介观孔隙中天然气水合物生成过程模拟

目前，对沉积物中天然气水合物（以下简称水合物）的生成过程研究大部分集中在实验研究以及水合物生成的本征动力学方面，在孔隙尺度方面缺乏对水合物生成过程的全方位考虑。为此，从介观角度出发，结合实验模拟结果，研究了沉积物孔隙中甲烷及二氧化碳水合物的生成过程。首先对甲烷水合物和二氧化碳水合物在沉积物中的生成过程进行了实验模拟（沉积物样品选用南海浅表层沉积物，粒径为60~100 目），结果表明：水合物在沉积物孔隙中的生成过程比较平缓，体系温度基本没有大的变化；在初始阶段水合物生成量比较大，随着反应的进行，水合物生成速率逐渐减小。在实验的基础上，以孔隙水中溶解气体的浓度为变量，从介观角度数值模拟了甲烷水合物和二氧化碳水合物在沉积物中的生成过程，并以单个沉积物孔隙空间的水合物生成表征沉积物体系内水合物的生成特性。结论认为：沉积物颗粒堆积孔隙内部的水合物先在沉积物壁面处生成，然后水合物层逐渐加厚，向孔隙中心生长，水合物呈层状生成，最后填满整个沉积物颗粒孔隙；伴随着水合物的生成，沉积物体系孔隙率降低。通过模拟计算得到的水合物转化率与实验结果进行对比，其误差范围介于3%~7%，表明该模型具有较强的可靠性。

A simulation study of natural gas hydrate generation in mesoscopic-scale pores

The studies of natural gas hydrate formation process in deposits are still at the experimental stage and mostly focus on that in terms of intrinsic kinetics, but rarely consider the scale of pores in the deposits. In view of this, we made an investigation, based on the simulation results, into the formation process of methane and carbon dioxide hydrate in mesoscopic scale pores of deposits. First, we conducted such an experimental simulation study, in which the shallow deposits in the South China Sea were chosen as the samples with the pore size of 60 ~ 100 meshes. The following results were achieved. a. The process of natural gas hydrate formation in the pores of the deposits is relatively stable and slow and the temperature of the system basically keeps stable without significant fluctuation. b. The rate of gas hydrate formation is relatively great at the initial stage and then decreases progressively along with the progress of reaction. On this basis, with the concentration of the dissolved gas in pore water as the variable, a mesoscopic pore space model was established to characterize the gas hydrate formation process in the pores of deposits. Thus, the other findings were obtained. a. Gas hydrate first is generated at the interface of a sediment particle, then becomes thickening and grows to the center of pore space, and finally forms in layers until it fills up the whole pore space. b. Relative porosity of sediments gradually decreases with the formation of gas hydrate. In comparison with the experimental data, the calculated results from the simulation modeling produce an error of 3 ~ 7%, indicating that the simulation model is of rather high reliability.