Multi-fractal analysis for the AE energy dissipation of CO_2 and CO_2 + brine/water treated low-clay shales under uniaxial compressive tests

摘要

Carbon dioxide (CO2) is a promising fluid which can be used for shale gas recovery. In this study, uniaxial compressive strength tests were performed on low-clay shale samples soaked in sub-/or super-critical CO2 and its dissolved water/brine solutions for 10, 20 and 30 days. The deformation and crack evolution during the experiments were analyzed by the multi-fractal method based on the acoustic emission (AE) signals. The results show that the value of.a (the width of the spectrum) increases with increasing soaking time, which is related to the alteration of shale's internal structure during imbibition. Sub-or super-critical CO2+ water/brine solutions significantly decrease the strength and change the brittleness of shales, which decrease the frequencies of large and small AE signals and present low values of.f (the relationship between small and large signal frequencies). The morphology analysis shows that samples with imbibition produce more large-scale cracks during the compression tests and tend to have right skewed distributions. From the crack closure stage to the stable/unstable crack propagation stage, the average value of.a for soaked samples decreases, which means the structure of the AE signals becomes simpler. Super-critical conditions, which decreases the complexity of shale deformation and weakens the role of small/high signals in the AE energy release, yield a lower value of.a than the corresponding sub-critical conditions. This study helps to understand the effect of CO2 and its saturated fluids on shales and the mechanism of cracks propagation during CO2 enhanced shale gas recovery and CO2 sequestration in shale reservoirs.