Numerical investigation on the long-term production behavior of horizontal well at the gas hydrate production site in South China Sea

The gas hydrates-bearing shaly silt reservoirs have the characteristics of high reserves and low permeability. The trial production test in South China Sea in 2020 shows that horizontal well has broad prospects for exploiting this kind of hydrate reservoir. However, due to the short trial production time, the long-term gas production behavior is still unknown and needs further investigation. Based on the trial production data, the geological model of multi-layer hydrate reservoir and multi-physical field coupling mathematical model are established. The influence of optimization of depressurization scheme and optimization of reservoir selection on the improvement of gas production efficiency and the influence of compaction effect on attenuation of gas production rate and reservoir stability are investigated. The results show that the pressure drop gradient and constant pressure duration are the key factors affecting step-wise depressurization. The constant pressure duration of single-stage depressurization is more appropriate between 5 and 10 days. When the pressure drops below the equilibrium pressure of hydrate phase, the pressure drop gradient of 0.2 MPa/d is the best for long-term stable gas production. On the other hand, the low permeability of HBL inhibits pressure drop propagation and heat transfer in this sublayer, keeping the gas production rate at a low level in the later stage of production. Further simulation shows that HBL with hydrate saturation of 19% ∼ 25% should be selected for the target reservoir in order to improve gas production efficiency. In addition, the compaction effect in the hydrate decomposition zone is obvious. Compared with the 30 days, the near-well effective permeability drops by more than 30% at 720 days, which is an important reason for the decrease of gas production rate in the later stage of production. Finally, the formation subsidence increases with the expansion of hydrate decomposition radius. The maximum formation subsidence is 0.31 m after 720 days of exploitation.