩石抗拉强度T采用空心岩柱液压致裂试验确定；③—平均差系数采用常用的统计学公式计算，以每个测试段的两个值σH1、σH2为样本进行计算
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