Experimental study on the scale effect of strength and deformation of Chang 7 shale in Ordos Basin

Shale has an obvious bedding structure and strong heterogeneity，and its strength and deformation characteristics have a significant scale effect，which makes the test results of small-scale rock samples in the laboratory to be limited in characterizing the mechanical properties of large-scale shale rock mass in the mine.In order to explore the scale effect law of shale strength and deformation，taking the continental shale outcrop of Yanchang Formation 7 in Ordos Basin as the research object，the triaxial compression tests of 11 rock samples with height diameter ratio of 2，diameter of 17，20，25 and 38 mm were carried out with RTR-1000 rock triaxial testing system.Based on the analysis of the scale effect law and its internal mechanism of shale strength and deformation，a new scale effect model for shale strength was proposed，and the reasonable test scale of indoor rock sample in characterizing shale field was preliminarily obtained.The results show that most of the mechanical parameters describing the strength and deformation of shale have significant scale effect，and the elastic modulus and Poisson’s ratio increase with the increase of rock sample size，and the peak strength，peak strain，residual strain，residual stress and fracture time decrease with the increase of rock sample size.Shale with different scales changes instantaneously from elastic deformation to brittle failure，and there is no obvious plastic yield section.Before failure,the deformation of shale is linear elastic deformation，and after fracture，it is tensile shear failure，but the shear angle is small，and it is within 10 degrees.The scale effect of shale strength and deformation parameters is caused by loading direction，bedding plane direction，mechanical properties of bedding plane and uneven distribution of micro defects.Based on Griffith micro-crack strength theory model，a new strength scale effect model for shale was obtained.The model is in good agreement with the test results and can accurately predict the strength of shale samples with different scales.Different mechanical parameters of shale rock mass correspond to different sizes of reasonable laboratory test rock samples，and when the height diameter ratio of rock sample is 2 and the length is more than 160 mm，the mechanical parameters of rock sample measured in the laboratory can be used to characterize the mechanical parameters of rock mass in the mine after field correction.The scale effect law of shale strength and deformation can be used to predict the fracture pressure，the deformation characteristics of wellbore during well drilling and the initiation and stable expansion pressure during fracturing.In the current volume fracturing operation,it is very beneficial to expand the fracturing scale of branch fractures along the bedding direction or for the large and thick shale reservoirs with extremely developed micro defects along the bedding direction，it can be considered to conduct the multi-cluster fracturing of vertical wells to fully release the mechanical advantages of the bedding surface conducive to fracture expansion，so as to improve the productivity of shale gas wells.