The Simulation of Hydrate Production by Depressurization in Deep Ocean

Abstract

Abstract:

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.

Key words: deep ocean, hydrate, depressurization, single well

CLC Number:

TE132.2

LI Xiao-Sen, CHEN Qi, LI Gang, CHEN Chao-Yang. The Simulation of Hydrate Production by Depressurization in Deep Ocean[J]. Geoscience, 2010, 24(3): 598-606.

References

（1）开采过程中水合物分解速率是一个升高—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。
（2）开采初期水合物分解产生的甲烷气体在地层中累积，导致水合物分解速率降低。开采后期由于“气穴现象”导致水合物分解速率发生波动，但总体水合物分解速率逐渐加快。
（3）开采初期井口抽出的大部分是水，由于气体不能被及时抽出而在地层中富集，导致地层中气体饱和度逐渐增大。开采后期地层中饱和度降低，井口产气速率增大，地层中甲烷气体的饱和度降低。
（4）水合物的分解过程同时发生在水合物层的上部、下部和水平方向，在井口附近会产生一个低温区，表明井口附近水合物分解较快。体系的压力波传递速度较快，在开采过程中整个体系压力迅速趋于一致。
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