Fracture propagation and failure mode characteristics of lamellar lacustrine shale under true triaxial compression conditions

Shale oil in China is mainly concentrated in lacustrine shale reservoir, which has poor structural continuity, strong heterogeneity and strong anisotropy. It is of theoretical significance to study the structural characteristics and mechanical properties of lacustrine shale to improve the exploitation effect of lacustrine shale oil in China. In this investigation, the lamellar lacustrine shale of Chang 7 member of Yanchang in Ordos Basin, which is rich in shale oil, is selected for study, the natural lamellar structures were characterized quantitatively and the true triaxial compression tests were concluded. It found that the thickness distribution of total, light, and dark laminae follows an exponential, power and transformed logarithmic function, which can be used to build lacustrine shale laminar structure models of different sizes more precisely. In addition, with increasing of intermediate principal stress, the ratios of crack initiation stress (sigma ci) and damage stress (sigma cd) to peak strength (sigma c) both decrease, and the connection degree between the activated natural cracks and new cracks is more obvious. Moreover, it found that laminae types and morphologies have effect on the type and number of new cracks, and natural crack characteristics affect the connection form between natural and new cracks. These illustrate that the inhomogeneity of initial structure characteristics and the stress condition of true triaxial loading jointly affect the crack propagation and failure pattern of lacustrine shale, the reduced stress ratio and fracture combination failure mode indicates that lacustrine shale is a favorable rock facies for fracturing. These conclusions can provide part essential data for establishing the geometric model of lacustrine shale and provide theory reference for the optimization design of fracturing scheme in lacustrine shale oil exploitation.