Estimation of anisotropic elastic parameters of clay and its application in rock physics modelling of shale reservoir

As a main component of shale reservoirs, clay minerals can influence the mechanical characteristics of reservoir rocks through the type, content, and orientation of clay minerals. Accurate elastic constants of clay minerals are crucial for high-precision seismic inversion and “sweet spots” prediction. However, conventional rock physics modelling often simplifies clay minerals as isotropic, ignoring their inherent anisotropy and discrete distribution characteristics, which leads to prediction distortion. In this study, the complete anisotropic elastic constants of kaolinite, smectite, and illite were calculated based on the first-principles. Then, a shale physical model considering clay anisotropy, geometric morphology, and discrete distribution was established. The morphological structure is characterized by introducing the ratio of the short diameter to the long diameter of clay platelets. Numerical analysis shows that the elastic constants C11 and C66 are inversely proportional to the ratio, while C33, C44, and C13 are relatively insensitive to this change. The inter-clay porosity is inversely proportional to C11, C13, C33, C44, and C66, especially C33. Ultrasonic data demonstrated that the developed model is reliable in predicting velocity and elastic anisotropy parameters. Based on well logging data from the Sichuan Basin, parameters such as interlayer porosity, lamination factor, and anisotropy of clay were inverted using a rock physics model. This study provides a basis for precise seismic data analysis and reliable detection of engineering “sweet spots” within shale formations.