含甲烷水合物沉积层的力学性质与南海陆坡稳定性的研究

Huge quantities of methane hydrates have been found in the submarine sediments on the continental slope in the Pearl River Mouth Basin, South China Sea. The hydrates have the potential to become a promising clean energy source, however, past records of massive submarine landslides and observations of pockmarks in this region related to active hydrate dissociation triggered by changes in environmental conditions, both natural and anthropogenic, have raised environmental concerns. Existing researches provide solid knowledge of the equilibrium and dissociation of hydrates in laboratory conditions, but in submarine sediments the equilibrium may shift due to anomalies from hydrate premelting at the grain boundaries and curved interface in confined pores. These anomalies facilitate the growth of segregated high-saturation hydrates-bearing layer and create weak patches poised for failure. In our proposed research, we will quantitively assess how environmental conditions affect the hydrate distribution in the submarine sediments and the transport of methane into the overlying ocean and atmosphere, and apply the results to estimate the stability of continental slopes in the northern South China Sea. Our approach focuses on mathematical models of the physical processes controlling the hydrate anomalies, weakening the sediment contacts, and initializing landslides and pockmarks. The infinite-slope analysis with rate-and-state formulations for stress perturbation along finite slipping patches due to hydrate dissociation is used to examine the landslide potential. As an important indicator of significant hydrate dissociation, the pockmarks and chimneys are studied to better understand potential mechanisms that generate these gas-escape features. Our work will test the hypothesis that convection in the sediments causes the regular-spacing pockmark field and relate the morphological features to the methane-producing microbial activities. Each component of the research is designed to fill critical gaps in understanding the hydrate resources in the South China Sea and explain their response to environmental changes, providing essential guidance for future exploitation of unconventional hydrocarbon and improving forecasts for submarine landslides that could threat our infrastructure, disrupt our hydrate reserves, and pollute the ocean and atmosphere with vast amount of methane in the Pearl River Mouth Basin.

甲烷水合物是很有潜力的清洁能源，在我国南海珠江口盆地的陆坡上储量巨大。自然或者人为因素导致的环境条件变化都会导致水合物迅速分解，触发海底滑坡和海底麻坑。前人对实验室中水合物的分解进行了详细的研究，但是在海底沉积物中，受水合物在颗粒表面预熔化和有限孔隙中的弯曲界面影响，水合物的平衡条件会和实验室中有所不同，促使高饱和含水合物层的生长，产生易断裂的低强度片区。我们计划研究海底沉积物中的水合物分布，定量描述水合物的分层生长和分解，解释沉积层局部弱化和滑坡与麻坑的产生，并结合有限滑动面上因速率—状态摩擦关系造成的应力扰动，分析滑坡的可能性，评估南海北部陆坡的稳定性。模型将探讨气体逸出地貌的形成机制，以及此类地貌和产甲烷微生物活动之间的联系。研究的每一部分都有助于我们更好地理解南海水合物对环境因素的稳定性，为在珠江口盆地一带的能源开发和地质灾害预防提供重要的指导。

甲烷水合物是很有潜力的清洁能源，在我国南海珠江口盆地的陆坡沉积物中储量巨大。自然或者人为因素导致的环境条件变化都会导致水合物迅速分解，触发海底滑坡和海底麻坑等现象。前人对实验室中水合物的稳定性进行了详细的研究，但是在海底沉积物中，受水合物在颗粒表面预熔化和有限孔隙中的弯曲水合物界面影响，水合物的平衡条件会和实验室中有所不同，促使高饱和含水合物层的生长，产生易断裂的低强度片区。为了更好地模拟沉积物中的不规则孔隙，我们用球形颗粒聚合体模拟颗粒沉积物，采用蒙特卡洛方法对颗粒间的不规则孔隙进行抽样统计。利用此方法，并与冰透镜做类比，我们描述因为孔隙水随着相变程度增加而减少的过程中，水–水合物（或冰）的界面曲率变化，以及由此产生的相平衡偏移。在冻土中，该关系可以用来预测盐类（包括氯盐和高氯酸盐）对冻结曲线的影响，对火星等地外环境中液态水的存在可能性有重要意义。例如，在含有大量高氯酸盐的火星表层土壤中，在零下数十度的环境中土壤孔隙中依然能够有大量液态孔隙水。在海底沉积物中水合物稳定区底部，我们抽样统计球形颗粒中的水合物相—液相与气相–液相边界曲率，得到在不同粒径的沉积物中，水合物–游离气–液相三相共存的区域相对于理论预测的稳定带底部的距离。在细粒沉积物中，这个区域的厚度可达数十到数百米，对解释水合物与下伏游离气之间的声反射界面 BSR 有一定帮助。在研究麻坑等海底气体逸出地貌时，我们发现油气领域常用的几个估算含水合物沉积层中的渗透率的公式彼此相差巨大，对模拟结果造成很大影响。为此我们提出了一种基于浮动随机游走法计算沉积层渗透率随水合物（或冰）含量变化的方法，这种方法对水合物（或冰）的体积分数采用了概率诠释，可以用来更好地模拟因孔隙水到水合物或冰的相变导致的颗粒介质的渗透率变化，为下一步模拟麻坑等现象打下良好的基础。