Methane adsorption and sealing effects of shale: A case study of relict synclines in the southeastern Chongqing

Universality and difference both exist in the methane adsorption of shale. This study concerns the influencing factors of shale gas occurrence, preservation and accumulation. The gas adsorption capacity of the Wufeng-Longmaxi shale in the relict synclinal district in the southeastern Chongqing was studied by means of isothermal adsorption, scanning electron microscopy, nitrogen adsorption and organic geochemical methods. The adsorption capacity of shale is controlled by some internal characteristics such as pore structure and organic matter features, and some external environment conditions such as temperature and pressure. The adsorption strength of shale under subsurface conditions can be predicted using a Langmuir model. The adsorption strength of Wufeng-Longmaxi shale from the relict synclinal district in the southeastern Chongqing has a critical depth at which adsorption strength reaches its peak value. Theore-tical analysis and experimental data of shale permeability determination under three axial stress concluded that the adsorption effect of shale resulted in the decrease of permeability which blocked the diffusion influences to shale gas flow, which was called as 'gas sealing effect'. The influences from this effect on gas preservation and enrichment boundary should be further studied.