考虑甲烷溶解的天然气水合物分解超孔隙水压力

天然气水合物的分解会生成大量气体和水,在土层渗透性较差的情况下,大量气体无法及时溢出,压力不易传导到外部,使土层的超孔隙水压力迅速增大,有效应力降低,同时水合物分解后其自身与沉积物的胶结作用大幅减小,导致沉积物强度降低,发生大规模变形或失稳,进而引发海底失稳等灾害。基于热力学基本理论,建立了考虑水合物分解产生的甲烷气体部分溶解的超孔隙水压力模型,并对其影响因素进行讨论,最后针对南海工况研究水合物分解对海床稳定性的影响。结果表明,水合物分解产生的超孔隙水压力随着水合物分解先迅速增大后缓慢增长最后趋于稳定;初始饱和度、塑性指数、水深、沉积物埋深、内摩擦角等是影响水合物分解产生的超孔隙水压力的关键影响因素;影响海床稳定性的关键不利因素是超孔隙水压力聚积而非土体强度弱化;生成的甲烷气体二次溶解使超孔隙水压力减小,一定程度上缓解了斜坡的滑动。

Excess pore pressure of gas hydrate dissociation considering methane dissolution

The dissociation of natural gas hydrate will generate a large amount of gas and water. In the case of poor permeability of the soil layer, a large amount of gas cannot escape in time, and the pressure is not easily transmitted to the outside, so that the excess pore water pressure in the soil layer increases rapidly and the effective stress decreases. At the same time, the cementation between hydrate and sediment is greatly reduced after hydrate dissociation. As a result, the sediment strength will decrease and large-scale deformation or instability will occur, which will lead to seabed instability and other disasters. Based on the basic theory of thermodynamics, an excess pore water pressure model considering the partial dissolution of methane gas produced by hydrate dissociation was established, and then its influencing factors were discussed in this paper. Finally, the influence of hydrate dissociation on the stability of seabed was studied in allusion to the condition of the South China Sea. The results show that the excess pore water pressure generated by hydrate dissociation first increases rapidly, then increases slowly, and finally tends to be stable; the initial saturation, plasticity index, water depth, burial depth of sediments, internal friction angle, etc. are key factors that influence the excess pore water pressure caused by hydrate dissociation; the key unfavorable factor that affects the stability of the seabed is the accumulation of excess pore water pressure rather than the weakening of strength of soil mass; the secondary dissolution of the methane gas generated reduces the excess pore water pressure, which alleviates the sliding of the slope to some extent.