Numerical simulations of the synthetic processes and consequences of secondary hydrates during depressurization of a horizontal well in the hydrates production

The depressurization method was implemented through a horizontal well in the production test at Shenhu, the South China Sea in 2020. The numerical model of a layered hydrate reservoir was established based on the in-field data of this test. The gas production rates measured was validated by comparing with the numerical results. The synthesis time, scopes, gas sources and maximum saturations of secondary hydrates were originally clarified by analyzing the data from different monitoring sections, which followed by the sensitivity analyses against the initial permeability of each hydrate-bearing layer. The effect of secondary hydrates on the short-term and long-term productions was quantified. The hydrates were resynthesized extensively above the interface between the natural gas hydrate layer and the mixing layer, which reduced the in-situ permeability and then the well gas production seriously. The areas affected by depressurization were constrained within the mixing layer. The gas sources were supplemented by the primary free gas and the gas decomposed by the hydrates in the upper mixing layer. The vertical extensions of the secondary hydrates were limited to about 5 m due to the lower permeability, while the horizontal distributions were approximately 78 m away from the horizontal well after 3 years. Given that the secondary hydrates were eliminated artificially, the cumulative gas volume produced in the horizontal well was increased by 24% after 3 years. The sensitivity analysis showed that the variations of permeability in the mixing layer had more obvious effects on secondary hydrates. Therefore, the hydraulic fracturing could be used to prevent the resynthesis of hydrates.