海底水合物矿藏降压开采与甲烷气体扩散过程的数值模拟

The Simulation of Hydrate Production by Depressurization in Deep Ocean

在深海条件下采用单井降压法开采的天然气水合物矿藏中,利用TOUGH+HYDRATE软件对其开采过程和甲烷气体扩散过程进行数值模拟。物理模型由上至下依次为上盖层、水合物沉积层和下盖层。将上、下盖层外边界的温度设为恒定,与含水合物沉积层之间有热量和质量交换,数值模型采用二维圆柱坐标网格。模拟结果表明开采过程中井口产气速率是一个升高—降低—波动升高的过程,水合物分解产生的气体有一部分通过上盖层溢出,能在一定程度上增加大气中温室气体的量。开采初期水合物分解速率降低的主要原因是水合物分解产生的甲烷气体在地层中大量累积,开采后期水合物分解速率产生波动的主要原因是发生"气穴现象"。井口附近由于压力变化较快水合物分解最为剧烈,其附近有个低温区存在。上、下盖层附近水合物分解速率也较快。

In the work,we employ the TOUGH＋ HYDRATE software from Lawrence Berkeley National Laboratory to simulate the exploitation process by depressurization to determine and analyze the significant characteristics and the key factors of the effect on the hydrate dissociation.The physical module of the reservoir is divided into three parts,the overburden,the hydrate reservoir and the underburden,and the numerical module is discretized in 2-D column coordination.In the process of hydrate dissociation,the temperatures of the burden boundaries are assumed to be constant and the burden can exchange the mass and heat with the hydrate reservoir.The results illustrate that the dissociation of the hydrate is a synchronous process in the whole hydrate bearing layer.However,it is more intensive in the region near the overburden and underburden on account of the heat transformation among the overburden,underburden and water.The hydrate near the well more easily dissociate because there is a bigger pressure drop in the region near the well.In the initial period of the hydrate production,the large amount of water in the hydrate layer is drilled out.Thus,the production rate of methane gas is relatively low,with the proceeding of the production and the production gas rate gradually increases with time.The dissociated gas in the hydrate reserve can not be drilled completely.Some of them permeated into the overburden and then released from the overburden eventually.In the initial period of the dissociation,the accumulation of gas in the reservoir induces the sharp declination of hydrate dissociation rate.In the later period,the phenomenon of ＂Gas cavity＂ has the great impact on the hydrate dissociation rate which results in a drastic fluctuation of the hydrate dissociation rate.