Experimental study of anisotropic gas permeability and its relationship with fracture structure of Longmaxi Shales, Sichuan Basin, China

We investigated the anisotropic permeability in three directions of three cubic shale samples from Lower Silurian Longmaxi Formation in southeastern Sichuan Basin, China. The relationship between anisotropic permeability and fracture structure as characterised by micro-computed tomography (CT) and scanning electron microscope (SEM) Maps was also investigated. At confining pressure of 3 MPa, permeability parallel to bedding of the three shales varies between 37.6 and 3042.4 nanodarcies (nd), while permeability perpendicular to bedding varies between 3.6 and 17.3 nd, using helium and methane. The permeability anisotropy ratio between the parallel and perpendicular to bedding directions varies from 5.2 to 510.5 for the three samples using the two gases. Moreover, permeability in two parallel to bedding directions also shows strong anisotropy. The CT and SEM Maps results suggest that microfractures are critical to permeability in the parallel to bedding direction; they also lead to a higher anisotropy ratio between the parallel and perpendicular to bedding directions, while reducing the anisotropy ratio between the parallel to bedding directions. The helium permeability is 1.4 to 3.3 times that of methane permeability, while the helium to methane permeability ratio decreases with the sample's total organic carbon content: an observation that requires further investigation.