Dual effects of retained fracturing fluid on methane diffusion in shale containing adsorbed methane

Retained fracturing fluids have dual effects on methane diffusion in organic-rich shale, which disrupt the methane transport. To understand these dual effects, the methane transport behavior was investigated before and after water entered the shale pore structure. We simulated the entry of the fracturing fluid into shale by changing the imbibition pressure difference. Meanwhile, nuclear magnetic resonance (NMR) was employed to analyze the distribution of water in the multiscale pore structure of shale. Results indicate that water is mainly distributed in pores with a diameter (D) of <50 nm, especially in pores with D of 2-10 nm. When water entered the shale pore structure, the methane diffusion rate decreased in pores with D > 10 nm and increased in pores with D < 10 nm. In the presence of water, the methane diffusion rate in shale macropores decreased by 13.8%-52.6%, and the methane diffusion rate in micropores increased by 67.9%-223.7%. These results indicate that although water retention is not beneficial to the slip flow and Knudson diffusion of methane, it facilitates the surface diffusion of methane. These conclusions hold high significance for determining the shut-in time and fracturing fluid flowback in shale gas wells.