Water-based fracking fluids are currently used to enhance the gas production from unconventional gas reservoirs, while the formation damage due to the interaction between rock matrix and residual water significantly reduce the pore space available for gas movement. In this paper, the effect of different fracturing fluids on fracture permeability is investigated using the same fractured siltstone sample in five rounds of cycling test (CO2-CO2-CO2-water-CO2). The experimental results reveal that the maximum confining pressure dominated fracture permeability. The fracture permeability for CO2 at 10MPa confining pressure decreased by around 75% after applying 40MPa confining pressure. After water flooding in the fourth round, the fracture permeability for CO2 was considerably reduced by around 70-85% compared with that in the second cycling test, and the influence of water flooding was much stronger at lower CO2 injection pressure. The experimental results showed that the fracture permeability for gas reduced significantly after water flooding due to formation damage. Thus, liquid CO2 is much more suitable than water as fracturing fluid for clay-abundant unconventional gas reservoirs.

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