Nanoindentation investigation of mechanical and creep properties of continental Triassic Yanchang Formation shale, Ordos Basin

Abstract

The mechanical and creep properties of shale have a great impact on wellbore stability and artificial fracturing performance during shale gas exploration. We have applied nanoindentation to study the preceding two aspects of the lacustrine Triassic Yanchang Formation shale in Ordos Basin on the microscale, which is the representative exploration target for shale gas occurring in continental shales in China. A massive nanoindentation campaign finds that the Yanchang shale is highly mechanically heterogeneous in local areas but statistically homogeneous in large areas. The sequentially increasing indentation loads (5–400 mN) result in a two-stage change of values of Young’s modulus and hardness, i.e., descending first and then remaining steady with a turning point at approximately 300 mN, which suggests that the large indentation load of not less than 300 mN can detect the mechanical response of bulk shale on the microscale. In addition, the indentation tests reveal a strong anisotropy of mechanical properties of Yanchang shale, as the measured Young’s modulus and hardness in the bedding plane parallel are much larger than those in the bedding plane normal direction. Furthermore, the Yanchang shale presents strong creep behavior and weak fracture toughness, which are different from those of current gas-producing marine shales worldwide. We attribute this to the higher content of clay minerals and relatively more loose texture of the Yanchang shale. In particular, the creep strain-rate sensitivity ( m) is calculated to be 0.102–0.134, suggesting that the dominant deformation mechanism of the lacustrine Yanchang shale may be dislocation creep.