Experimental study on tensile strength of sandstone under biaxial compressive stress
JU Yingtong, CHEN Mian, YANG Shuai
1 College of Petroleum Engineering, China University of Petroleum-Beijing, Beijing 102249, China 2 PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
The tensile strength of rock material is an important and basic parameter when it comes to wellbore stability. In general, for metal, we can obtain the tensile strength in a direct way – the Tensile Test. However, for geotechnical materials, this method is impractical from the first step of sample preparation. Generally, the most commonly used method to obtain the tensile strength of rocks in the laboratory is using the Brazilian Test, which is an indirect approach. Now, this method is recommended by ISRM and has been extensively accepted by the whole community. However, this method has certain limitations. First, since it cannot restore the initial compacted state of the rock, there may be deviations when determining the tensile strength of ultra-deep rock formation. Additionally, the underlying assumption of this method is that the rock undergoes brittle failure directly after the linear elastic stage, which does not align with the actual deformation behavior of ultra-deep sandstone. Based on the problems mentioned above, in this paper, the real tensile yield strength and ultimate tensile strength of sandstone are determined through Biaxial Compression Tests. The real tensile yield strength and ultimate tensile strength of underground sandstone can be determined by a Biaxial Compression Test. The results show that they both increase along with the confining pressure, which accords with the common understanding that the denser the rock material, the higher the strength. The equivalent uniaxial tensile yield strength of sandstone can be obtained by fitting-extrapolation, which is very close to the results of Brazilian Tests, verifying the reliability of this method. By studying the tensile strength of sandstone under biaxial compressive stress, it can provide a more accurate basis for checking tensile failure under unloading conditions, as well as the theoretical and experimental support for the stability of sandstone wellbores.
