Micro scopic heterogeneity and mechanical anisotropy of the laminated shale in Longmaxi Formation

As a clean energy, the commercial development and utilization of shale gas affect the global energy landscape. The microscopic heterogeneity and mechanical anisotropy of laminated shale have crucial significance for studying wellbore stability and the hydraulic fracture (HF) geometry. In order to provide experimental and theoretical bases for the optimization of drilling and fracturing parameters in the field, the microscopic heterogeneity and mechanical anisotropy of Longmaxi laminated shale were studied. The uniaxial compressive experiments, field-emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) observations and wave velocity tests were conducted on shale samples cored at different angles. Moreover, the effect of microscopic heterogeneity and mechanical anisotropy on the geometry of HFs was discussed. The results suggest that the bedding planes influence microscopic heterogeneity and mechanical anisotropy in the Longmaxi laminated shales. For the microscopic heterogeneity, with increasing angle between observation direction and bedding planes, the development degree of micropores increased. As observed from FE-SEM and AFM images, the distribution of mineralogical components and organic matter-hosted pores shows strong heterogeneity in microscopic scale, indicative of increasing gas storage capacity (Fig. 1, 2). 1 FE-SEM images of shale samples with different coring angles (β is the angle between the observation direction and bedding direction in the FE-SEM images. The figures show that in the direction parallel to the beddings, the shale matrix is cemented well. With increasing bedding angle, the development degree of micro pore structure increases gradually) 2 AFM images of shale samples with different coring angles (γ is the angle between the observation direction and the bedding direction in the AFM tests. The results are similar to the FE-SEM tests) As for the mechanical anisotropy, under the uniaxial compression, the failure mode and mechanical parameters were different due to the different bedding angles. With increasing angle (θ) between the loading direction and bedding direction, the failure mode gradually changed from tensile failure perpendicular to the bedding planes, to shear failure, and then to the "splitting-shearing" mixed failure (Fig. 3). With increasing θ, the uniaxial compressive strength and Poisson's ratio of Longmaxi laminated shales display a "U-shaped" anisotropic model that is characterized by a first decrease and a subsequent increase. While the elastic modulus and S-P wave velocity shows a decreasing trend, the bedding planes of shale with weak cementation will be damaged before the rock matrix, which will significantly affect mechanical properties of the whole rock. The microscopic heterogeneity of shales influences the anisotropy of mechanical properties to a certain extent. The varying development degree of micropore structure in different bedding directions will indirectly affect the mechanical properties by affecting the strain and cementation degree of matrix during the uniaxial compression experiments. 3 Typical failure patterns and fracture geometry of shale samples with different coring angles 4 Effect of shale anisotropy on the initiation of hydraulic fracture (a.The HF propagates along the bedding plane near the well after initiation; b.The HF propagates directly to the sample boundary after initiation) The microscopic heterogeneity and mechanical anisotropy of shales can affect the HF behavior during hydraulic fracturing, and the fluid seepage flow paths under the shutoff of pumps. The development of natural fracture planes near the wellbore will induce HF propagation. It is suggested that micro-fractures are relatively developed in the direction perpendicular to beddings. The developed micro-fractures not only create conditions for the initiation of HFs, but also provide channels for the seepage of fracturing fluid after the pump is stopped. The results provide theoretical basis for the parameter optimization of laminated shale hydraulic fracturing.