Effect of polymer on gas flow behavior in microfractures of unconventional gas reservoirs

Tight sandstone and gas shale exhibit ultra-low permeability, so hydraulic stimulation is a crucial factor in unblocking production potential of unconventional gas reservoirs. Slickwater fracturing has been widely applied in field cases due to its low cost and good effect, and polymer friction reducer serves as one of the primary additives in the fracturing fluid to reduce the fluid friction associated with high pump rates. In spite of the flowback post fracturing treatment, large volumes of friction reducer would remain in microfractures that result in possible formation and fracture damage. However, most of former emphases were placed on unconventional natural gas production increment due to hydraulic stimulation, and very few results were shown in the literature on how polymer friction reducer impacts gas flow behavior in microfractures. We fabricated two tight sandstone fracture models and one shale fracture model with different fracture widths in this study. A core flooding apparatus was set up to conduct gas-injection experiments before and after polymer treatment. An approach was developed to determine residual resistance factor for gas (Frr,gas). Experimental results indicate the Frr,gas in the first cycle after polymer treatment decreases with an increase in superficial gas velocity, and the relationship can be fitted well with a power-law equation. The Frr,gas tends to be stabilized in the subsequent cycles. The Frr,gas is smaller in tight sandstone fracture model compared to the shale fracture model with the same fracture width. Surprisingly, polymer treatment does not resist gas flow, and may even improve it in the wider fracture.