Abstract: Natural gas hydrates (NGH)in the northern South China Sea (SCS)are mainly distributed in argillaceous silt, with rich reserves but low production efficiency. The whole production process is accompanied by the dissociation of natural gas hydrate, which is an endothermic process. In order to achieve efficient production, it is critical to precisely determine the conditions and heat absorption capacity of NGH dissociation. The argillaceous silt cores of quartz powder, montmorillonite and SCS soil were prepared by simulating the reservoir conditions in Shenhu area. The conditions and heat absorption capacity of methane hydrate dissociation in the core were determined by high pressure differential scanning calorimetry (DSC). It is found that the dissociation temperature of methane hydrate in argillaceous silt is lower than bulk hydrate, showing the characteristics of multistage dissociation. The offsets of the initial dissociation temperature of methane hydrate in montmorillonite and SCS core are -8.53~-6.37 K and -2.46~-2.25 K, respectively, and the offsets of the end dissociation temperature are -1.04~-0.75 K and -0.56~-0.37 K, respectively. The dissociation heat adsorption capacity of methane hydrate in quartz powder, montmorillonite and SCS soil were 78.2 kJ/L, 31.6 kJ/L and 34.6 kJ/L, respectively (hydrate saturation was 36.7 % ~37.3 %, 13.9 % ~22.3 % and 20.2 % ~23.0 %, respectively). Secondary hydrate formation was generally observed during the heating process. A fast prediction method of hydrate dissociation conditions in cores based on water activity measurement was established, and the difference between the predicted and the experimental temperature was less than 0.68 K. The analysis shows that the pore structure and hydration of argillaceous silt have an important influence on the conditions and heat absorption capacity of methane hydrate dissociation.

Key words: methane hydrate, argillaceous silt, dissociation conditions, heat absorption capacity, water activity

摘要： 中国南海北部天然气水合物主要分布在泥质粉砂中,储量丰富但开采效率低。开采全程伴随着天然气水合物的分解,是一个吸热过程。要实现高效开采,必须准确掌握天然气水合物分解的条件及吸热量。通过模拟神狐海域的储层条件,制备了石英粉、蒙脱土和南海土等泥质粉砂岩心,利用高压差示扫描量热仪测定了岩心中甲烷水合物分解的条件和吸热量。研究发现,泥质粉砂中甲烷水合物的分解温度低于非孔隙中水合物,呈多级分解的特征。蒙脱土和南海土岩心中甲烷水合物开始分解温度的偏移量分别为-8.53~-6.37 K和-2.46~-2.25 K,结束分解温度的偏移量分别为-1.04~-0.75 K和-0.56~-0.37 K。石英粉、蒙脱土、南海土中甲烷水合物分解的吸热量分别为78.2 kJ/L、31.6 kJ/L和34.6 kJ/L (水合物饱和度分别为36.7%~37.3%、13.9%~22.3%和20.2%~23.0%)。升温过程中普遍观测到水合物二次生成。建立了基于水活度测量的岩心中水合物分解条件快速预测方法,预测温度与实验值之差小于0.68 K。研究结果表明,泥质粉砂的孔隙结构和水化作用对甲烷水合物分解的条件和吸热量有重要影响。

关键词: 甲烷水合物, 泥质粉砂, 分解条件, 吸热量, 水活度