Abstract

A short abstract Submarine natural gas hydrate production presents many challenges which include gas productivity and potential geologic disaster. Stratum subsidence, marine slope slide, sand production and wellbore instability are the typical geological risks. Due to the complexity and high expense of the field test, the numerical simulation is developed coupling with geostress, thermal field and flow field based on ABAQUS. According to current work in Shenhu area in South China Sea, a vertical well and the depressurization method are adopted in the model. Results show that the hydrate decomposition region is tapered from hydrate-bearing stratum (HBS) bottom to top in spatial distribution. The gas production is insufficient from hydrate using the depressurization method for commercial mining. The depressurization method increases the effective hydrate stress in HBS, which is roughly equivalent to the value of the pressure drop. With the cooperation of the excess pore pressure during water expansion from hydrate formation and the decrease of additional strength, they cause the hydrate formation to be compressed and eventually leads to seafloor subsidence. The reduction of production pressure can effectively increase the hydrate yield, but the resulting formation subsidence is also more significant.

Introduction

Natural gas hydrates (NGH) are ice-like crystalline components formed from water and hydrocarbon gas (Chen et al., 2017). NGH widely presents in ocean and permafrost area with a huge amount (Makogon, 1966, 1981, 1982). Some estimations think that the energy stored in NGH reservoirs are twice than the sum of fossil fuel deposits (Feng et al., 2012). And many countries try to drill it to produce natural gas. NGH is the potential resource for its enormous reservoirs.

NGH is stabilised in conditions of high pressure and low temperature (Lu et al., 2017). When the condition is broken, the dissociation will appear. 1m3 NGH can dissociate into 0.80 m3 water and 150~180 m3 gas approximately in normal condition (Lu et al., 2017; Esmaeilzadeh et al., 2008). During the dissociation process, NGH will form a heterogeneous surface because of heterogeneous phase reaction (Feng et al., 2012). Meanwhile, the process can directly reduce the strength of hydrate-bearing stratum (HBS) with the cooperation of the excess pore pressure and the decrease of additional strength provided by NGH and then lead to geological disasters and engineering damages, such as the destruction of seabed, wellhole and foundation (Lu et al., 2017; Hu et al., 2004).

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