ABSTRACT
With the beneficial of the physical properties, super-critical carbon dioxide is a suitable non-aqueous fluid for shale gas exploitation. The interactions among CO2, drilling/fracking fluids and shale formations will influence shale’s mechanical properties, which is of great importance for fracturing. In this paper, the fractal characteristics and acoustic emission of shale are investigated by conducting a series of compressive tests together with Acoustic Emission (AE) technology. Shale samples were imbibed in sub-critical/supercritical CO2+water for different time (10 days, 20 days and 30 days). The results show that AE energy is mainly created at the unstable crack propagation stage, while AE counts are mostly occurred at both the stable and unstable crack propagation stages. Longer imbibition time creates higher maximum cumulative AE energy. Shale samples with or without imbibition present very good correlation during the R/S analysis. Values of Hurst index are all higher than 0.5. AE signals show an enhanced trend with increasingly loading time. The treatment of sub-/super-critical CO2+water slightly affects shale’s fractal dimension. Both sub-critical and super-critical imbibition conditions could decrease the fractal dimension of shale, and make shale samples more regular for deformation and crack propagation.
