海底天然气渗漏系统水合物成藏过程及控制因素

FORMATION AND ITS CONTROLLING FACTORS OF GAS HYDRATE RESERVOIR IN MARINE GAS VENT SYSTEM

在海底天然气渗漏系统沉淀水合物的动力学基础上,建立了水合物沉淀与分解的化学动力学模型。应用该模型分析了美国墨西哥湾布什山天然气渗漏系统水合物的成藏过程,探讨了水合物沉淀、稳定性影响因素。在渗漏通量为每年400kg·m-2的单个通道中,约需425a才能导致水合物稳定带沉积层约30%孔隙完全被水合物充填,渗漏通道被堵塞,沉淀的水合物在剖面上从稳定带底部向海底趋于富C3+C4。在渗漏通道天然气流量由弱到强再到弱的演化过程中,渗漏速度增大过程中形成的水合物在渗漏速度减小过程中将分解,总量约10%的水合物将被分解。如果分解产生的天然气可快速迁移出渗漏系统,海底温度的升高可引起约40%的水合物在20d内分解,并导致海底渗漏速度的急剧增大。

A chemical kinetic model of hydrate crystallization and dissolution in a gas vent system was constructed based on the kinetic model of hydrates precipitating from a gas steam. This model was used to analyze how the gas hydrate reservoir was formed, how the variations of venting rate crystallized hydrates, and how bottom water temperature variations could destabilize the precipitated hydrate and increase the gas venting rate in the Bush Hill gas vent system in the Gulf of Mexico. If hydrates crystallize from 8 m diameter local vent at a venting rate of 400 kg·m -2 per year, sediments with about 30% porosity in the hydrate stability zone will be completely filled with hydrates in about 425 years, thus the individual channel may be plugged. Accumulated hydrates have more enriched C_ 3 +C_ 4 at the seafloor than at the bottom of gas hydrate stable zone. In the evolution of venting rate from low to fast then low again in a gas vent channel, if hydrate dissolution is much faster than crystallization, about 10% hydrates precipitating during the ramp up period of venting rate will dissociate during the ramp down period of venting rate. If the dissolved hydrate gas can escape from the gas vent system, the seafloor temperature increase will cause about 40% of precipitated hydrates to dissociate within 20 days and the gas venting rate to increase rapidly.