Abstract

Montmorillonites are commonly existing in shales and understanding the anisotropic elastic properties of montmorillonite is very crucial because they provide important geomechanical information for the discovery and development of shale gas. Layer charges with varying densities and locations are naturally associated with montmorillonites and it has been demonstrated that these layer charge characteristics can dramatically influence the anisotropic elastic properties of dry montmorillonites. However, montmorillonites are generally hydrated by water under reservoir conditions and whether the existence of water will impact the effect of the layer charge characteristics on the anisotropic elastic properties is still poorly understood. To gain such knowledge, this paper investigated the anisotropic elastic parameters of hydrated montmorillonites with various layer charge characteristics via molecular dynamic (MD) simulations. The results demonstrated that the existence of water reduces all the elastic factors of montmorillonites, and weakens the variation of the elastic factors with different layer charge locations (LCLs), in comparison with dry montmorillonites. It was also found that when hydrated, the influence of layer charge density (LCD) on the in-plane compressional factors tends to be diminished, and the out-of-plane compressional and shear factors tend to increase consistently with increasing LCD, whereas their dependence on the LCD can be disordered in different dry montmorillonite models. The variations in the basal spacing and the interactions between the atoms caused by different layer charge characteristics when water is present were found to plausibly explain and interpret the simulation results. The results not only illustrate the link between the anisotropic elastic properties and layer charge characteristics in hydrated montmorillonites but also provide crucial geomechanical information to help the successful discovery and development of shale gas reservoirs containing hydrated montmorillonite.

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