In order to study water transfer characteristics inside non-saturated media during methane hydrate formation and dissociation processes,water changes on the top,middle and bottom locations of experimental media during the reaction processes were continuously followed with a novel apparatus with three pF-meter sensors.Coarse sand,fine sand and loess were chosen as experimental media.It was experimentally observed that methane hydrate was easier formed inside coarse sand and fine sand than inside loess.Methane hydrate formation configuration and water transfer characteristics during methane hydrate formation processes were very different among the different non-saturated media,which were important for understanding methane hydrate formation and dissociation mechanism inside sediments in nature.
Peng Zhang Qingbai Wu Guanli Jiang Jing Zhan Yingmei Wang
In order to study the nature of gas hydrate in porous media,the formation and dissociation processes of methane hydrate in loess were investigated.Five cooling rates were applied to form methane hydrate.The nucleation times of methane hydrate formation at each cooling rate were measured for comparison.The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion.Under the same initial conditions,the faster the cooling rate,the shorter the nucleation time,and the lower the methane gas conversion.Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess.The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature,the faster the dissociating rates of methane hydrate.
Gas hydrates formation and dissociation processes inside porous media are always accompanied by water transfer behavior, which is similar to the water behavior of ice freezing and thawing processes. These processes have been studied by many researchers, but all the studies are so far on the water transfer characteristics outside porous media and the water transfer characteristics inside porous media have been little known. In this study, in order to study the water transfer characteristics inside porous media during methane hydrate formation and dissociation processes, a novel apparatus with three pF-meter sensors which can detect water content changes inside porous media was applied. It was experimentally observed that methane hydrate formation processes were accompanied by water transfer from bottom to top inside porous media, however, the water behavior during hydrate dissociation processes was abnormal, for which more studies are needed to find out the real reason in our future work.
Peng Zhang, Qingbai Wu, Yibin Pu, Guanli Jiang, Jing Zhan, Yingmei Wang State Key Laboratory, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, Gansu, China
Hydrate formation and dissociation processes are always accompanied by water migration in porous media, which is similar to the ice. In our study, a novel pF-meter sensor which could detect the changes of water content inside sand was first applied to hydrate formation and dissociation processes. It also can study the water change characteristics in the core scale of a partially saturated silica sand sample and compare the differences of water changes between the processes of formation and dissociation of methane hydrate and freezing and thawing of ice. The experimental results showed that the water changes in the processes of formation and dissociation of methane hydrate were basically similar to that of the freezing and thawing of ice in sand. When methane hydrate or ice was formed, water changes showed the decrease in water content on the whole and the pF values rose following the formation processes. However, there were very obvious differences between the ice thawing and hydrate dissociation.
With a new apparatus designed and assembled by ourselves, the matrix potential of non-saturated loess was firstly measured and studied during methane hydrate formation processes. The experimental results showed that during two formation processes, the matrix potential changes of the loess all presented a good linear relationship with water conversion ratios. In addition, although it was well known that the secondary gas hydrate formation was easier than the initial, our experimental results showed that the initial hydrate formation efficiency in non-saturated loess was higher than that of the secondary.
