Comparative Analysis Of Maximum Horizontal Principal Stress Estimated By Tensile Strength Of Rock

The compliance of the hydraulic fracturing in-situ stress measuring system will produce great influence on the results of maximum horizontal principal stress and improving the measurement accuracy of the maximum horizontal principal stress has important significances on various underground engineering design.So we try to estimate maximum horizontal principal stress from the tensile strength of rock measured by laboratory test,and make data comparative analysis with the maximum horizontal principal stress data calculated by using the classical hydro-fracturing formula.It is expected to find an alternative method to avoid the influence of system flexibility and improve the testing accuracy of the maximum horizontal principal stress.In the process of this study,the work is from the following aspects respectively: Arrange the research status about the domestic and foreign rock tensile strength experimental methods and comprehensive comparison;Carry out indoor double rings direct tensile test and analysis experimental results,preliminary study the three optimal ranges of inner ring diameter / specimen diameter,outer ring diameter / specimen diameter,inner ring diameter / outer ring diameter about double ring direct tensile test.carry out the numerical simulation and comparative analysis of the specimens with different sizes and verify laboratory results.Comparative analysis of laboratory data and field measured data.Based on the above research,the main results are as follows:(1)The tensile strength values of rocks measured by different test methods in the same area are very different.The theory of direct tensile,bending tensile,brazil split,hollow rock column test is perfect,the experimental guidance system has become mature.The principle of direct tensile test is most closed to the definition of rock tensile strength.The measured value of the bending tensile test is large,which is closely related to the fracture mechanism,and it has many influence factors.There is a great controversy in brazil splitting test,it has many influence factors but high recognition.In recent years,the experiment of hollow rock column is developing rapidly.The research on the double ring direct tensile test is very little,lacks of systematic experimental guidance.(2)Design part of inner and outer ring diameter double rings direct tensile test specimens and carry out laboratory tests,find that when the ratio of inner ring diameter and specimen diameter is 0.32~0.47,the ratio of outer ring diameter and specimen diameter is 0.53~0.69,the ratio of inner ring diameter and outer ring diameter is 0.58~0.69,the tensile strength of rock is stable and reliable.(3)Using ABAQUS numerical simulation software simulates 54 double rings direct tensile specimens with different sizes and that shows when the ratio of outer ring diameter / specimen diameter is 0.53~0.74,the ratio of inner ring diameter / specimen diameter is 0.32~0.48,the stress distribution of the specimen is relatively uniform,it is possible to obtain stable and reliable rock tensile strength theoretically.At the same time,the rationality of the ratio of inner and outer ring diameter/ specimen diameter about indoor double rings direct tensile test were verified.It should be emphasized that when choosing the ratio of outer ring diameter/ specimen diameter is lager,the ratio of inner ring diameter / specimen diameter should be larger.(4)The maximum horizontal principal stressestimated by indoor test,the rocks obtained from Honghe in Shandong and Yixian in Hebei,is comparative analysis with that calculated by classical hydraulic fracturing formulato find that the maximum horizontal principal stressestimated by bending tensile test,disc splitting test(strip method,2:1),hollow rock column test(1:1,oil)is higher 50% or even over 100% than the field stress.The maximum horizontal principal stressestimated by disc splitting test(pyramid method,1:1),direct tensile test,hollow rock column(1:1,water)is slightly larger thanthe field stress.The maximum horizontal principal stressestimated by the double rings direct tensile test is slightly less than that measured in the field.Which are all in acceptable range.The tensile strength of rocks obtained by disc splitting test(pyramid method,2:1)is large discreteness and is unstable and unreliable.The comparative result of hollow rock column test(1:2,water)is not the same in two areas and needs further research.