Rock Physics Modeling Studies on the Elastic and Anisotropic Properties of Organic-Rich Shale

Shale gas reservoirs have a large amount of resources, a wide range of burial, and great development potential. In order to evaluate the elastic properties of the shale, elastic wave velocity and anisotropy measurements of Longmaxi shale samples were carried out in the laboratory. Combined with the results of back scattering scanning electron microscopy (SEM) and digital mineral composition tests, the relationship between the anisotropy and the mineral components of the shale samples is discussed. It is found that the clay and kerogen combination distributed with an inorganic mineral background is the main cause of anisotropy. Then, the elastic properties of the organic-rich shale are analyzed with the anisotropic differential equivalent medium model (DEM). The clay and kerogen combination is established with kerogen as the background medium and clay mineral as the additive phase. The bond transformation is used to rotate the combination so that its directional arrangement is consistent with the real sedimentary situation of the stratum. Then, the clay and kerogen combination is added to the inorganic mineral matrix, with the organic and inorganic pores added to characterize the anisotropy of the shale to the greatest extent. It is found that the error between the wave velocity results calculated from the model and measured in the laboratory is less than 10%, which means the model is reliable. Finally, the effects of the microcracks and aspect ratio, kerogen content, and maturity on the elastic and anisotropic properties of shale rocks are simulated and analyzed with this model. The degree of anisotropy increases with the decrease in the pore aspect ratio and the increase in the microcracks content. The greater the kerogen content and maturity, the greater the anisotropy of rock. This study is of great significance for predicting the "sweet spot" of shale gas and optimizing hydraulic fracturing layers.