Experimental study of fines migration in gap-graded soils with gas production: Implications to hydrate production from unconsolidated reservoirs

Methane hydrate production from unconsolidated reservoirs could be hampered by excessive sanding, which has been particularly noticed during gas production from hydrate-bearing sediments in the deep ocean. The sanding process involves the interaction of liquid, gas, particles, and phase change. Here we present an experimental study of production-induced fines migration under K0 pressure in a gap-graded sediment by using a transparent horizontal permeameter that allows the sand production process to be recorded. Effervescent powders, which release carbon dioxide when exposed to water, are used to simulate hydrate, producing gas in unconsolidated sediments to reproduce the phase change process. Our results show that permeability decreases due to pores being filled by gas in the initial stage of pressure variation, which further leads to the sudden collapse of the specimen, manifested by a burst period on the erosion curve. For the gap-graded soil with low fines content, gas fills the pores and accelerates the flow to wash away more fine grains. As the fines content increases, the effect of gas on the finial erosion weight decreases. The erosion localization is more severe, and the formation time of the flow channel is shorter in the soil with gas generation and high fines content. Once the flow channel is formed, erosion in the remaining areas is reduced as water flows converge into the flow channel. Sensitivity analysis shows that the final erosion weight increases with increasing grain size ratio, fine grain content, and effervescent powder content. As the size of effervescent powder increases, the final erosion weight also increases.