Anisotropic characteristics of crack initiation and damage thresholds of deep black shale in Longmaxi Formation

To investigate the anisotropic characteristics of crack initiation mechanism and evolution law during the progressive shale failure process, uniaxial compression tests have been conducted on the black shale at the depth of 1974 m in Longmaxi formation of Dingye 1 well in Dingshan Dongxi area.Two stress thresholds(crack initiation stress σ_ci and crack damage stress σ_cd) are identified based on the energy dissipation evolution mechanism.The reasons for the anisotropy of crack initiation stress and crack damage stress are explained in detail.The anisotropic characteristics of mutual relationships among the peak stress and the two stress thresholds are discussed.The anisotropic characteristics of the strain energy and elastic strain energy conversion efficiency corresponding to two stress thresholds and peak stress are analyzed.The results show that the bedding angle has little influence on crack initiation stress, but it has significant influence on crack damage stress.With the bedding angle increasing, the ratios σ_ci/σ_c and σ_cd/σ_c increase first and then decrease, while the ratios σ_ci/σ_cd have no obvious change law with the bedding angle increasing.The ratios σ_cd/σ_c can be used as an early warning index of rock failure.The larger is σ_cd/σ_c,the shorter the warning time before the sample failure.When the bedding angle is 45° and 60°,the sample failure is destroyed rapidly, and failure pattern generally is macro-crack slipping along the bedding plane.The strain energy decreases first and then increases with the bedding angle.The minimum strain energy is required for failure at 60° and the maximum strain energy is required for failure at 0° or 90°.Elastic strain energy conversion efficiency is less sensitive to the bedding angle at the crack initiation stress, while elastic strain energy conversion efficiency is more sensitive to the bedding angle at the crack damage stress and peak stress.