海底天然气渗漏区热传递对天然气水合物形成的影响

EFFECTS OF HEAT TRANSITION ON HYDRATE ACCUMULATION IN GAS VENT SITES

天然气水合物的沉淀/分解作用是一种放热/吸热反应,海底天然气渗漏是从高温区向低温区运移而且携带热量,这2种热量(水合物生成热和渗漏天然气热容热)会导致海底温度场的变化并影响水合物的形成。以美国墨西哥湾布什山水合物丘为例,应用渗漏天然气形成水合物的动力学模型,探讨了水合物生成热和渗漏天然气热容热对水合物稳定性的影响:在布什山,水合物天然气渗漏量为1.8 kg/(m2.a)和10%的渗漏天然气沉淀为水合物条件下,10 ka内水合物生成热和渗漏天然气热容热使海底表层的地温梯度增加了3℃/km,在1 km深处的沉积层地温梯度则降低了1.4℃/km左右,温度最大的扰动发生于400 m左右深的沉积层里(增加了0.4℃),这样的温度场变化使水合物稳定带厚度减少了12 m,使0.06 kg/m2的水合物分解。

Gas hydrate crystallization/dissolution are exothermic/endothermic reactions, and gas venting from higher temperatures to the lower produces capacity heat, which results in the change bottom water temperature and consequently affects gas hydrate accumulation. We use the compositional kinetic model to analyze how hydrate latent heat and water capacity heat affect the gas hydrate stability in the Bush Hill. We take filter loss of venting gas as 1.8kg/（m^2· a） and there is 10G vent gas deposited as gas hydrate in the subsurface. Modeling shows that gas hydrate latent heat and water capacity heat within 10ka can perturb the temperature field in the subsurface of the Bush Hill, and thermal gradient increases 3℃/km at the surface of the seafloor and decreases -1.4 ℃/km at 1km deep. The most obvious temperature variation, which is about 0.4℃ ,occurs in the middle part of the gas hydrate stability zone（400 mbsf deep）, resulting in the reduction of hydrate stability zone by 12m and the dissolution of 0.06kg/m^2 of gas hydrate.