Abstract: To study the effects of different sizes of shale with the same material and different dips of laminae on its mechanical properties and damage characteristics,in order to provide important theoretical guidance for the shale gas extraction from the Lower Cambrian brown shale under hydraulic fracturing in Fenggang Three Areas of Qianbei Region. The shale model was constructed based on RFPA2D-Flow software,and the controlled perimeter pressure was 10MPa,the water pressure was 8MPa,and the quartz content was fixed at 40%. Four sets of aspect ratios of 1,2,3 and 4 were set,and the inclination angles of different layers in each set were 0°,30°,60° and 90°. The total number of experimental groups was 16,and the effects of laminar inclination angle and size effect on the stress-strain curve characteristics,peak strength,deformation characteristics,and damage characteristics of shale were analyzed. The study shows that the elastic modulus and peak strength of shale matrix have the size effect,and the elastic modulus and peak strength of shale are significantly affected by the angle of inclination of the laminae,and both show obvious anisotropy. Under the same aspect ratio,the compressive strength shows a "V" curve with the increase of laminar inclination. Based on the crack distribution of specimens with different laminar inclination angles and aspect ratios,the damage modes can be categorized into three types,i. e. ,Ⅰ-type,polyflexural,and flame-type damage,and Ⅰ-type damage modes dominate in all models. Tensile damage mainly occurs at aspect ratios of 1,3,and 4,accompanied by a small amount of shear damage,indicating that the specimen is characterized by compressive but not tensile resistance,and at an aspect ratio of 2,tensile damage is balanced with shear damage,indicating that the specimen is neither compressive nor tensile.

Key words: shale,size effects,laminar dip,mechanical properties,damage characteristics